Fix crash when cursor ends up at invalid position

ci/licenses_golden/licenses_flutter

@@ -190,6 +190,9 @@ FILE: ../../../flutter/fml/platform/darwin/scoped_block.h
 FILE: ../../../flutter/fml/platform/darwin/scoped_block.mm
 FILE: ../../../flutter/fml/platform/darwin/scoped_nsobject.h
 FILE: ../../../flutter/fml/platform/darwin/scoped_nsobject.mm
+FILE: ../../../flutter/fml/platform/darwin/string_range_sanitization.h
+FILE: ../../../flutter/fml/platform/darwin/string_range_sanitization.mm
+FILE: ../../../flutter/fml/platform/darwin/string_range_sanitization_unittests.mm
 FILE: ../../../flutter/fml/platform/fuchsia/paths_fuchsia.cc
 FILE: ../../../flutter/fml/platform/linux/message_loop_linux.cc
 FILE: ../../../flutter/fml/platform/linux/message_loop_linux.h

fml/BUILD.gn

@@ -106,6 +106,8 @@ source_set("fml") {
       "platform/darwin/scoped_block.mm",
       "platform/darwin/scoped_nsobject.h",
       "platform/darwin/scoped_nsobject.mm",
+      "platform/darwin/string_range_sanitization.h",
+      "platform/darwin/string_range_sanitization.mm",
     ]
 
     libs += [ "Foundation.framework" ]
@@ -188,6 +190,7 @@ executable("fml_unittests") {
     "message_loop_unittests.cc",
     "message_unittests.cc",
     "paths_unittests.cc",
+    "platform/darwin/string_range_sanitization_unittests.mm",
     "string_view_unittest.cc",
     "synchronization/count_down_latch_unittests.cc",
     "synchronization/semaphore_unittest.cc",

fml/platform/darwin/string_range_sanitization.h

@@ -0,0 +1,29 @@
+// 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.
+
+#ifndef FLUTTER_FML_STRING_RANGE_SANITIZATION_H_
+#define FLUTTER_FML_STRING_RANGE_SANITIZATION_H_
+
+#include <Foundation/Foundation.h>
+
+namespace fml {
+
+// Returns a range encompassing the grapheme cluster in which |index| is located.
+//
+// A nil |text| or an index greater than or equal to text.length will result in
+// `NSRange(NSNotFound, 0)`.
+NSRange RangeForCharacterAtIndex(NSString* text, NSUInteger index);
+
+// Returns a range encompassing the grapheme clusters falling in |range|.
+//
+// This method will not alter the length of the input range, but will ensure
+// that the range's location is not in the middle of a multi-byte unicode
+// sequence.
+//
+// An invalid range will result in `NSRange(NSNotFound, 0)`.
+NSRange RangeForCharactersInRange(NSString* text, NSRange range);
+
+}  // namespace fml
+
+#endif  // FLUTTER_FML_STRING_RANGE_SANITIZATION_H_

fml/platform/darwin/string_range_sanitization.mm

@@ -0,0 +1,29 @@
+// 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.
+
+#include "flutter/fml/platform/darwin/string_range_sanitization.h"
+
+namespace fml {
+
+NSRange RangeForCharacterAtIndex(NSString* text, NSUInteger index) {
+  if (text == nil || index >= text.length) {
+    return NSMakeRange(NSNotFound, 0);
+  }
+  if (index < text.length)
+    return [text rangeOfComposedCharacterSequenceAtIndex:index];
+  return NSMakeRange(index, 0);
+}
+
+NSRange RangeForCharactersInRange(NSString* text, NSRange range) {
+  if (text == nil || range.location + range.length > text.length) {
+    return NSMakeRange(NSNotFound, 0);
+  }
+  NSRange sanitizedRange = [text rangeOfComposedCharacterSequencesForRange:range];
+  // We don't want to override the length, we just want to make sure we don't
+  // select into the middle of a multi-byte character. Taking the
+  // `sanitizedRange`'s length will end up altering the actual selection.
+  return NSMakeRange(sanitizedRange.location, range.length);
+}
+
+}  // namespace fml

