From 21a3eb62650507179f79cf89ff09127dabe7f91f Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Tue, 8 Feb 2022 15:05:28 -0800
Subject: [PATCH 01/10] [camera]remove self ref pointer for save photo
 delegate, and make sure thread safety"

---
 packages/camera/camera/CHANGELOG.md           |   1 +
 .../ios/Runner.xcodeproj/project.pbxproj      |  14 +-
 .../ios/RunnerTests/FLTCamPhotoCaptureTests.m | 172 ++++++++++++++++++
 ...QueueTests.m => FLTCamSampleBufferTests.m} |   4 +-
 .../RunnerTests/FLTSavePhotoDelegateTests.m   |  93 +++++-----
 packages/camera/camera/ios/Classes/FLTCam.m   |  28 ++-
 .../camera/camera/ios/Classes/FLTCam_Test.h   |  15 ++
 .../camera/ios/Classes/FLTSavePhotoDelegate.h |  25 +--
 .../camera/ios/Classes/FLTSavePhotoDelegate.m |  24 ++-
 .../ios/Classes/FLTSavePhotoDelegate_Test.h   |   5 +
 10 files changed, 303 insertions(+), 78 deletions(-)
 create mode 100644 packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
 rename packages/camera/camera/example/ios/RunnerTests/{SampleBufferQueueTests.m => FLTCamSampleBufferTests.m} (93%)

diff --git a/packages/camera/camera/CHANGELOG.md b/packages/camera/camera/CHANGELOG.md
index d2b2f907a6fb..f14522c5ab40 100644
--- a/packages/camera/camera/CHANGELOG.md
+++ b/packages/camera/camera/CHANGELOG.md
@@ -1,5 +1,6 @@
 ## 0.9.4+12
 
+* Updates iOS camera's photo capture delegate reference on a background queue to prevent potential race conditions, and some related internal code cleanup.
 * Skips unnecessary AppDelegate setup for unit tests on iOS.
 * Internal code cleanup for stricter analysis options.
 
diff --git a/packages/camera/camera/example/ios/Runner.xcodeproj/project.pbxproj b/packages/camera/camera/example/ios/Runner.xcodeproj/project.pbxproj
index ac39de2e3f84..5f788fc9b9f9 100644
--- a/packages/camera/camera/example/ios/Runner.xcodeproj/project.pbxproj
+++ b/packages/camera/camera/example/ios/Runner.xcodeproj/project.pbxproj
@@ -3,7 +3,7 @@
 	archiveVersion = 1;
 	classes = {
 	};
-	objectVersion = 50;
+	objectVersion = 46;
 	objects = {
 
 /* Begin PBXBuildFile section */
@@ -23,11 +23,12 @@
 		E01EE4A82799F3A5008C1950 /* QueueHelperTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E01EE4A72799F3A5008C1950 /* QueueHelperTests.m */; };
 		E032F250279F5E94009E9028 /* CameraCaptureSessionQueueRaceConditionTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E032F24F279F5E94009E9028 /* CameraCaptureSessionQueueRaceConditionTests.m */; };
 		E04F108627A87CA600573D0C /* FLTSavePhotoDelegateTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E04F108527A87CA600573D0C /* FLTSavePhotoDelegateTests.m */; };
+		E071CF7227B3061B006EF3BA /* FLTCamPhotoCaptureTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E071CF7127B3061B006EF3BA /* FLTCamPhotoCaptureTests.m */; };
+		E071CF7427B31DE4006EF3BA /* FLTCamSampleBufferTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E071CF7327B31DE4006EF3BA /* FLTCamSampleBufferTests.m */; };
 		E0C6E2002770F01A00EA6AA3 /* ThreadSafeMethodChannelTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0C6E1FD2770F01A00EA6AA3 /* ThreadSafeMethodChannelTests.m */; };
 		E0C6E2012770F01A00EA6AA3 /* ThreadSafeTextureRegistryTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0C6E1FE2770F01A00EA6AA3 /* ThreadSafeTextureRegistryTests.m */; };
 		E0C6E2022770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0C6E1FF2770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m */; };
 		E0F95E3D27A32AB900699390 /* CameraPropertiesTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0F95E3C27A32AB900699390 /* CameraPropertiesTests.m */; };
-		E0F95E4427A36B9200699390 /* SampleBufferQueueTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0F95E4327A36B9200699390 /* SampleBufferQueueTests.m */; };
 		E487C86026D686A10034AC92 /* CameraPreviewPauseTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E487C85F26D686A10034AC92 /* CameraPreviewPauseTests.m */; };
 		F6EE622F2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m in Sources */ = {isa = PBXBuildFile; fileRef = F6EE622E2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m */; };
 /* End PBXBuildFile section */
@@ -85,11 +86,12 @@
 		E01EE4A72799F3A5008C1950 /* QueueHelperTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = QueueHelperTests.m; sourceTree = "<group>"; };
 		E032F24F279F5E94009E9028 /* CameraCaptureSessionQueueRaceConditionTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = CameraCaptureSessionQueueRaceConditionTests.m; sourceTree = "<group>"; };
 		E04F108527A87CA600573D0C /* FLTSavePhotoDelegateTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = FLTSavePhotoDelegateTests.m; sourceTree = "<group>"; };
+		E071CF7127B3061B006EF3BA /* FLTCamPhotoCaptureTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = FLTCamPhotoCaptureTests.m; sourceTree = "<group>"; };
+		E071CF7327B31DE4006EF3BA /* FLTCamSampleBufferTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = FLTCamSampleBufferTests.m; sourceTree = "<group>"; };
 		E0C6E1FD2770F01A00EA6AA3 /* ThreadSafeMethodChannelTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ThreadSafeMethodChannelTests.m; sourceTree = "<group>"; };
 		E0C6E1FE2770F01A00EA6AA3 /* ThreadSafeTextureRegistryTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ThreadSafeTextureRegistryTests.m; sourceTree = "<group>"; };
 		E0C6E1FF2770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ThreadSafeEventChannelTests.m; sourceTree = "<group>"; };
 		E0F95E3C27A32AB900699390 /* CameraPropertiesTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = CameraPropertiesTests.m; sourceTree = "<group>"; };
-		E0F95E4327A36B9200699390 /* SampleBufferQueueTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = SampleBufferQueueTests.m; sourceTree = "<group>"; };
 		E487C85F26D686A10034AC92 /* CameraPreviewPauseTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = CameraPreviewPauseTests.m; sourceTree = "<group>"; };
 		F63F9EED27143B19002479BF /* MockFLTThreadSafeFlutterResult.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = MockFLTThreadSafeFlutterResult.h; sourceTree = "<group>"; };
 		F6EE622E2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = MockFLTThreadSafeFlutterResult.m; sourceTree = "<group>"; };
@@ -126,8 +128,9 @@
 				E0C6E1FF2770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m */,
 				E0C6E1FD2770F01A00EA6AA3 /* ThreadSafeMethodChannelTests.m */,
 				E0C6E1FE2770F01A00EA6AA3 /* ThreadSafeTextureRegistryTests.m */,
-				E0F95E4327A36B9200699390 /* SampleBufferQueueTests.m */,
 				E04F108527A87CA600573D0C /* FLTSavePhotoDelegateTests.m */,
+				E071CF7127B3061B006EF3BA /* FLTCamPhotoCaptureTests.m */,
+				E071CF7327B31DE4006EF3BA /* FLTCamSampleBufferTests.m */,
 				E01EE4A72799F3A5008C1950 /* QueueHelperTests.m */,
 				E487C85F26D686A10034AC92 /* CameraPreviewPauseTests.m */,
 				F6EE622E2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m */,
@@ -396,12 +399,13 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
-				E0F95E4427A36B9200699390 /* SampleBufferQueueTests.m in Sources */,
 				03F6F8B226CBB4670024B8D3 /* ThreadSafeFlutterResultTests.m in Sources */,
 				033B94BE269C40A200B4DF97 /* CameraMethodChannelTests.m in Sources */,
+				E071CF7227B3061B006EF3BA /* FLTCamPhotoCaptureTests.m in Sources */,
 				E0F95E3D27A32AB900699390 /* CameraPropertiesTests.m in Sources */,
 				03BB766B2665316900CE5A93 /* CameraFocusTests.m in Sources */,
 				E487C86026D686A10034AC92 /* CameraPreviewPauseTests.m in Sources */,
+				E071CF7427B31DE4006EF3BA /* FLTCamSampleBufferTests.m in Sources */,
 				E04F108627A87CA600573D0C /* FLTSavePhotoDelegateTests.m in Sources */,
 				F6EE622F2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m in Sources */,
 				334733EA2668111C00DCC49E /* CameraOrientationTests.m in Sources */,
diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
new file mode 100644
index 000000000000..8f6f6494dbf8
--- /dev/null
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
@@ -0,0 +1,172 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@import camera;
+@import camera.Test;
+@import AVFoundation;
+@import XCTest;
+#import <OCMock/OCMock.h>
+
+@interface FLTCamPhotoCaptureTests : XCTestCase
+
+@end
+
+@implementation FLTCamPhotoCaptureTests
+
+- (void)testCaptureToFile_savePhotoDelegateReferencesMustBeAccessedOnCaptureSessionQueue {
+  XCTestExpectation *setReferenceExpectation =
+      [self expectationWithDescription:
+                @"FLTSavePhotoDelegate references must be set on capture session queue."];
+  XCTestExpectation *clearReferenceExpectation =
+      [self expectationWithDescription:
+                @"FLTSavePhotoDelegate references must be cleared on capture session queue."];
+
+  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
+  const char *captureSessionQueueSpecific = "capture_session_queue";
+  dispatch_queue_set_specific(captureSessionQueue, captureSessionQueueSpecific,
+                              (void *)captureSessionQueueSpecific, NULL);
+  FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
+
+  // settings.uniqueID is used as the key for `inProgressSavePhotoDelegates` dictionary
+  AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
+  id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
+  OCMStub([mockSettings photoSettings]).andReturn(settings);
+
+  // We need to make sure the delegate reference is actually saved in a real dictionary, so that we
+  // can call its completion handler later. Must use a partial mock, in order to forward invocation
+  // to the real object.
+  id mockDelegates = OCMPartialMock([NSMutableDictionary dictionary]);
+  OCMStub([mockDelegates setObject:OCMOCK_ANY forKeyedSubscript:OCMOCK_ANY])
+      .andDo(^(NSInvocation *invocation) {
+        if (dispatch_get_specific(captureSessionQueueSpecific)) {
+          FLTSavePhotoDelegate *delegate;
+          // Index 0 and 1 are `self` and `_cmd`.
+          [invocation getArgument:&delegate atIndex:2];
+          if (delegate) {
+            [setReferenceExpectation fulfill];
+          } else {
+            [clearReferenceExpectation fulfill];
+          }
+        }
+      })
+      .andForwardToRealObject();
+  cam.inProgressSavePhotoDelegates = mockDelegates;
+
+  id mockOutput = OCMClassMock([AVCapturePhotoOutput class]);
+  OCMStub([mockOutput capturePhotoWithSettings:OCMOCK_ANY delegate:OCMOCK_ANY])
+      .andDo(^(NSInvocation *invocation) {
+        FLTSavePhotoDelegate *delegate = cam.inProgressSavePhotoDelegates[@(settings.uniqueID)];
+        XCTAssertNotNil(delegate, @"Delegate reference must be saved to the dictionary.");
+        // Completion runs on IO queue.
+        dispatch_queue_t ioQueue = dispatch_queue_create("io_queue", NULL);
+        dispatch_async(ioQueue, ^{
+          NSString *filePath = @"test";
+          delegate.completionHandler(nil, filePath);
+        });
+      });
+  cam.capturePhotoOutput = mockOutput;
+
+  // `FLTCam::captureToFile` runs on capture session queue.
+  dispatch_async(captureSessionQueue, ^{
+    [cam captureToFile:OCMClassMock([FLTThreadSafeFlutterResult class])];
+  });
+
+  [self waitForExpectationsWithTimeout:1 handler:nil];
+}
+
+- (void)testCaptureToFile_mustReportErrorToResultIfSavePhotoDelegateCompletionsWithError {
+  XCTestExpectation *errorExpectation =
+      [self expectationWithDescription:
+                @"Must send error to result if save photo delegate completes with error."];
+
+  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
+  FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
+
+  AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
+  id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
+  OCMStub([mockSettings photoSettings]).andReturn(settings);
+
+  NSError *error = [NSError errorWithDomain:@"test" code:0 userInfo:nil];
+  id mockResult = OCMClassMock([FLTThreadSafeFlutterResult class]);
+  OCMStub([mockResult sendError:error]).andDo(^(NSInvocation *invocation) {
+    [errorExpectation fulfill];
+  });
+
+  id mockOutput = OCMClassMock([AVCapturePhotoOutput class]);
+  OCMStub([mockOutput capturePhotoWithSettings:OCMOCK_ANY delegate:OCMOCK_ANY])
+      .andDo(^(NSInvocation *invocation) {
+        FLTSavePhotoDelegate *delegate = cam.inProgressSavePhotoDelegates[@(settings.uniqueID)];
+        // Completion runs on IO queue.
+        dispatch_queue_t ioQueue = dispatch_queue_create("io_queue", NULL);
+        dispatch_async(ioQueue, ^{
+          delegate.completionHandler(error, nil);
+        });
+      });
+  cam.capturePhotoOutput = mockOutput;
+
+  // `FLTCam::captureToFile` runs on capture session queue.
+  dispatch_async(captureSessionQueue, ^{
+    [cam captureToFile:mockResult];
+  });
+
+  [self waitForExpectationsWithTimeout:1 handler:nil];
+}
+
+- (void)testCaptureToFile_mustReportPathToResultIfSavePhotoDelegateCompletionsWithPath {
+  XCTestExpectation *pathExpectation =
+      [self expectationWithDescription:
+                @"Must send file path to result if save photo delegate completes with file path."];
+
+  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
+  FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
+
+  AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
+  id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
+  OCMStub([mockSettings photoSettings]).andReturn(settings);
+
+  NSString *filePath = @"test";
+  id mockResult = OCMClassMock([FLTThreadSafeFlutterResult class]);
+  OCMStub([mockResult sendSuccessWithData:filePath]).andDo(^(NSInvocation *invocation) {
+    [pathExpectation fulfill];
+  });
+
+  id mockOutput = OCMClassMock([AVCapturePhotoOutput class]);
+  OCMStub([mockOutput capturePhotoWithSettings:OCMOCK_ANY delegate:OCMOCK_ANY])
+      .andDo(^(NSInvocation *invocation) {
+        FLTSavePhotoDelegate *delegate = cam.inProgressSavePhotoDelegates[@(settings.uniqueID)];
+        // Completion runs on IO queue.
+        dispatch_queue_t ioQueue = dispatch_queue_create("io_queue", NULL);
+        dispatch_async(ioQueue, ^{
+          delegate.completionHandler(nil, filePath);
+        });
+      });
+  cam.capturePhotoOutput = mockOutput;
+
+  // `FLTCam::captureToFile` runs on capture session queue.
+  dispatch_async(captureSessionQueue, ^{
+    [cam captureToFile:mockResult];
+  });
+  [self waitForExpectationsWithTimeout:1 handler:nil];
+}
+
+/// Creates an `FLTCam` that runs its operations on a given capture session queue.
+- (FLTCam *)createFLTCamWithCaptureSessionQueue:(dispatch_queue_t)captureSessionQueue {
+  id inputMock = OCMClassMock([AVCaptureDeviceInput class]);
+  OCMStub([inputMock deviceInputWithDevice:[OCMArg any] error:[OCMArg setTo:nil]])
+      .andReturn(inputMock);
+
+  id sessionMock = OCMClassMock([AVCaptureSession class]);
+  OCMStub([sessionMock alloc]).andReturn(sessionMock);
+  OCMStub([sessionMock addInputWithNoConnections:[OCMArg any]]);  // no-op
+  OCMStub([sessionMock canSetSessionPreset:[OCMArg any]]).andReturn(YES);
+
+  return [[FLTCam alloc] initWithCameraName:@"camera"
+                           resolutionPreset:@"medium"
+                                enableAudio:true
+                                orientation:UIDeviceOrientationPortrait
+                        captureSessionQueue:captureSessionQueue
+                                      error:nil];
+}
+
+@end
diff --git a/packages/camera/camera/example/ios/RunnerTests/SampleBufferQueueTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTCamSampleBufferTests.m
similarity index 93%
rename from packages/camera/camera/example/ios/RunnerTests/SampleBufferQueueTests.m
rename to packages/camera/camera/example/ios/RunnerTests/FLTCamSampleBufferTests.m
index 19cead905f61..ccc8de5b23bd 100644
--- a/packages/camera/camera/example/ios/RunnerTests/SampleBufferQueueTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTCamSampleBufferTests.m
@@ -8,11 +8,11 @@
 @import XCTest;
 #import <OCMock/OCMock.h>
 
