Add the ability to have translucent gesture detectors

A translucent gesture detector still listens for gestures but also lets the
content visually behind the detector receive events.

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -977,6 +977,12 @@ class RenderCustomPaint extends RenderProxyBox {
 
 typedef void PointerEventListener(PointerInputEvent e);
 
+enum HitTestBehavior {
+  deferToChild,
+  opaque,
+  translucent,
+}
+
 /// Invokes the callbacks in response to pointer events.
 class RenderPointerListener extends RenderProxyBox {
   RenderPointerListener({
@@ -984,6 +990,7 @@ class RenderPointerListener extends RenderProxyBox {
     this.onPointerMove,
     this.onPointerUp,
     this.onPointerCancel,
+    this.behavior: HitTestBehavior.deferToChild,
     RenderBox child
   }) : super(child);
 
@@ -991,6 +998,20 @@ class RenderPointerListener extends RenderProxyBox {
   PointerEventListener onPointerMove;
   PointerEventListener onPointerUp;
   PointerEventListener onPointerCancel;
+  HitTestBehavior behavior;
+
+  bool hitTest(HitTestResult result, { Point position }) {
+    bool hitTarget = false;
+    if (position.x >= 0.0 && position.x < size.width &&
+        position.y >= 0.0 && position.y < size.height) {
+      hitTarget = hitTestChildren(result, position: position) || hitTestSelf(position);
+      if (hitTarget || behavior == HitTestBehavior.translucent)
+        result.add(new BoxHitTestEntry(this, position));
+    }
+    return hitTarget;
+  }
+
+  bool hitTestSelf(Point position) => behavior == HitTestBehavior.opaque;
 
   void handleEvent(InputEvent event, HitTestEntry entry) {
     if (onPointerDown != null && event.type == 'pointerdown')
@@ -1017,6 +1038,8 @@ class RenderPointerListener extends RenderProxyBox {
     if (listeners.isEmpty)
       listeners.add('<none>');
     settings.add('listeners: ${listeners.join(", ")}');
+    if (behavior != HitTestBehavior.deferToChild)
+      settings.add('behavior: $behavior');
   }
 }
 

packages/flutter/lib/src/widgets/basic.dart

@@ -31,6 +31,7 @@ export 'package:flutter/rendering.dart' show
     FontWeight,
     FractionalOffset,
     Gradient,
+    HitTestBehavior,
     ImageFit,
     ImageRepeat,
     InputEvent,
@@ -1229,19 +1230,24 @@ class Listener extends OneChildRenderObjectWidget {
     this.onPointerDown,
     this.onPointerMove,
     this.onPointerUp,
-    this.onPointerCancel
-  }) : super(key: key, child: child);
+    this.onPointerCancel,
+    this.behavior: HitTestBehavior.deferToChild
+  }) : super(key: key, child: child) {
+    assert(behavior != null);
+  }
 
   final PointerEventListener onPointerDown;
   final PointerEventListener onPointerMove;
   final PointerEventListener onPointerUp;
   final PointerEventListener onPointerCancel;
+  final HitTestBehavior behavior;
 
   RenderPointerListener createRenderObject() => new RenderPointerListener(
     onPointerDown: onPointerDown,
     onPointerMove: onPointerMove,
     onPointerUp: onPointerUp,
-    onPointerCancel: onPointerCancel
+    onPointerCancel: onPointerCancel,
+    behavior: behavior
   );
 
   void updateRenderObject(RenderPointerListener renderObject, Listener oldWidget) {
@@ -1249,6 +1255,7 @@ class Listener extends OneChildRenderObjectWidget {
     renderObject.onPointerMove = onPointerMove;
     renderObject.onPointerUp = onPointerUp;
     renderObject.onPointerCancel = onPointerCancel;
+    renderObject.behavior = behavior;
   }
 }
 

packages/flutter/lib/src/widgets/gesture_detector.dart

@@ -48,7 +48,8 @@ class GestureDetector extends StatefulComponent {
     this.onPanEnd,
     this.onScaleStart,
     this.onScaleUpdate,
-    this.onScaleEnd
+    this.onScaleEnd,
+    this.behavior
   }) : super(key: key);
 
   final Widget child;
@@ -77,6 +78,8 @@ class GestureDetector extends StatefulComponent {
   final GestureScaleUpdateCallback onScaleUpdate;
   final GestureScaleEndCallback onScaleEnd;
 
+  final HitTestBehavior behavior;
+
   _GestureDetectorState createState() => new _GestureDetectorState();
 }
 
@@ -224,9 +227,14 @@ class _GestureDetectorState extends State<GestureDetector> {
       _scale.addPointer(event);
   }
 
+  HitTestBehavior get _defaultBehavior {
+    return config.child == null ? HitTestBehavior.translucent : HitTestBehavior.deferToChild;
+  }
+
   Widget build(BuildContext context) {
     return new Listener(
       onPointerDown: _handlePointerDown,
+      behavior: config.behavior ?? _defaultBehavior,
       child: config.child
     );
   }

packages/unit/test/widget/gesture_detector_test.dart

@@ -134,4 +134,69 @@ void main() {
       expect(didEndPan, isTrue);
     });
   });
+
+  test('Translucent', () {
+    testWidgets((WidgetTester tester) {
+      bool didReceivePointerDown;
+      bool didTap;
+
+      void pumpWidgetTree(HitTestBehavior behavior) {
+        tester.pumpWidget(
+          new Stack([
+            new Listener(
+              onPointerDown: (_) {
+                didReceivePointerDown = true;
+              },
+              child: new Container(
+                width: 100.0,
+                height: 100.0,
+                decoration: const BoxDecoration(
+                  backgroundColor: const Color(0xFF00FF00)
+                )
+              )
+            ),
+            new Container(
+              width: 100.0,
+              height: 100.0,
+              child: new GestureDetector(
+                onTap: () {
+                  didTap = true;
+                },
+                behavior: behavior
+              )
+            )
+          ])
+        );
+      }
+
+      didReceivePointerDown = false;
+      didTap = false;
+      pumpWidgetTree(null);
+      tester.tapAt(new Point(10.0, 10.0));
+      expect(didReceivePointerDown, isTrue);
+      expect(didTap, isTrue);
+
+      didReceivePointerDown = false;
+      didTap = false;
+      pumpWidgetTree(HitTestBehavior.deferToChild);
+      tester.tapAt(new Point(10.0, 10.0));
+      expect(didReceivePointerDown, isTrue);
+      expect(didTap, isFalse);
+
+      didReceivePointerDown = false;
+      didTap = false;
+      pumpWidgetTree(HitTestBehavior.opaque);
+      tester.tapAt(new Point(10.0, 10.0));
+      expect(didReceivePointerDown, isFalse);
+      expect(didTap, isTrue);
+
+      didReceivePointerDown = false;
+      didTap = false;
+      pumpWidgetTree(HitTestBehavior.translucent);
+      tester.tapAt(new Point(10.0, 10.0));
+      expect(didReceivePointerDown, isTrue);
+      expect(didTap, isTrue);
+
+    });
+  });
 }