Prevent duplicate plugin registration in FlutterEnginePluginRegistry. (#49365)

shell/platform/android/BUILD.gn

@@ -421,6 +421,7 @@ action("robolectric_tests") {
     "test/io/flutter/embedding/android/FlutterViewTest.java",
     "test/io/flutter/embedding/android/RobolectricFlutterActivity.java",
     "test/io/flutter/embedding/engine/FlutterEngineCacheTest.java",
+    "test/io/flutter/embedding/engine/FlutterEnginePluginRegistryTest.java",
     "test/io/flutter/embedding/engine/FlutterEngineTest.java",
     "test/io/flutter/embedding/engine/FlutterJNITest.java",
     "test/io/flutter/embedding/engine/FlutterShellArgsTest.java",

shell/platform/android/io/flutter/embedding/engine/FlutterEnginePluginRegistry.java

@@ -118,6 +118,12 @@ public void destroy() {
 
   @Override
   public void add(@NonNull FlutterPlugin plugin) {
+    if (has(plugin.getClass())) {
+      Log.w(TAG, "Attempted to register plugin (" + plugin +") but it was "
+          + "already registered with this FlutterEngine (" + flutterEngine + ").");
+      return;
+    }
+
     Log.v(TAG, "Adding plugin: " + plugin);
     // Add the plugin to our generic set of plugins and notify the plugin
     // that is has been attached to an engine.

shell/platform/android/test/io/flutter/FlutterTestSuite.java

@@ -13,6 +13,7 @@
 import io.flutter.embedding.android.FlutterFragmentTest;
 import io.flutter.embedding.android.FlutterViewTest;
 import io.flutter.embedding.engine.FlutterEngineCacheTest;
+import io.flutter.embedding.engine.FlutterEnginePluginRegistryTest;
 import io.flutter.embedding.engine.FlutterJNITest;
 import io.flutter.embedding.engine.RenderingComponentTest;
 import io.flutter.embedding.engine.plugins.shim.ShimPluginRegistryTest;
@@ -35,6 +36,7 @@
     FlutterActivityTest.class,
     FlutterAndroidComponentTest.class,
     FlutterEngineCacheTest.class,
+    FlutterEnginePluginRegistryTest.class,
     FlutterEngineTest.class,
     FlutterFragmentTest.class,
     FlutterJNITest.class,

shell/platform/android/test/io/flutter/embedding/engine/FlutterEnginePluginRegistryTest.java

@@ -0,0 +1,80 @@
+package io.flutter.embedding.engine;
+
+import android.content.Context;
+import android.support.annotation.NonNull;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.annotation.Config;
+
+import io.flutter.embedding.engine.loader.FlutterLoader;
+import io.flutter.embedding.engine.plugins.FlutterPlugin;
+import io.flutter.plugin.platform.PlatformViewsController;
+
+import static junit.framework.TestCase.assertTrue;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+// Run with Robolectric so that Log calls don't crash.
+@Config(manifest=Config.NONE)
+@RunWith(RobolectricTestRunner.class)
+public class FlutterEnginePluginRegistryTest {
+  @Test
+  public void itDoesNotRegisterTheSamePluginTwice() {
+    Context context = mock(Context.class);
+
+    FlutterEngine flutterEngine = mock(FlutterEngine.class);
+    PlatformViewsController platformViewsController = mock(PlatformViewsController.class);
+    when(flutterEngine.getPlatformViewsController()).thenReturn(platformViewsController);
+
+    FlutterLoader flutterLoader = mock(FlutterLoader.class);
+
+    FakeFlutterPlugin fakePlugin1 = new FakeFlutterPlugin();
+    FakeFlutterPlugin fakePlugin2 = new FakeFlutterPlugin();
+
+    FlutterEnginePluginRegistry registry = new FlutterEnginePluginRegistry(
+        context,
+        flutterEngine,
+        flutterLoader
+    );
+
+    // Verify that the registry doesn't think it contains our plugin yet.
+    assertFalse(registry.has(fakePlugin1.getClass()));
+
+    // Add our plugin to the registry.
+    registry.add(fakePlugin1);
+
+    // Verify that the registry now thinks it contains our plugin.
+    assertTrue(registry.has(fakePlugin1.getClass()));
+    assertEquals(1, fakePlugin1.attachmentCallCount);
+
+    // Add a different instance of the same plugin class.
+    registry.add(fakePlugin2);
+
+    // Verify that the registry did not detach the 1st plugin, and
+    // it did not attach the 2nd plugin.
+    assertEquals(1, fakePlugin1.attachmentCallCount);
+    assertEquals(0, fakePlugin1.detachmentCallCount);
+
+    assertEquals(0, fakePlugin2.attachmentCallCount);
+    assertEquals(0, fakePlugin2.detachmentCallCount);
+  }
+
+  private static class FakeFlutterPlugin implements FlutterPlugin {
+    public int attachmentCallCount = 0;
+    public int detachmentCallCount = 0;
+
+    @Override
+    public void onAttachedToEngine(@NonNull FlutterPluginBinding binding) {
+      attachmentCallCount += 1;
+    }
+
+    @Override
+    public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
+      detachmentCallCount += 1;
+    }
+  }
+}