-@interface SampleBufferQueueTests : XCTestCase
+@interface FLTCamSampleBufferTests : XCTestCase
 
 @end
 
-@implementation SampleBufferQueueTests
+@implementation FLTCamSampleBufferTests
 
 - (void)testSampleBufferCallbackQueueMustBeCaptureSessionQueue {
   id inputMock = OCMClassMock([AVCaptureDeviceInput class]);
diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m
index b6ea84da449c..cb8ae3f1742f 100644
--- a/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m
@@ -14,40 +14,44 @@ @interface FLTSavePhotoDelegateTests : XCTestCase
 
 @implementation FLTSavePhotoDelegateTests
 
-- (void)testHandlePhotoCaptureResult_mustSendErrorIfFailedToCapture {
-  NSError *error = [NSError errorWithDomain:@"test" code:0 userInfo:nil];
-  dispatch_queue_t ioQueue = dispatch_queue_create("test", NULL);
-  id mockResult = OCMClassMock([FLTThreadSafeFlutterResult class]);
-  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc] initWithPath:@"test"
-                                                                       result:mockResult
-                                                                      ioQueue:ioQueue];
+- (void)testHandlePhotoCaptureResult_mustCompleteWithErrorIfFailedToCapture {
+  XCTestExpectation *completionExpectation =
+      [self expectationWithDescription:@"Must complete with error if failed to capture photo."];
 
-  [delegate handlePhotoCaptureResultWithError:error
+  NSError *captureError = [NSError errorWithDomain:@"test" code:0 userInfo:nil];
+  dispatch_queue_t ioQueue = dispatch_queue_create("test", NULL);
+  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
+           initWithPath:@"test"
+                ioQueue:ioQueue
+      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+        XCTAssertEqualObjects(captureError, error);
+        XCTAssertNil(path);
+        [completionExpectation fulfill];
+      }];
+
+  [delegate handlePhotoCaptureResultWithError:captureError
                             photoDataProvider:^NSData * {
                               return nil;
                             }];
-  OCMVerify([mockResult sendError:error]);
+  [self waitForExpectationsWithTimeout:1 handler:nil];
 }
 
-- (void)testHandlePhotoCaptureResult_mustSendErrorIfFailedToWrite {
-  XCTestExpectation *resultExpectation =
-      [self expectationWithDescription:@"Must send IOError to the result if failed to write file."];
+- (void)testHandlePhotoCaptureResult_mustCompleteWithErrorIfFailedToWrite {
+  XCTestExpectation *completionExpectation =
+      [self expectationWithDescription:@"Must complete with error if failed to write file."];
   dispatch_queue_t ioQueue = dispatch_queue_create("test", NULL);
-  id mockResult = OCMClassMock([FLTThreadSafeFlutterResult class]);
 
