Support dragging native platform views

shell/platform/android/BUILD.gn

@@ -439,6 +439,7 @@ action("robolectric_tests") {
     "test/io/flutter/embedding/engine/dart/DartExecutorTest.java",
     "test/io/flutter/embedding/engine/loader/ApplicationInfoLoaderTest.java",
     "test/io/flutter/embedding/engine/loader/FlutterLoaderTest.java",
+    "test/io/flutter/embedding/engine/mutatorsstack/FlutterMutatorViewTest.java",
     "test/io/flutter/embedding/engine/plugins/shim/ShimPluginRegistryTest.java",
     "test/io/flutter/embedding/engine/renderer/FlutterRendererTest.java",
     "test/io/flutter/embedding/engine/systemchannels/KeyEventChannelTest.java",

shell/platform/android/io/flutter/embedding/engine/mutatorsstack/FlutterMutatorView.java

@@ -19,6 +19,8 @@ public class FlutterMutatorView extends FrameLayout {
   private float screenDensity;
   private int left;
   private int top;
+  private int prevLeft;
+  private int prevTop;
 
   private final AndroidTouchProcessor androidTouchProcessor;
 
@@ -122,11 +124,25 @@ public boolean onTouchEvent(MotionEvent event) {
       return super.onTouchEvent(event);
     }
 
-    // Mutator view itself doesn't rotate, scale, skew, etc.
-    // we only need to account for translation.
-    Matrix screenMatrix = new Matrix();
-    screenMatrix.postTranslate(left, top);
-
+    final Matrix screenMatrix = new Matrix();
+
+    switch (event.getAction()) {
+      case MotionEvent.ACTION_DOWN:
+        prevLeft = left;
+        prevTop = top;
+        screenMatrix.postTranslate(left, top);
+        break;
+      case MotionEvent.ACTION_MOVE:
+        // While the view is dragged, use the left and top positions as
+        // they were at the moment the touch event fired.
+        screenMatrix.postTranslate(prevLeft, prevTop);
+        prevLeft = left;
+        prevTop = top;
+        break;
+      case MotionEvent.ACTION_UP:
+        screenMatrix.postTranslate(left, top);
+        break;
+    }
     return androidTouchProcessor.onTouchEvent(event, screenMatrix);
   }
 }

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

@@ -18,6 +18,7 @@
 import io.flutter.embedding.engine.RenderingComponentTest;
 import io.flutter.embedding.engine.loader.ApplicationInfoLoaderTest;
 import io.flutter.embedding.engine.loader.FlutterLoaderTest;
+import io.flutter.embedding.engine.mutatorsstack.FlutterMutatorViewTest;
 import io.flutter.embedding.engine.plugins.shim.ShimPluginRegistryTest;
 import io.flutter.embedding.engine.renderer.FlutterRendererTest;
 import io.flutter.embedding.engine.systemchannels.KeyEventChannelTest;
@@ -60,6 +61,7 @@
   FlutterJNITest.class,
   FlutterLaunchTests.class,
   FlutterLoaderTest.class,
+  FlutterMutatorViewTest.class,
   FlutterShellArgsTest.class,
   FlutterRendererTest.class,
   FlutterShellArgsTest.class,

shell/platform/android/test/io/flutter/embedding/engine/mutatorsstack/FlutterMutatorViewTest.java

@@ -0,0 +1,79 @@
+package io.flutter.embedding.engine.mutatorsstack;
+
+import static junit.framework.TestCase.*;
+import static org.mockito.Mockito.*;
+
+import android.graphics.Matrix;
+import android.view.MotionEvent;
+import io.flutter.embedding.android.AndroidTouchProcessor;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.ArgumentCaptor;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.RuntimeEnvironment;
+import org.robolectric.annotation.Config;
+
+@Config(manifest = Config.NONE)
+@RunWith(RobolectricTestRunner.class)
+public class FlutterMutatorViewTest {
+
+  @Test
+  public void canDragViews() {
+    final AndroidTouchProcessor touchProcessor = mock(AndroidTouchProcessor.class);
+    final FlutterMutatorView view =
+        new FlutterMutatorView(RuntimeEnvironment.systemContext, 1.0f, touchProcessor);
+    final FlutterMutatorsStack mutatorStack = mock(FlutterMutatorsStack.class);
+
+    assertTrue(view.onInterceptTouchEvent(mock(MotionEvent.class)));
+
+    {
+      view.readyToDisplay(mutatorStack, /*left=*/ 1, /*top=*/ 2, /*width=*/ 0, /*height=*/ 0);
+      view.onTouchEvent(MotionEvent.obtain(0, 0, MotionEvent.ACTION_DOWN, 0.0f, 0.0f, 0));
+      final ArgumentCaptor<Matrix> matrixCaptor = ArgumentCaptor.forClass(Matrix.class);
+      verify(touchProcessor).onTouchEvent(any(), matrixCaptor.capture());
+
+      final Matrix screenMatrix = new Matrix();
+      screenMatrix.postTranslate(1, 2);
+      assertTrue(matrixCaptor.getValue().equals(screenMatrix));
+    }
+
+    reset(touchProcessor);
+
+    {
+      view.readyToDisplay(mutatorStack, /*left=*/ 3, /*top=*/ 4, /*width=*/ 0, /*height=*/ 0);
+      view.onTouchEvent(MotionEvent.obtain(0, 0, MotionEvent.ACTION_MOVE, 0.0f, 0.0f, 0));
+      final ArgumentCaptor<Matrix> matrixCaptor = ArgumentCaptor.forClass(Matrix.class);
+      verify(touchProcessor).onTouchEvent(any(), matrixCaptor.capture());
+
+      final Matrix screenMatrix = new Matrix();
+      screenMatrix.postTranslate(1, 2);
+      assertTrue(matrixCaptor.getValue().equals(screenMatrix));
+    }
+
+    reset(touchProcessor);
+
+    {
+      view.readyToDisplay(mutatorStack, /*left=*/ 5, /*top=*/ 6, /*width=*/ 0, /*height=*/ 0);
+      view.onTouchEvent(MotionEvent.obtain(0, 0, MotionEvent.ACTION_MOVE, 0.0f, 0.0f, 0));
+      final ArgumentCaptor<Matrix> matrixCaptor = ArgumentCaptor.forClass(Matrix.class);
+      verify(touchProcessor).onTouchEvent(any(), matrixCaptor.capture());
+
+      final Matrix screenMatrix = new Matrix();
+      screenMatrix.postTranslate(3, 4);
+      assertTrue(matrixCaptor.getValue().equals(screenMatrix));
+    }
+
+    reset(touchProcessor);
+
+    {
+      view.readyToDisplay(mutatorStack, /*left=*/ 7, /*top=*/ 8, /*width=*/ 0, /*height=*/ 0);
+      view.onTouchEvent(MotionEvent.obtain(0, 0, MotionEvent.ACTION_DOWN, 0.0f, 0.0f, 0));
+      final ArgumentCaptor<Matrix> matrixCaptor = ArgumentCaptor.forClass(Matrix.class);
+      verify(touchProcessor).onTouchEvent(any(), matrixCaptor.capture());
+
+      final Matrix screenMatrix = new Matrix();
+      screenMatrix.postTranslate(7, 8);
+      assertTrue(matrixCaptor.getValue().equals(screenMatrix));
+    }
+  }
+}