fml/platform/darwin/string_range_sanitization_unittests.mm

@@ -0,0 +1,30 @@
+// 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.
+
+#include <Foundation/Foundation.h>
+
+#include "flutter/fml/platform/darwin/string_range_sanitization.h"
+#include "gtest/gtest.h"
+
+TEST(StringRangeSanitizationTest, CanHandleUnicode) {
+  auto result = fml::RangeForCharacterAtIndex(@"😠", 1);
+  EXPECT_EQ(result.location, 0UL);
+  EXPECT_EQ(result.length, 2UL);
+}
+
+TEST(StringRangeSanitizationTest, HandlesInvalidRanges) {
+  auto ns_not_found = static_cast<unsigned long>(NSNotFound);
+  EXPECT_EQ(fml::RangeForCharacterAtIndex(@"😠", 3).location, ns_not_found);
+  EXPECT_EQ(fml::RangeForCharacterAtIndex(@"😠", -1).location, ns_not_found);
+  EXPECT_EQ(fml::RangeForCharacterAtIndex(nil, 0).location, ns_not_found);
+  EXPECT_EQ(fml::RangeForCharactersInRange(@"😠", NSMakeRange(1, 2)).location, ns_not_found);
+  EXPECT_EQ(fml::RangeForCharactersInRange(@"😠", NSMakeRange(3, 0)).location, ns_not_found);
+  EXPECT_EQ(fml::RangeForCharactersInRange(nil, NSMakeRange(0, 0)).location, ns_not_found);
+}
+
+TEST(StringRangeSanitizationTest, CanHandleUnicodeRange) {
+  auto result = fml::RangeForCharactersInRange(@"😠", NSMakeRange(1, 0));
+  EXPECT_EQ(result.location, 0UL);
+  EXPECT_EQ(result.length, 0UL);
+}

shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.h"
+#include "flutter/fml/platform/darwin/string_range_sanitization.h"
 
 #include <Foundation/Foundation.h>
 #include <UIKit/UIKit.h>
@@ -283,7 +284,13 @@ - (void)setSelectedTextRange:(UITextRange*)selectedTextRange {
 - (void)setSelectedTextRange:(UITextRange*)selectedTextRange updateEditingState:(BOOL)update {
   if (_selectedTextRange != selectedTextRange) {
     UITextRange* oldSelectedRange = _selectedTextRange;
-    _selectedTextRange = [selectedTextRange copy];
+    if (self.hasText) {
+      FlutterTextRange* flutterTextRange = (FlutterTextRange*)selectedTextRange;
+      _selectedTextRange = [[FlutterTextRange
+          rangeWithNSRange:fml::RangeForCharactersInRange(self.text, flutterTextRange.range)] copy];
+    } else {
+      _selectedTextRange = [selectedTextRange copy];
+    }
     [oldSelectedRange release];
 
     if (update)
@@ -415,20 +422,12 @@ - (UITextRange*)textRangeFromPosition:(UITextPosition*)fromPosition
   return [FlutterTextRange rangeWithNSRange:NSMakeRange(fromIndex, toIndex - fromIndex)];
 }
 
-/** Returns the range of the character sequence at the specified index in the
- * text. */
-- (NSRange)rangeForCharacterAtIndex:(NSUInteger)index {
-  if (index < self.text.length)
-    return [self.text rangeOfComposedCharacterSequenceAtIndex:index];
-  return NSMakeRange(index, 0);
-}
-
 - (NSUInteger)decrementOffsetPosition:(NSUInteger)position {
-  return [self rangeForCharacterAtIndex:MAX(0, position - 1)].location;
+  return fml::RangeForCharacterAtIndex(self.text, MAX(0, position - 1)).location;
 }
 
 - (NSUInteger)incrementOffsetPosition:(NSUInteger)position {
-  NSRange charRange = [self rangeForCharacterAtIndex:position];
+  NSRange charRange = fml::RangeForCharacterAtIndex(self.text, position);
   return MIN(position + charRange.length, self.text.length);
 }
 
@@ -565,7 +564,7 @@ - (UITextPosition*)closestPositionToPoint:(CGPoint)point withinRange:(UITextRang
 - (UITextRange*)characterRangeAtPoint:(CGPoint)point {
   // TODO(cbracken) Implement.
   NSUInteger currentIndex = ((FlutterTextPosition*)_selectedTextRange.start).index;
-  return [FlutterTextRange rangeWithNSRange:[self rangeForCharacterAtIndex:currentIndex]];
+  return [FlutterTextRange rangeWithNSRange:fml::RangeForCharacterAtIndex(self.text, currentIndex)];
 }
 
 - (void)beginFloatingCursorAtPoint:(CGPoint)point {