   NSError *ioError = [NSError errorWithDomain:@"IOError"
                                          code:0
                                      userInfo:@{NSLocalizedDescriptionKey : @"Localized IO Error"}];
-
-  OCMStub([mockResult sendErrorWithCode:@"IOError"
-                                message:@"Unable to write file"
-                                details:ioError.localizedDescription])
-      .andDo(^(NSInvocation *invocation) {
-        [resultExpectation fulfill];
-      });
-  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc] initWithPath:@"test"
-                                                                       result:mockResult
-                                                                      ioQueue:ioQueue];
+  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
+           initWithPath:@"test"
+                ioQueue:ioQueue
+      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+        XCTAssertEqualObjects(ioError, error);
+        XCTAssertNil(path);
+        [completionExpectation fulfill];
+      }];
 
   // We can't use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
   // `XCTRunnerIDESession::logDebugMessage:`) on a private queue.
@@ -63,23 +67,25 @@ - (void)testHandlePhotoCaptureResult_mustSendErrorIfFailedToWrite {
   [self waitForExpectationsWithTimeout:1 handler:nil];
 }
 
-- (void)testHandlePhotoCaptureResult_mustSendSuccessIfSuccessToWrite {
-  XCTestExpectation *resultExpectation = [self
-      expectationWithDescription:@"Must send file path to the result if success to write file."];
+- (void)testHandlePhotoCaptureResult_mustCompleteWithFilePathIfSuccessToWrite {
+  XCTestExpectation *completionExpectation =
+      [self expectationWithDescription:@"Must complete with file path if success to write file."];
 
   dispatch_queue_t ioQueue = dispatch_queue_create("test", NULL);
-  id mockResult = OCMClassMock([FLTThreadSafeFlutterResult class]);
-  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc] initWithPath:@"test"
-                                                                       result:mockResult
-                                                                      ioQueue:ioQueue];
-  OCMStub([mockResult sendSuccessWithData:delegate.path]).andDo(^(NSInvocation *invocation) {
-    [resultExpectation fulfill];
-  });
+  NSString *filePath = @"test";
+  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
+           initWithPath:filePath
+                ioQueue:ioQueue
+      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+        XCTAssertNil(error);
+        XCTAssertEqualObjects(filePath, path);
+        [completionExpectation fulfill];
+      }];
 
   // We can't use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
   // `XCTRunnerIDESession::logDebugMessage:`) on a private queue.
   id mockData = OCMPartialMock([NSData data]);
-  OCMStub([mockData writeToFile:OCMOCK_ANY options:NSDataWritingAtomic error:[OCMArg setTo:nil]])
+  OCMStub([mockData writeToFile:filePath options:NSDataWritingAtomic error:[OCMArg setTo:nil]])
       .andReturn(YES);
 
   [delegate handlePhotoCaptureResultWithError:nil
@@ -94,16 +100,12 @@ - (void)testHandlePhotoCaptureResult_bothProvideDataAndSaveFileMustRunOnIOQueue
       [self expectationWithDescription:@"Data provider must run on io queue."];
   XCTestExpectation *writeFileQueueExpectation =
       [self expectationWithDescription:@"File writing must run on io queue"];
-  XCTestExpectation *resultExpectation = [self
-      expectationWithDescription:@"Must send file path to the result if success to write file."];
+  XCTestExpectation *completionExpectation =
+      [self expectationWithDescription:@"Must complete with file path if success to write file."];
 
   dispatch_queue_t ioQueue = dispatch_queue_create("test", NULL);
   const char *ioQueueSpecific = "io_queue_specific";
   dispatch_queue_set_specific(ioQueue, ioQueueSpecific, (void *)ioQueueSpecific, NULL);
-  id mockResult = OCMClassMock([FLTThreadSafeFlutterResult class]);
-  OCMStub([mockResult sendSuccessWithData:OCMOCK_ANY]).andDo(^(NSInvocation *invocation) {
-    [resultExpectation fulfill];
-  });
 
   // We can't use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
   // `XCTRunnerIDESession::logDebugMessage:`) on a private queue.
@@ -116,9 +118,14 @@ - (void)testHandlePhotoCaptureResult_bothProvideDataAndSaveFileMustRunOnIOQueue
       })
       .andReturn(YES);
 
-  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc] initWithPath:@"test"
-                                                                       result:mockResult
-                                                                      ioQueue:ioQueue];
+  NSString *filePath = @"test";
+  FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
+           initWithPath:filePath
+                ioQueue:ioQueue
+      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+        [completionExpectation fulfill];
+      }];
+
   [delegate handlePhotoCaptureResultWithError:nil
                             photoDataProvider:^NSData * {
                               if (dispatch_get_specific(ioQueueSpecific)) {
diff --git a/packages/camera/camera/ios/Classes/FLTCam.m b/packages/camera/camera/ios/Classes/FLTCam.m
index 94f985066675..d5a30561ce95 100644
--- a/packages/camera/camera/ios/Classes/FLTCam.m
+++ b/packages/camera/camera/ios/Classes/FLTCam.m
@@ -50,7 +50,6 @@ @interface FLTCam () <AVCaptureVideoDataOutputSampleBufferDelegate,
 @property(nonatomic) FLTImageStreamHandler *imageStreamHandler;
 @property(readonly, nonatomic) AVCaptureSession *captureSession;
 
-@property(readonly, nonatomic) AVCapturePhotoOutput *capturePhotoOutput API_AVAILABLE(ios(10));
 @property(readonly, nonatomic) AVCaptureInput *captureVideoInput;
 @property(readonly) CVPixelBufferRef volatile latestPixelBuffer;
 @property(readonly, nonatomic) CGSize captureSize;
@@ -110,6 +109,7 @@ - (instancetype)initWithCameraName:(NSString *)cameraName
   _lockedCaptureOrientation = UIDeviceOrientationUnknown;
   _deviceOrientation = orientation;
   _videoFormat = kCVPixelFormatType_32BGRA;
+  _inProgressSavePhotoDelegates = [NSMutableDictionary dictionary];
 
   NSError *localError = nil;
   _captureVideoInput = [AVCaptureDeviceInput deviceInputWithDevice:_captureDevice
@@ -220,11 +220,27 @@ - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)
     return;
   }
 
-  [_capturePhotoOutput
-      capturePhotoWithSettings:settings
-                      delegate:[[FLTSavePhotoDelegate alloc] initWithPath:path
-                                                                   result:result
-                                                                  ioQueue:self.photoIOQueue]];
+  FLTSavePhotoDelegate *savePhotoDelegate = [[FLTSavePhotoDelegate alloc]
+           initWithPath:path
+                ioQueue:self.photoIOQueue
+      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+        dispatch_async(self.captureSessionQueue, ^{
+          // Dispatch back to capture session queue to delete reference.
+          // Retain cycle is broken after the dictionary entry is cleared.
+          self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = nil;
+        });
+
+        if (error) {
+          [result sendError:error];
+        } else {
+          NSAssert(path, @"Path must not be nil if no error.");
+          [result sendSuccessWithData:path];
+        }
+      }];
+
+  // Already on capture session queue.
+  self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = savePhotoDelegate;
+  [self.capturePhotoOutput capturePhotoWithSettings:settings delegate:savePhotoDelegate];
 }
 
 - (AVCaptureVideoOrientation)getVideoOrientationForDeviceOrientation:
diff --git a/packages/camera/camera/ios/Classes/FLTCam_Test.h b/packages/camera/camera/ios/Classes/FLTCam_Test.h
index 556578eba361..5b326658744a 100644
--- a/packages/camera/camera/ios/Classes/FLTCam_Test.h
+++ b/packages/camera/camera/ios/Classes/FLTCam_Test.h
@@ -3,10 +3,25 @@
 // found in the LICENSE file.
 
 #import "FLTCam.h"
+#import "FLTSavePhotoDelegate.h"
 
 // APIs exposed for unit testing.
 @interface FLTCam ()
 
+/// The output for video capturing.
 @property(readonly, nonatomic) AVCaptureVideoDataOutput *captureVideoOutput;
 
+/// The output for photo capturing. Exposed setter for unit tests.
+@property(strong, nonatomic) AVCapturePhotoOutput *capturePhotoOutput API_AVAILABLE(ios(10));
+
+/// A dictionary to retain all in-progress FLTSavePhotoDelegates. The key of the dictionary is the
+/// AVCapturePhotoSettings's uniqueID for each photo capture operation, and the value is the
+/// FLTSavePhotoDelegate that handles the result of each photo capture operation. Note that photo
+/// capture operations may overlap, so we have to keep track of multiple delegates in progress,
+/// instead of just a single delegate reference.
+/// Exposed setter for unit tests. Do not use `copy` becauase we need to replace it with a mock
+/// object to verify that the reference values are accessed on the right dispatch queue.
+@property(strong, nonatomic)
+    NSMutableDictionary<NSNumber *, FLTSavePhotoDelegate *> *inProgressSavePhotoDelegates;
+
 @end
diff --git a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h
index a773b4613931..9536dfbe4343 100644
--- a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h
+++ b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h
@@ -4,34 +4,35 @@
 
 @import AVFoundation;
 @import Foundation;
-@import Flutter;
 
 #import "FLTThreadSafeFlutterResult.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
+/// The completion handler block for save photo operations.
+/// Can be called from either main queue or IO queue.
+/// If success, `error` will be present and `path` will be nil. Otherewise, `error` will be nil and
+/// `path` will be present.
+/// @param error photo capture error or IO error.
+/// @param path the path for successfully saved photo file.
+typedef void (^FLTSavePhotoDelegateCompletionHandler)(NSError *_Nullable error,
+                                                      NSString *_Nullable path);
+
 /**
  Delegate object that handles photo capture results.
  */
 @interface FLTSavePhotoDelegate : NSObject <AVCapturePhotoCaptureDelegate>
-/// The file path for the captured photo.
-@property(readonly, nonatomic) NSString *path;
-/// The thread safe flutter result wrapper to report the result.
-@property(readonly, nonatomic) FLTThreadSafeFlutterResult *result;
-/// The queue on which captured photos are wrote to disk.
-@property(strong, nonatomic) dispatch_queue_t ioQueue;
-/// Used to keep the delegate alive until didFinishProcessingPhotoSampleBuffer.
-@property(strong, nonatomic, nullable) FLTSavePhotoDelegate *selfReference;
 
 /**
  * Initialize a photo capture delegate.
  * @param path the path for captured photo file.
- * @param result the thread safe flutter result wrapper to report the result.
  * @param ioQueue the queue on which captured photos are wrote to disk.
+ * @param completionHandler The completion handler block for save photo operations. Can
+ * be called from either main queue or IO queue.
  */
 - (instancetype)initWithPath:(NSString *)path
-                      result:(FLTThreadSafeFlutterResult *)result
-                     ioQueue:(dispatch_queue_t)ioQueue;
+                     ioQueue:(dispatch_queue_t)ioQueue
+           completionHandler:(FLTSavePhotoDelegateCompletionHandler)completionHandler;
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m
index 8dadfec7fecd..76ce55fcface 100644
--- a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m
+++ b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m
@@ -3,37 +3,41 @@
 // found in the LICENSE file.
 
 #import "FLTSavePhotoDelegate.h"
+#import "FLTSavePhotoDelegate_Test.h"
+
+@interface FLTSavePhotoDelegate ()
+/// The file path for the captured photo.
+@property(readonly, nonatomic) NSString *path;
+/// The queue on which captured photos are wrote to disk.
+@property(readonly, nonatomic) dispatch_queue_t ioQueue;
+@end
 
 @implementation FLTSavePhotoDelegate
 
 - (instancetype)initWithPath:(NSString *)path
-                      result:(FLTThreadSafeFlutterResult *)result
-                     ioQueue:(dispatch_queue_t)ioQueue {
+                     ioQueue:(dispatch_queue_t)ioQueue
+           completionHandler:(FLTSavePhotoDelegateCompletionHandler)completionHandler {
   self = [super init];
   NSAssert(self, @"super init cannot be nil");
   _path = path;
-  _selfReference = self;
-  _result = result;
   _ioQueue = ioQueue;
+  _completionHandler = completionHandler;
   return self;
 }
 
 - (void)handlePhotoCaptureResultWithError:(NSError *)error
                         photoDataProvider:(NSData * (^)(void))photoDataProvider {
-  self.selfReference = nil;
   if (error) {
-    [self.result sendError:error];
+    self.completionHandler(error, nil);
     return;
   }
   dispatch_async(self.ioQueue, ^{
     NSData *data = photoDataProvider();
     NSError *ioError;
     if ([data writeToFile:self.path options:NSDataWritingAtomic error:&ioError]) {
-      [self.result sendSuccessWithData:self.path];
+      self.completionHandler(nil, self.path);
     } else {
-      [self.result sendErrorWithCode:@"IOError"
-                             message:@"Unable to write file"
-                             details:ioError.localizedDescription];
+      self.completionHandler(ioError, nil);
     }
   });
 }
diff --git a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate_Test.h b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate_Test.h
index c0b77c7bac83..2d0d4f96be9d 100644
--- a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate_Test.h
+++ b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate_Test.h
@@ -9,6 +9,11 @@
  */
 @interface FLTSavePhotoDelegate ()
 
+/// The completion handler block for capture and save photo operations.
+/// Can be called from either main queue or IO queue.
+/// Exposed for unit tests to manually trigger the completion.
+@property(readonly, nonatomic) FLTSavePhotoDelegateCompletionHandler completionHandler;
+
 /// Handler to write captured photo data into a file.
 /// @param error the capture error.
 /// @param photoDataProvider a closure that provides photo data.

From 8cc0bce108a96c5f032017924b4641cfcf283dc3 Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Fri, 11 Feb 2022 11:07:24 -0800
Subject: [PATCH 02/10] remove queue test

---
 .../ios/RunnerTests/FLTCamPhotoCaptureTests.m | 61 -------------------
 1 file changed, 61 deletions(-)

diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
index 8f6f6494dbf8..85e92453cab7 100644
--- a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
@@ -14,67 +14,6 @@ @interface FLTCamPhotoCaptureTests : XCTestCase
 
 @implementation FLTCamPhotoCaptureTests
 
-- (void)testCaptureToFile_savePhotoDelegateReferencesMustBeAccessedOnCaptureSessionQueue {
-  XCTestExpectation *setReferenceExpectation =
-      [self expectationWithDescription:
-                @"FLTSavePhotoDelegate references must be set on capture session queue."];
-  XCTestExpectation *clearReferenceExpectation =
-      [self expectationWithDescription:
-                @"FLTSavePhotoDelegate references must be cleared on capture session queue."];
-
-  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
-  const char *captureSessionQueueSpecific = "capture_session_queue";
-  dispatch_queue_set_specific(captureSessionQueue, captureSessionQueueSpecific,
-                              (void *)captureSessionQueueSpecific, NULL);
-  FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
-
-  // settings.uniqueID is used as the key for `inProgressSavePhotoDelegates` dictionary
-  AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
-  id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
-  OCMStub([mockSettings photoSettings]).andReturn(settings);
-
-  // We need to make sure the delegate reference is actually saved in a real dictionary, so that we
-  // can call its completion handler later. Must use a partial mock, in order to forward invocation
-  // to the real object.
-  id mockDelegates = OCMPartialMock([NSMutableDictionary dictionary]);
-  OCMStub([mockDelegates setObject:OCMOCK_ANY forKeyedSubscript:OCMOCK_ANY])
-      .andDo(^(NSInvocation *invocation) {
-        if (dispatch_get_specific(captureSessionQueueSpecific)) {
-          FLTSavePhotoDelegate *delegate;
-          // Index 0 and 1 are `self` and `_cmd`.
-          [invocation getArgument:&delegate atIndex:2];
-          if (delegate) {
-            [setReferenceExpectation fulfill];
-          } else {
-            [clearReferenceExpectation fulfill];
-          }
-        }
-      })
-      .andForwardToRealObject();
-  cam.inProgressSavePhotoDelegates = mockDelegates;
-
-  id mockOutput = OCMClassMock([AVCapturePhotoOutput class]);
-  OCMStub([mockOutput capturePhotoWithSettings:OCMOCK_ANY delegate:OCMOCK_ANY])
-      .andDo(^(NSInvocation *invocation) {
-        FLTSavePhotoDelegate *delegate = cam.inProgressSavePhotoDelegates[@(settings.uniqueID)];
-        XCTAssertNotNil(delegate, @"Delegate reference must be saved to the dictionary.");
-        // Completion runs on IO queue.
-        dispatch_queue_t ioQueue = dispatch_queue_create("io_queue", NULL);
-        dispatch_async(ioQueue, ^{
-          NSString *filePath = @"test";
-          delegate.completionHandler(nil, filePath);
-        });
-      });
-  cam.capturePhotoOutput = mockOutput;
-
-  // `FLTCam::captureToFile` runs on capture session queue.
-  dispatch_async(captureSessionQueue, ^{
-    [cam captureToFile:OCMClassMock([FLTThreadSafeFlutterResult class])];
-  });
-
-  [self waitForExpectationsWithTimeout:1 handler:nil];
-}
-
 - (void)testCaptureToFile_mustReportErrorToResultIfSavePhotoDelegateCompletionsWithError {
   XCTestExpectation *errorExpectation =
       [self expectationWithDescription:

From 384b4cb4e3ad117c71b9e3f4fb6cef16971fe0f5 Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Fri, 11 Feb 2022 11:09:09 -0800
Subject: [PATCH 03/10] swap path error order

---
 .../example/ios/RunnerTests/FLTCamPhotoCaptureTests.m     | 4 ++--
 .../example/ios/RunnerTests/FLTSavePhotoDelegateTests.m   | 8 ++++----
 packages/camera/camera/ios/Classes/FLTCam.m               | 7 ++++---
 packages/camera/camera/ios/Classes/FLTCam_Test.h          | 4 +---
 packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h | 8 ++++----
 packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m | 8 ++++----
 6 files changed, 19 insertions(+), 20 deletions(-)

diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
index 85e92453cab7..03c68fb514a2 100644
--- a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
@@ -39,7 +39,7 @@ - (void)testCaptureToFile_mustReportErrorToResultIfSavePhotoDelegateCompletionsW
         // Completion runs on IO queue.
         dispatch_queue_t ioQueue = dispatch_queue_create("io_queue", NULL);
         dispatch_async(ioQueue, ^{
-          delegate.completionHandler(error, nil);
+          delegate.completionHandler(nil, error);
         });
       });
   cam.capturePhotoOutput = mockOutput;
@@ -77,7 +77,7 @@ - (void)testCaptureToFile_mustReportPathToResultIfSavePhotoDelegateCompletionsWi
         // Completion runs on IO queue.
         dispatch_queue_t ioQueue = dispatch_queue_create("io_queue", NULL);
         dispatch_async(ioQueue, ^{
-          delegate.completionHandler(nil, filePath);
+          delegate.completionHandler(filePath, nil);
         });
       });
   cam.capturePhotoOutput = mockOutput;
diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m
index cb8ae3f1742f..9e8e2441f0b9 100644
--- a/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m
@@ -23,7 +23,7 @@ - (void)testHandlePhotoCaptureResult_mustCompleteWithErrorIfFailedToCapture {
   FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
            initWithPath:@"test"
                 ioQueue:ioQueue
-      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+      completionHandler:^(NSString *_Nullable path, NSError *_Nullable error) {
         XCTAssertEqualObjects(captureError, error);
         XCTAssertNil(path);
         [completionExpectation fulfill];
@@ -47,7 +47,7 @@ - (void)testHandlePhotoCaptureResult_mustCompleteWithErrorIfFailedToWrite {
   FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
            initWithPath:@"test"
                 ioQueue:ioQueue
-      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+      completionHandler:^(NSString *_Nullable path, NSError *_Nullable error) {
         XCTAssertEqualObjects(ioError, error);
         XCTAssertNil(path);
         [completionExpectation fulfill];
@@ -76,7 +76,7 @@ - (void)testHandlePhotoCaptureResult_mustCompleteWithFilePathIfSuccessToWrite {
   FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
            initWithPath:filePath
                 ioQueue:ioQueue
-      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+      completionHandler:^(NSString *_Nullable path, NSError *_Nullable error) {
         XCTAssertNil(error);
         XCTAssertEqualObjects(filePath, path);
         [completionExpectation fulfill];
@@ -122,7 +122,7 @@ - (void)testHandlePhotoCaptureResult_bothProvideDataAndSaveFileMustRunOnIOQueue
   FLTSavePhotoDelegate *delegate = [[FLTSavePhotoDelegate alloc]
            initWithPath:filePath
                 ioQueue:ioQueue
-      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+      completionHandler:^(NSString *_Nullable path, NSError *_Nullable error) {
         [completionExpectation fulfill];
       }];
 
diff --git a/packages/camera/camera/ios/Classes/FLTCam.m b/packages/camera/camera/ios/Classes/FLTCam.m
index d5a30561ce95..b85d2e2af017 100644
--- a/packages/camera/camera/ios/Classes/FLTCam.m
+++ b/packages/camera/camera/ios/Classes/FLTCam.m
@@ -75,8 +75,8 @@ @interface FLTCam () <AVCaptureVideoDataOutputSampleBufferDelegate,
 @property AVAssetWriterInputPixelBufferAdaptor *videoAdaptor;
 /// All FLTCam's state access and capture session related operations should be on run on this queue.
 @property(strong, nonatomic) dispatch_queue_t captureSessionQueue;
-/// The queue on which captured photos (not videos) are wrote to disk.
-/// Videos are wrote to disk by `videoAdaptor` on an internal queue managed by AVFoundation.
+/// The queue on which captured photos (not videos) are written to disk.
+/// Videos are written to disk by `videoAdaptor` on an internal queue managed by AVFoundation.
 @property(strong, nonatomic) dispatch_queue_t photoIOQueue;
 @property(assign, nonatomic) UIDeviceOrientation deviceOrientation;
 @end
@@ -223,10 +223,11 @@ - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)
   FLTSavePhotoDelegate *savePhotoDelegate = [[FLTSavePhotoDelegate alloc]
            initWithPath:path
                 ioQueue:self.photoIOQueue
-      completionHandler:^(NSError *_Nullable error, NSString *_Nullable path) {
+      completionHandler:^(NSString *_Nullable path, NSError *_Nullable error) {
         dispatch_async(self.captureSessionQueue, ^{
           // Dispatch back to capture session queue to delete reference.
           // Retain cycle is broken after the dictionary entry is cleared.
+          // Retain cycle is to keep the original behavior with our previous `selfReference` approach in the FLTSavePhotoDelegate, where delegate is released only after capture completion.
           self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = nil;
         });
 
diff --git a/packages/camera/camera/ios/Classes/FLTCam_Test.h b/packages/camera/camera/ios/Classes/FLTCam_Test.h
index 5b326658744a..db885d0d0773 100644
--- a/packages/camera/camera/ios/Classes/FLTCam_Test.h
+++ b/packages/camera/camera/ios/Classes/FLTCam_Test.h
@@ -19,9 +19,7 @@
 /// FLTSavePhotoDelegate that handles the result of each photo capture operation. Note that photo
 /// capture operations may overlap, so we have to keep track of multiple delegates in progress,
 /// instead of just a single delegate reference.
-/// Exposed setter for unit tests. Do not use `copy` becauase we need to replace it with a mock
-/// object to verify that the reference values are accessed on the right dispatch queue.
-@property(strong, nonatomic)
+@property(readonly, nonatomic)
     NSMutableDictionary<NSNumber *, FLTSavePhotoDelegate *> *inProgressSavePhotoDelegates;
 
 @end
diff --git a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h
index 9536dfbe4343..40e4562e4483 100644
--- a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h
+++ b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.h
@@ -13,10 +13,10 @@ NS_ASSUME_NONNULL_BEGIN
 /// Can be called from either main queue or IO queue.
 /// If success, `error` will be present and `path` will be nil. Otherewise, `error` will be nil and
 /// `path` will be present.
-/// @param error photo capture error or IO error.
 /// @param path the path for successfully saved photo file.
-typedef void (^FLTSavePhotoDelegateCompletionHandler)(NSError *_Nullable error,
-                                                      NSString *_Nullable path);
+/// @param error photo capture error or IO error.
+typedef void (^FLTSavePhotoDelegateCompletionHandler)(NSString *_Nullable path,
+                                                      NSError *_Nullable error);
 
 /**
  Delegate object that handles photo capture results.
@@ -26,7 +26,7 @@ typedef void (^FLTSavePhotoDelegateCompletionHandler)(NSError *_Nullable error,
 /**
  * Initialize a photo capture delegate.
  * @param path the path for captured photo file.
- * @param ioQueue the queue on which captured photos are wrote to disk.
+ * @param ioQueue the queue on which captured photos are written to disk.
  * @param completionHandler The completion handler block for save photo operations. Can
  * be called from either main queue or IO queue.
  */
diff --git a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m
index 76ce55fcface..ced3cb5e407f 100644
--- a/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m
+++ b/packages/camera/camera/ios/Classes/FLTSavePhotoDelegate.m
@@ -8,7 +8,7 @@
 @interface FLTSavePhotoDelegate ()
 /// The file path for the captured photo.
 @property(readonly, nonatomic) NSString *path;
-/// The queue on which captured photos are wrote to disk.
+/// The queue on which captured photos are written to disk.
 @property(readonly, nonatomic) dispatch_queue_t ioQueue;
 @end
 
@@ -28,16 +28,16 @@ - (instancetype)initWithPath:(NSString *)path
 - (void)handlePhotoCaptureResultWithError:(NSError *)error
                         photoDataProvider:(NSData * (^)(void))photoDataProvider {
   if (error) {
-    self.completionHandler(error, nil);
+    self.completionHandler(nil, error);
     return;
   }
   dispatch_async(self.ioQueue, ^{
     NSData *data = photoDataProvider();
     NSError *ioError;
     if ([data writeToFile:self.path options:NSDataWritingAtomic error:&ioError]) {
-      self.completionHandler(nil, self.path);
+      self.completionHandler(self.path, nil);
     } else {
-      self.completionHandler(ioError, nil);
+      self.completionHandler(nil, ioError);
     }
   });
 }

From 3e9a5bb696ae5d9a3a4e2af3a0fae4fa52e1a408 Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Fri, 11 Feb 2022 11:59:32 -0800
Subject: [PATCH 04/10] add asserts capture queue specific

---
 .../ios/RunnerTests/FLTCamPhotoCaptureTests.m |  3 ++-
 .../ios/RunnerTests/QueueHelperTests.m        | 15 +++++++++++++++
 .../camera/camera/ios/Classes/CameraPlugin.m  |  3 +++
 packages/camera/camera/ios/Classes/FLTCam.m   |  4 +++-
 .../camera/camera/ios/Classes/QueueHelper.h   | 19 +++++++++++++++++++
 .../camera/camera/ios/Classes/QueueHelper.m   | 12 ++++++++++++
 6 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
index 03c68fb514a2..9fcb69362955 100644
--- a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
@@ -20,8 +20,8 @@ - (void)testCaptureToFile_mustReportErrorToResultIfSavePhotoDelegateCompletionsW
                 @"Must send error to result if save photo delegate completes with error."];
 
   dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
+  [QueueHelper setSpecific:FLTCaptureSessionQueueSpecific forQueue:captureSessionQueue];
   FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
-
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
   id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
   OCMStub([mockSettings photoSettings]).andReturn(settings);
@@ -58,6 +58,7 @@ - (void)testCaptureToFile_mustReportPathToResultIfSavePhotoDelegateCompletionsWi
                 @"Must send file path to result if save photo delegate completes with file path."];
 
   dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
+  [QueueHelper setSpecific:FLTCaptureSessionQueueSpecific forQueue:captureSessionQueue];
   FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
 
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
diff --git a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
index c5f377f7efa9..7078103bd12c 100644
--- a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
@@ -35,4 +35,19 @@ - (void)testShouldDispatchToMainQueueIfCalledFromBackgroundQueue {
   [self waitForExpectationsWithTimeout:1 handler:nil];
 }
 
+- (void)testSetAndCheckQueueSpecific {
+  XCTestExpectation *expectation = [self expectationWithDescription:@"Complete test"];
+  const char *specific = "specific";
+  dispatch_queue_t queue = dispatch_queue_create("test", NULL);
+  [QueueHelper setSpecific: specific forQueue:queue];
+  
+  XCTAssertFalse([QueueHelper isCurrentlyOnQueueWithSpecific:specific], @"Must not be on the test queue before dispatched to it.");
+  dispatch_async(queue, ^{
+    XCTAssert([QueueHelper isCurrentlyOnQueueWithSpecific:specific], @"Must be on the test queue after dispatched to it.");
+    [expectation fulfill];
+  });
+  
+  [self waitForExpectationsWithTimeout:1 handler:nil];
+}
+
 @end
diff --git a/packages/camera/camera/ios/Classes/CameraPlugin.m b/packages/camera/camera/ios/Classes/CameraPlugin.m
index 97e995429e14..1b58cf9d7128 100644
--- a/packages/camera/camera/ios/Classes/CameraPlugin.m
+++ b/packages/camera/camera/ios/Classes/CameraPlugin.m
@@ -13,6 +13,7 @@
 #import "FLTThreadSafeFlutterResult.h"
 #import "FLTThreadSafeMethodChannel.h"
 #import "FLTThreadSafeTextureRegistry.h"
+#import "QueueHelper.h"
 
 @interface CameraPlugin ()
 @property(readonly, nonatomic) FLTThreadSafeTextureRegistry *registry;
@@ -38,6 +39,8 @@ - (instancetype)initWithRegistry:(NSObject<FlutterTextureRegistry> *)registry
   _registry = [[FLTThreadSafeTextureRegistry alloc] initWithTextureRegistry:registry];
   _messenger = messenger;
   _captureSessionQueue = dispatch_queue_create("io.flutter.camera.captureSessionQueue", NULL);
+  [QueueHelper setSpecific:FLTCaptureSessionQueueSpecific forQueue:_captureSessionQueue];
+  
   [self initDeviceEventMethodChannel];
   [self startOrientationListener];
   return self;
diff --git a/packages/camera/camera/ios/Classes/FLTCam.m b/packages/camera/camera/ios/Classes/FLTCam.m
index b85d2e2af017..a93846b1fa11 100644
--- a/packages/camera/camera/ios/Classes/FLTCam.m
+++ b/packages/camera/camera/ios/Classes/FLTCam.m
@@ -5,6 +5,7 @@
 #import "FLTCam.h"
 #import "FLTCam_Test.h"
 #import "FLTSavePhotoDelegate.h"
+#import "QueueHelper.h"
 
 @import CoreMotion;
 #import <libkern/OSAtomic.h>
@@ -201,6 +202,7 @@ - (void)updateOrientation:(UIDeviceOrientation)orientation
 }
 
 - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)) {
+  
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
   if (_resolutionPreset == FLTResolutionPresetMax) {
     [settings setHighResolutionPhotoEnabled:YES];
@@ -239,7 +241,7 @@ - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)
         }
       }];
 
-  // Already on capture session queue.
+  NSAssert([QueueHelper isCurrentlyOnQueueWithSpecific:FLTCaptureSessionQueueSpecific], @"save photo delegate references must be updated on the capture session queue");
   self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = savePhotoDelegate;
   [self.capturePhotoOutput capturePhotoWithSettings:settings delegate:savePhotoDelegate];
 }
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.h b/packages/camera/camera/ios/Classes/QueueHelper.h
index c2548148c499..f7410e8d039f 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.h
+++ b/packages/camera/camera/ios/Classes/QueueHelper.h
@@ -6,8 +6,27 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+/// Queue-specific context data to be associated with the capture session queue.
+extern const char *FLTCaptureSessionQueueSpecific;
+
+/// A class that contains dispatch queue related helper functions.
 @interface QueueHelper : NSObject
+
+/// Ensure the given block to be run on the main queue.
+/// If caller site is already on the main queue, the block will be run synchronously. Otherwise, the block will be dispatch asynchronously to the main queue.
+/// @param block the block to be run on the main queue.
 + (void)ensureToRunOnMainQueue:(void (^)(void))block;
+
+
+/// Sets the queue-specific context data for a given queue.
+/// @param specific the queue-specific context data.
+/// @param queue the queue to be associated with the context data.
++ (void)setSpecific: (const char *)specific forQueue: (dispatch_queue_t) queue;
+
+/// Check if the caller is on a certain queue specified by its queue-specifc context data.
+/// @returns true if the queue is on a certain queue specified by its queue-specific context data. false otherwise.
++ (BOOL)isCurrentlyOnQueueWithSpecific: (const char *)specific;
+
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.m b/packages/camera/camera/ios/Classes/QueueHelper.m
index 194dfa9cfb95..7c09d0234e9c 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.m
+++ b/packages/camera/camera/ios/Classes/QueueHelper.m
@@ -4,7 +4,10 @@
 
 #import "QueueHelper.h"
 
+const char *FLTCaptureSessionQueueSpecific = "capture_session_queue";
+
 @implementation QueueHelper
+
 + (void)ensureToRunOnMainQueue:(void (^)(void))block {
   if (!NSThread.isMainThread) {
     dispatch_async(dispatch_get_main_queue(), block);
@@ -12,4 +15,13 @@ + (void)ensureToRunOnMainQueue:(void (^)(void))block {
     block();
   }
 }
+
++ (void)setSpecific: (const char *)specific forQueue: (dispatch_queue_t) queue {
+  dispatch_queue_set_specific(queue, specific, (void *)specific, NULL);
+}
+
++ (BOOL)isCurrentlyOnQueueWithSpecific: (const char *)specific {
+  return dispatch_get_specific(specific);
+}
+
 @end

From 3c6dc574d60e56520ac7697dfe1c35e042fec495 Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Fri, 11 Feb 2022 11:59:56 -0800
Subject: [PATCH 05/10] format

format
---
 .../ios/RunnerTests/QueueHelperTests.m        | 23 +++++++++++--------
 .../camera/camera/ios/Classes/CameraPlugin.m  |  2 +-
 packages/camera/camera/ios/Classes/FLTCam.m   |  7 +++---
 .../camera/camera/ios/Classes/QueueHelper.h   | 11 +++++----
 .../camera/camera/ios/Classes/QueueHelper.m   |  4 ++--
 5 files changed, 26 insertions(+), 21 deletions(-)

diff --git a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
index 7078103bd12c..40e4737bcd24 100644
--- a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
@@ -36,18 +36,21 @@ - (void)testShouldDispatchToMainQueueIfCalledFromBackgroundQueue {
 }
 
 - (void)testSetAndCheckQueueSpecific {
-  XCTestExpectation *expectation = [self expectationWithDescription:@"Complete test"];
-  const char *specific = "specific";
   dispatch_queue_t queue = dispatch_queue_create("test", NULL);
-  [QueueHelper setSpecific: specific forQueue:queue];
-  
-  XCTAssertFalse([QueueHelper isCurrentlyOnQueueWithSpecific:specific], @"Must not be on the test queue before dispatched to it.");
-  dispatch_async(queue, ^{
-    XCTAssert([QueueHelper isCurrentlyOnQueueWithSpecific:specific], @"Must be on the test queue after dispatched to it.");
-    [expectation fulfill];
+  const char *specific = "specific";
+  [QueueHelper setSpecific:specific forQueue:queue];
+
+  XCTAssertFalse([QueueHelper isCurrentlyOnQueueWithSpecific:specific],
+                 @"Must not be on the test queue before dispatching to it.");
+
+  // Note: sync call
+  dispatch_sync(queue, ^{
+    XCTAssert([QueueHelper isCurrentlyOnQueueWithSpecific:specific],
+              @"Must be on the test queue after dispatching to it.");
   });
-  
-  [self waitForExpectationsWithTimeout:1 handler:nil];
+
+  XCTAssertFalse([QueueHelper isCurrentlyOnQueueWithSpecific:specific],
+                 @"Must not be on the test queue outside of dispatch_sync block.");
 }
 
 @end
diff --git a/packages/camera/camera/ios/Classes/CameraPlugin.m b/packages/camera/camera/ios/Classes/CameraPlugin.m
index 1b58cf9d7128..10d2af9bbb5f 100644
--- a/packages/camera/camera/ios/Classes/CameraPlugin.m
+++ b/packages/camera/camera/ios/Classes/CameraPlugin.m
@@ -40,7 +40,7 @@ - (instancetype)initWithRegistry:(NSObject<FlutterTextureRegistry> *)registry
   _messenger = messenger;
   _captureSessionQueue = dispatch_queue_create("io.flutter.camera.captureSessionQueue", NULL);
   [QueueHelper setSpecific:FLTCaptureSessionQueueSpecific forQueue:_captureSessionQueue];
-  
+
   [self initDeviceEventMethodChannel];
   [self startOrientationListener];
   return self;
diff --git a/packages/camera/camera/ios/Classes/FLTCam.m b/packages/camera/camera/ios/Classes/FLTCam.m
index a93846b1fa11..639df41e2eed 100644
--- a/packages/camera/camera/ios/Classes/FLTCam.m
+++ b/packages/camera/camera/ios/Classes/FLTCam.m
@@ -202,7 +202,6 @@ - (void)updateOrientation:(UIDeviceOrientation)orientation
 }
 
 - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)) {
-  
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
   if (_resolutionPreset == FLTResolutionPresetMax) {
     [settings setHighResolutionPhotoEnabled:YES];
@@ -229,7 +228,8 @@ - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)
         dispatch_async(self.captureSessionQueue, ^{
           // Dispatch back to capture session queue to delete reference.
           // Retain cycle is broken after the dictionary entry is cleared.
-          // Retain cycle is to keep the original behavior with our previous `selfReference` approach in the FLTSavePhotoDelegate, where delegate is released only after capture completion.
+          // This is to keep the behavior with the previous `selfReference` approach in the
+          // FLTSavePhotoDelegate, where delegate is released only after capture completion.
           self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = nil;
         });
 
@@ -241,7 +241,8 @@ - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)
         }
       }];
 
-  NSAssert([QueueHelper isCurrentlyOnQueueWithSpecific:FLTCaptureSessionQueueSpecific], @"save photo delegate references must be updated on the capture session queue");
+  NSAssert([QueueHelper isCurrentlyOnQueueWithSpecific:FLTCaptureSessionQueueSpecific],
+           @"save photo delegate references must be updated on the capture session queue");
   self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = savePhotoDelegate;
   [self.capturePhotoOutput capturePhotoWithSettings:settings delegate:savePhotoDelegate];
 }
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.h b/packages/camera/camera/ios/Classes/QueueHelper.h
index f7410e8d039f..751905ab325e 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.h
+++ b/packages/camera/camera/ios/Classes/QueueHelper.h
@@ -13,19 +13,20 @@ extern const char *FLTCaptureSessionQueueSpecific;
 @interface QueueHelper : NSObject
 
 /// Ensure the given block to be run on the main queue.
-/// If caller site is already on the main queue, the block will be run synchronously. Otherwise, the block will be dispatch asynchronously to the main queue.
+/// If caller site is already on the main queue, the block will be run synchronously. Otherwise, the
+/// block will be dispatch asynchronously to the main queue.
 /// @param block the block to be run on the main queue.
 + (void)ensureToRunOnMainQueue:(void (^)(void))block;
 
-
 /// Sets the queue-specific context data for a given queue.
 /// @param specific the queue-specific context data.
 /// @param queue the queue to be associated with the context data.
-+ (void)setSpecific: (const char *)specific forQueue: (dispatch_queue_t) queue;
++ (void)setSpecific:(const char *)specific forQueue:(dispatch_queue_t)queue;
 
 /// Check if the caller is on a certain queue specified by its queue-specifc context data.
-/// @returns true if the queue is on a certain queue specified by its queue-specific context data. false otherwise.
-+ (BOOL)isCurrentlyOnQueueWithSpecific: (const char *)specific;
+/// @returns YES if the caller is on a certain queue specified by its queue-specific context data.
+/// NO otherwise.
++ (BOOL)isCurrentlyOnQueueWithSpecific:(const char *)specific;
 
 @end
 
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.m b/packages/camera/camera/ios/Classes/QueueHelper.m
index 7c09d0234e9c..d6f73f0a12ed 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.m
+++ b/packages/camera/camera/ios/Classes/QueueHelper.m
@@ -16,11 +16,11 @@ + (void)ensureToRunOnMainQueue:(void (^)(void))block {
   }
 }
 
-+ (void)setSpecific: (const char *)specific forQueue: (dispatch_queue_t) queue {
++ (void)setSpecific:(const char *)specific forQueue:(dispatch_queue_t)queue {
   dispatch_queue_set_specific(queue, specific, (void *)specific, NULL);
 }
 
-+ (BOOL)isCurrentlyOnQueueWithSpecific: (const char *)specific {
++ (BOOL)isCurrentlyOnQueueWithSpecific:(const char *)specific {
   return dispatch_get_specific(specific);
 }
 

From 04de9947cab546f6d2782798721bb478328477ce Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Fri, 11 Feb 2022 12:34:30 -0800
Subject: [PATCH 06/10] create queue helper

---
 .../ios/RunnerTests/FLTCamPhotoCaptureTests.m  | 10 ++++++----
 .../example/ios/RunnerTests/QueueHelperTests.m | 13 ++++++++++---
 .../camera/camera/ios/Classes/CameraPlugin.m   |  4 ++--
 .../camera/camera/ios/Classes/QueueHelper.h    | 18 ++++++++++--------
 .../camera/camera/ios/Classes/QueueHelper.m    |  4 +++-
 5 files changed, 31 insertions(+), 18 deletions(-)

diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
index 9fcb69362955..26e8a0516b38 100644
--- a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
@@ -19,8 +19,9 @@ - (void)testCaptureToFile_mustReportErrorToResultIfSavePhotoDelegateCompletionsW
       [self expectationWithDescription:
                 @"Must send error to result if save photo delegate completes with error."];
 
-  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
-  [QueueHelper setSpecific:FLTCaptureSessionQueueSpecific forQueue:captureSessionQueue];
+  dispatch_queue_t captureSessionQueue =
+      [QueueHelper createQueueWithLabel:"capture_session_queue"
+                               specific:FLTCaptureSessionQueueSpecific];
   FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
   id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
@@ -57,8 +58,9 @@ - (void)testCaptureToFile_mustReportPathToResultIfSavePhotoDelegateCompletionsWi
       [self expectationWithDescription:
                 @"Must send file path to result if save photo delegate completes with file path."];
 
-  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
-  [QueueHelper setSpecific:FLTCaptureSessionQueueSpecific forQueue:captureSessionQueue];
+  dispatch_queue_t captureSessionQueue =
+      [QueueHelper createQueueWithLabel:"capture_session_queue"
+                               specific:FLTCaptureSessionQueueSpecific];
   FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
 
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
diff --git a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
index 40e4737bcd24..1c894e454082 100644
--- a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
@@ -35,11 +35,18 @@ - (void)testShouldDispatchToMainQueueIfCalledFromBackgroundQueue {
   [self waitForExpectationsWithTimeout:1 handler:nil];
 }
 
-- (void)testSetAndCheckQueueSpecific {
-  dispatch_queue_t queue = dispatch_queue_create("test", NULL);
+- (void)testCreateQueue {
+  const char *label = "label";
   const char *specific = "specific";
-  [QueueHelper setSpecific:specific forQueue:queue];
+  dispatch_queue_t queue = [QueueHelper createQueueWithLabel:label specific:specific];
+  XCTAssert(0 == strcmp(label, dispatch_queue_get_label(queue)), "Must set the correct label.");
+  XCTAssert(0 == strcmp(specific, dispatch_queue_get_specific(queue, specific)),
+            "Must set the correct specific.");
+}
 
+- (void)testIsCurrentlyOnQueueWithSpecific {
+  const char *specific = "specific";
+  dispatch_queue_t queue = [QueueHelper createQueueWithLabel:"test" specific:specific];
   XCTAssertFalse([QueueHelper isCurrentlyOnQueueWithSpecific:specific],
                  @"Must not be on the test queue before dispatching to it.");
 
diff --git a/packages/camera/camera/ios/Classes/CameraPlugin.m b/packages/camera/camera/ios/Classes/CameraPlugin.m
index 10d2af9bbb5f..9b629679567f 100644
--- a/packages/camera/camera/ios/Classes/CameraPlugin.m
+++ b/packages/camera/camera/ios/Classes/CameraPlugin.m
@@ -38,8 +38,8 @@ - (instancetype)initWithRegistry:(NSObject<FlutterTextureRegistry> *)registry
   NSAssert(self, @"super init cannot be nil");
   _registry = [[FLTThreadSafeTextureRegistry alloc] initWithTextureRegistry:registry];
   _messenger = messenger;
-  _captureSessionQueue = dispatch_queue_create("io.flutter.camera.captureSessionQueue", NULL);
-  [QueueHelper setSpecific:FLTCaptureSessionQueueSpecific forQueue:_captureSessionQueue];
+  _captureSessionQueue = [QueueHelper createQueueWithLabel:"io.flutter.camera.captureSessionQueue"
+                                                  specific:FLTCaptureSessionQueueSpecific];
 
   [self initDeviceEventMethodChannel];
   [self startOrientationListener];
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.h b/packages/camera/camera/ios/Classes/QueueHelper.h
index 751905ab325e..42bc0d71ef1c 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.h
+++ b/packages/camera/camera/ios/Classes/QueueHelper.h
@@ -12,19 +12,21 @@ extern const char *FLTCaptureSessionQueueSpecific;
 /// A class that contains dispatch queue related helper functions.
 @interface QueueHelper : NSObject
 
-/// Ensure the given block to be run on the main queue.
+/// Ensures the given block to be run on the main queue.
 /// If caller site is already on the main queue, the block will be run synchronously. Otherwise, the
-/// block will be dispatch asynchronously to the main queue.
+/// block will be dispatched asynchronously to the main queue.
 /// @param block the block to be run on the main queue.
 + (void)ensureToRunOnMainQueue:(void (^)(void))block;
 
-/// Sets the queue-specific context data for a given queue.
-/// @param specific the queue-specific context data.
-/// @param queue the queue to be associated with the context data.
-+ (void)setSpecific:(const char *)specific forQueue:(dispatch_queue_t)queue;
+/// Creates a dispatch queue with a label and queue-specific context data.
+/// @param label label for debugging purpose.
+/// @param specific queue-specific context data.
+/// @return the created dispatch queue with the given label and queue-specific context data.
++ (dispatch_queue_t)createQueueWithLabel:(const char *)label specific:(const char *)specific;
 
-/// Check if the caller is on a certain queue specified by its queue-specifc context data.
-/// @returns YES if the caller is on a certain queue specified by its queue-specific context data.
+/// Checks if the caller is on a certain queue specified by its queue-specifc context data.
+/// @param specific queue-specific context data.
+/// @return YES if the caller is on a certain queue specified by its queue-specific context data;
 /// NO otherwise.
 + (BOOL)isCurrentlyOnQueueWithSpecific:(const char *)specific;
 
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.m b/packages/camera/camera/ios/Classes/QueueHelper.m
index d6f73f0a12ed..673ad0cfba8b 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.m
+++ b/packages/camera/camera/ios/Classes/QueueHelper.m
@@ -16,8 +16,10 @@ + (void)ensureToRunOnMainQueue:(void (^)(void))block {
   }
 }
 
-+ (void)setSpecific:(const char *)specific forQueue:(dispatch_queue_t)queue {
++ (dispatch_queue_t)createQueueWithLabel:(const char *)label specific:(const char *)specific {
+  dispatch_queue_t queue = dispatch_queue_create(label, NULL);
   dispatch_queue_set_specific(queue, specific, (void *)specific, NULL);
+  return queue;
 }
 
 + (BOOL)isCurrentlyOnQueueWithSpecific:(const char *)specific {

From e561222f7ca88c0501e11cec661b701153e16d90 Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Tue, 15 Feb 2022 21:05:56 -0800
Subject: [PATCH 07/10] remove queue helper functions that sets and getes queue
 specific data

---
 .../ios/RunnerTests/FLTCamPhotoCaptureTests.m | 12 ++++-----
 .../ios/RunnerTests/QueueHelperTests.m        | 25 -------------------
 .../camera/camera/ios/Classes/CameraPlugin.m  |  5 ++--
 packages/camera/camera/ios/Classes/FLTCam.m   |  2 +-
 .../camera/camera/ios/Classes/QueueHelper.h   | 12 ---------
 .../camera/camera/ios/Classes/QueueHelper.m   | 10 --------
 6 files changed, 10 insertions(+), 56 deletions(-)

diff --git a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
index 26e8a0516b38..fdb2abd4933e 100644
--- a/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m
@@ -19,9 +19,9 @@ - (void)testCaptureToFile_mustReportErrorToResultIfSavePhotoDelegateCompletionsW
       [self expectationWithDescription:
                 @"Must send error to result if save photo delegate completes with error."];
 
-  dispatch_queue_t captureSessionQueue =
-      [QueueHelper createQueueWithLabel:"capture_session_queue"
-                               specific:FLTCaptureSessionQueueSpecific];
+  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
+  dispatch_queue_set_specific(captureSessionQueue, FLTCaptureSessionQueueSpecific,
+                              (void *)FLTCaptureSessionQueueSpecific, NULL);
   FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
   id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
@@ -58,9 +58,9 @@ - (void)testCaptureToFile_mustReportPathToResultIfSavePhotoDelegateCompletionsWi
       [self expectationWithDescription:
                 @"Must send file path to result if save photo delegate completes with file path."];
 
-  dispatch_queue_t captureSessionQueue =
-      [QueueHelper createQueueWithLabel:"capture_session_queue"
-                               specific:FLTCaptureSessionQueueSpecific];
+  dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
+  dispatch_queue_set_specific(captureSessionQueue, FLTCaptureSessionQueueSpecific,
+                              (void *)FLTCaptureSessionQueueSpecific, NULL);
   FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
 
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
diff --git a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
index 1c894e454082..c5f377f7efa9 100644
--- a/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
+++ b/packages/camera/camera/example/ios/RunnerTests/QueueHelperTests.m
@@ -35,29 +35,4 @@ - (void)testShouldDispatchToMainQueueIfCalledFromBackgroundQueue {
   [self waitForExpectationsWithTimeout:1 handler:nil];
 }
 
-- (void)testCreateQueue {
-  const char *label = "label";
-  const char *specific = "specific";
-  dispatch_queue_t queue = [QueueHelper createQueueWithLabel:label specific:specific];
-  XCTAssert(0 == strcmp(label, dispatch_queue_get_label(queue)), "Must set the correct label.");
-  XCTAssert(0 == strcmp(specific, dispatch_queue_get_specific(queue, specific)),
-            "Must set the correct specific.");
-}
-
-- (void)testIsCurrentlyOnQueueWithSpecific {
-  const char *specific = "specific";
-  dispatch_queue_t queue = [QueueHelper createQueueWithLabel:"test" specific:specific];
-  XCTAssertFalse([QueueHelper isCurrentlyOnQueueWithSpecific:specific],
-                 @"Must not be on the test queue before dispatching to it.");
-
-  // Note: sync call
-  dispatch_sync(queue, ^{
-    XCTAssert([QueueHelper isCurrentlyOnQueueWithSpecific:specific],
-              @"Must be on the test queue after dispatching to it.");
-  });
-
-  XCTAssertFalse([QueueHelper isCurrentlyOnQueueWithSpecific:specific],
-                 @"Must not be on the test queue outside of dispatch_sync block.");
-}
-
 @end
diff --git a/packages/camera/camera/ios/Classes/CameraPlugin.m b/packages/camera/camera/ios/Classes/CameraPlugin.m
index 9b629679567f..14d94d0760cb 100644
--- a/packages/camera/camera/ios/Classes/CameraPlugin.m
+++ b/packages/camera/camera/ios/Classes/CameraPlugin.m
@@ -38,8 +38,9 @@ - (instancetype)initWithRegistry:(NSObject<FlutterTextureRegistry> *)registry
   NSAssert(self, @"super init cannot be nil");
   _registry = [[FLTThreadSafeTextureRegistry alloc] initWithTextureRegistry:registry];
   _messenger = messenger;
-  _captureSessionQueue = [QueueHelper createQueueWithLabel:"io.flutter.camera.captureSessionQueue"
-                                                  specific:FLTCaptureSessionQueueSpecific];
+  _captureSessionQueue = dispatch_queue_create(io.flutter.camera.captureSessionQueue, NULL);
+  dispatch_queue_set_specific(_captureSessionQueue, FLTCaptureSessionQueueSpecific,
+                              (void *)FLTCaptureSessionQueueSpecific, NULL);
 
   [self initDeviceEventMethodChannel];
   [self startOrientationListener];
diff --git a/packages/camera/camera/ios/Classes/FLTCam.m b/packages/camera/camera/ios/Classes/FLTCam.m
index 639df41e2eed..7d637a2d1c01 100644
--- a/packages/camera/camera/ios/Classes/FLTCam.m
+++ b/packages/camera/camera/ios/Classes/FLTCam.m
@@ -241,7 +241,7 @@ - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)
         }
       }];
 
-  NSAssert([QueueHelper isCurrentlyOnQueueWithSpecific:FLTCaptureSessionQueueSpecific],
+  NSAssert(dispatch_get_specific(FLTCaptureSessionQueueSpecific),
            @"save photo delegate references must be updated on the capture session queue");
   self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = savePhotoDelegate;
   [self.capturePhotoOutput capturePhotoWithSettings:settings delegate:savePhotoDelegate];
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.h b/packages/camera/camera/ios/Classes/QueueHelper.h
index 42bc0d71ef1c..dc7373220521 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.h
+++ b/packages/camera/camera/ios/Classes/QueueHelper.h
@@ -18,18 +18,6 @@ extern const char *FLTCaptureSessionQueueSpecific;
 /// @param block the block to be run on the main queue.
 + (void)ensureToRunOnMainQueue:(void (^)(void))block;
 
-/// Creates a dispatch queue with a label and queue-specific context data.
-/// @param label label for debugging purpose.
-/// @param specific queue-specific context data.
-/// @return the created dispatch queue with the given label and queue-specific context data.
-+ (dispatch_queue_t)createQueueWithLabel:(const char *)label specific:(const char *)specific;
-
-/// Checks if the caller is on a certain queue specified by its queue-specifc context data.
-/// @param specific queue-specific context data.
-/// @return YES if the caller is on a certain queue specified by its queue-specific context data;
-/// NO otherwise.
-+ (BOOL)isCurrentlyOnQueueWithSpecific:(const char *)specific;
-
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/packages/camera/camera/ios/Classes/QueueHelper.m b/packages/camera/camera/ios/Classes/QueueHelper.m
index 673ad0cfba8b..2cef7b677bfa 100644
--- a/packages/camera/camera/ios/Classes/QueueHelper.m
+++ b/packages/camera/camera/ios/Classes/QueueHelper.m
@@ -16,14 +16,4 @@ + (void)ensureToRunOnMainQueue:(void (^)(void))block {
   }
 }
 
-+ (dispatch_queue_t)createQueueWithLabel:(const char *)label specific:(const char *)specific {
-  dispatch_queue_t queue = dispatch_queue_create(label, NULL);
-  dispatch_queue_set_specific(queue, specific, (void *)specific, NULL);
-  return queue;
-}
-
-+ (BOOL)isCurrentlyOnQueueWithSpecific:(const char *)specific {
-  return dispatch_get_specific(specific);
-}
-
 @end

From 9ca4084595f5bb5ce23b0204fa9641b37566ebf9 Mon Sep 17 00:00:00 2001
From: hellohuanlin <41930132+hellohuanlin@users.noreply.github.com>
Date: Wed, 16 Feb 2022 12:17:58 -0800
Subject: [PATCH 08/10] Update packages/camera/camera/ios/Classes/FLTCam.m

Co-authored-by: Jenn Magder <magder@google.com>
---
 packages/camera/camera/ios/Classes/FLTCam.m | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/camera/camera/ios/Classes/FLTCam.m b/packages/camera/camera/ios/Classes/FLTCam.m
index 7d637a2d1c01..31a9decd59b9 100644
--- a/packages/camera/camera/ios/Classes/FLTCam.m
+++ b/packages/camera/camera/ios/Classes/FLTCam.m
@@ -230,7 +230,7 @@ - (void)captureToFile:(FLTThreadSafeFlutterResult *)result API_AVAILABLE(ios(10)
           // Retain cycle is broken after the dictionary entry is cleared.
           // This is to keep the behavior with the previous `selfReference` approach in the
           // FLTSavePhotoDelegate, where delegate is released only after capture completion.
-          self.inProgressSavePhotoDelegates[@(settings.uniqueID)] = nil;
+          [self.inProgressSavePhotoDelegates removeObjectForKey:@(settings.uniqueID)];
         });
 
         if (error) {

From 99a0b87ab5cf23bd682f56e12474cfcce220cf6a Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Wed, 16 Feb 2022 13:33:11 -0800
Subject: [PATCH 09/10] fix compiler error

---
 packages/camera/camera/ios/Classes/CameraPlugin.m | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/camera/camera/ios/Classes/CameraPlugin.m b/packages/camera/camera/ios/Classes/CameraPlugin.m
index 14d94d0760cb..fcea190de705 100644
--- a/packages/camera/camera/ios/Classes/CameraPlugin.m
+++ b/packages/camera/camera/ios/Classes/CameraPlugin.m
@@ -38,7 +38,7 @@ - (instancetype)initWithRegistry:(NSObject<FlutterTextureRegistry> *)registry
   NSAssert(self, @"super init cannot be nil");
   _registry = [[FLTThreadSafeTextureRegistry alloc] initWithTextureRegistry:registry];
   _messenger = messenger;
-  _captureSessionQueue = dispatch_queue_create(io.flutter.camera.captureSessionQueue, NULL);
+  _captureSessionQueue = dispatch_queue_create("io.flutter.camera.captureSessionQueue", NULL);
   dispatch_queue_set_specific(_captureSessionQueue, FLTCaptureSessionQueueSpecific,
                               (void *)FLTCaptureSessionQueueSpecific, NULL);
 

From 2d4a4ab1ac7a33483c65761812a61bffd15e60f0 Mon Sep 17 00:00:00 2001
From: Huan Lin <hellohuanlin@gmail.com>
Date: Wed, 16 Feb 2022 15:26:59 -0800
Subject: [PATCH 10/10] bump version

---
 packages/camera/camera/CHANGELOG.md | 5 ++++-
 packages/camera/camera/pubspec.yaml | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/packages/camera/camera/CHANGELOG.md b/packages/camera/camera/CHANGELOG.md
index f14522c5ab40..0089130b5e90 100644
--- a/packages/camera/camera/CHANGELOG.md
+++ b/packages/camera/camera/CHANGELOG.md
@@ -1,6 +1,9 @@
-## 0.9.4+12
+## 0.9.4+13
 
 * Updates iOS camera's photo capture delegate reference on a background queue to prevent potential race conditions, and some related internal code cleanup.
+
+## 0.9.4+12
+
 * Skips unnecessary AppDelegate setup for unit tests on iOS.
 * Internal code cleanup for stricter analysis options.
 
diff --git a/packages/camera/camera/pubspec.yaml b/packages/camera/camera/pubspec.yaml
index 7421a7738ddd..80d0393087d8 100644
--- a/packages/camera/camera/pubspec.yaml
+++ b/packages/camera/camera/pubspec.yaml
@@ -4,7 +4,7 @@ description: A Flutter plugin for controlling the camera. Supports previewing
   Dart.
 repository: https://github.com/flutter/plugins/tree/main/packages/camera/camera
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+camera%22
-version: 0.9.4+12
+version: 0.9.4+13
 
 environment:
   sdk: ">=2.14.0 <3.0.0"