Tap above LongPressDraggable should work (#6348)

Previously, we were nulling out the ArenaEntry in MultiDragPointerState
too early, which was prevent MultiDragPointerState from rejecting the
gesture in `dispose` if we hadn't accepted by the time the pointer went
up. Now we properly reject the gesture, which causes the tap gesture to
win during `sweep` in the arena.

Also, add a bunch of docs and annotations based on information I learned
while studying this issue. Finally, rename a private member of tap
recognizer to a name that would have confused me less in my
investigation.

Fixes #1186

packages/flutter/lib/src/gestures/arena.dart

@@ -92,6 +92,18 @@ class GestureArenaManager {
   }
 
   /// Forces resolution of the arena, giving the win to the first member.
+  ///
+  /// Sweep is typically after all the other processing for a [PointerUpEvent]
+  /// have taken place. It ensures that multiple passive gestures do not cause a
+  /// stalemate that prevents the user from interacting with the app.
+  ///
+  /// Recognizers that wish to delay resolving an arena past [PointerUpEvent]
+  /// should call [hold] to delay sweep until [release] is called.
+  ///
+  /// See also:
+  ///
+  ///  * [hold]
+  ///  * [release]
   void sweep(int pointer) {
     _GestureArena state = _arenas[pointer];
     if (state == null)
@@ -112,6 +124,17 @@ class GestureArenaManager {
   }
 
   /// Prevents the arena from being swept.
+  ///
+  /// Typically, a winner is chosen in an arena after all the other
+  /// [PointerUpEvent] processing by [sweep]. If a recognizer wishes to delay
+  /// resolving an arena past [PointerUpEvent], the recognizer can [hold] the
+  /// arena open using this function. To release such a hold and let the arena
+  /// resolve, call [release].
+  ///
+  /// See also:
+  ///
+  ///  * [sweep]
+  ///  * [release]
   void hold(int pointer) {
     _GestureArena state = _arenas[pointer];
     if (state == null)
@@ -123,6 +146,11 @@ class GestureArenaManager {
   ///
   /// If a sweep was attempted on a held arena, the sweep will be done
   /// on release.
+  ///
+  /// See also:
+  ///
+  ///  * [sweep]
+  ///  * [hold]
   void release(int pointer) {
     _GestureArena state = _arenas[pointer];
     if (state == null)

packages/flutter/lib/src/gestures/multidrag.dart

@@ -73,6 +73,7 @@ abstract class MultiDragPointerState {
 
   /// Resolve this pointer's entry in the [GestureArenaManager] with the given disposition.
   @protected
+  @mustCallSuper
   void resolve(GestureDisposition disposition) {
     _arenaEntry.resolve(disposition);
   }
@@ -131,7 +132,6 @@ abstract class MultiDragPointerState {
 
   void _up() {
     assert(_arenaEntry != null);
-    _arenaEntry = null;
     if (_client != null) {
       assert(pendingDelta == null);
       final DragEndDetails details = new DragEndDetails(velocity: _velocityTracker.getVelocity() ?? Velocity.zero);
@@ -147,7 +147,6 @@ abstract class MultiDragPointerState {
 
   void _cancel() {
     assert(_arenaEntry != null);
-    _arenaEntry = null;
     if (_client != null) {
       assert(pendingDelta == null);
       final Drag client = _client;
@@ -165,6 +164,7 @@ abstract class MultiDragPointerState {
   @mustCallSuper
   void dispose() {
     _arenaEntry?.resolve(GestureDisposition.rejected);
+    _arenaEntry = null;
     assert(() { _pendingDelta = null; return true; });
   }
 }

packages/flutter/lib/src/gestures/recognizer.dart

@@ -62,7 +62,8 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   final Map<int, GestureArenaEntry> _entries = <int, GestureArenaEntry>{};
   final Set<int> _trackedPointers = new HashSet<int>();
 
-  /// Called when a pointer event is routed to this recongizer.
+  /// Called when a pointer event is routed to this recognizer.
+  @protected
   void handleEvent(PointerEvent event);
 
   @override
@@ -75,9 +76,12 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   ///
   /// The given pointer ID is the ID of the last pointer this recognizer was
   /// tracking.
+  @protected
   void didStopTrackingLastPointer(int pointer);
 
   /// Resolves this recognizer's participation in each gesture arena with the given disposition.
+  @protected
+  @mustCallSuper
   void resolve(GestureDisposition disposition) {
     List<GestureArenaEntry> localEntries = new List<GestureArenaEntry>.from(_entries.values);
     _entries.clear();
@@ -100,6 +104,7 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   /// The pointer events are delivered to [handleEvent].
   ///
   /// Use [stopTrackingPointer] to remove the route added by this function.
+  @protected
   void startTrackingPointer(int pointer) {
     GestureBinding.instance.pointerRouter.addRoute(pointer, handleEvent);
     _trackedPointers.add(pointer);
@@ -113,6 +118,7 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
   /// call [didStopTrackingLastPointer] synchronously.
   ///
   /// Use [startTrackingPointer] to add the routes in the first place.
+  @protected
   void stopTrackingPointer(int pointer) {
     if (_trackedPointers.contains(pointer)) {
       GestureBinding.instance.pointerRouter.removeRoute(pointer, handleEvent);
@@ -124,6 +130,7 @@ abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
 
   /// Stops tracking the pointer associated with the given event if the event is
   /// a [PointerUpEvent] or a [PointerCancelEvent] event.
+  @protected
   void stopTrackingIfPointerNoLongerDown(PointerEvent event) {
     if (event is PointerUpEvent || event is PointerCancelEvent)
       stopTrackingPointer(event.pointer);
@@ -202,11 +209,13 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
   }
 
   /// Override to provide behavior for the primary pointer when the gesture is still possible.
+  @protected
   void handlePrimaryPointer(PointerEvent event);
 
   /// Override to be notified when [deadline] is exceeded.
   ///
   /// You must override this method if you supply a [deadline].
+  @protected
   void didExceedDeadline() {
     assert(deadline == null);
   }

packages/flutter/lib/src/gestures/tap.dart

@@ -59,7 +59,7 @@ typedef void GestureTapCancelCallback();
 /// [TapGestureRecognizer] considers all the pointers involved in the pointer
 /// event sequence as contributing to one gesture. For this reason, extra
 /// pointer interactions during a tap sequence are not recognized as additional
-/// taps. Fo example, down-1, down-2, up-1, up-2 produces only one tap on up-1.
+/// taps. For example, down-1, down-2, up-1, up-2 produces only one tap on up-1.
 ///
 /// See also:
 ///
@@ -84,7 +84,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
   GestureTapCancelCallback onTapCancel;
 
   bool _sentTapDown = false;
-  bool _wonArena = false;
+  bool _wonArenaForPrimaryPointer = false;
   Point _finalPosition;
 
   @override
@@ -97,7 +97,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
 
   @override
   void resolve(GestureDisposition disposition) {
-    if (_wonArena && disposition == GestureDisposition.rejected) {
+    if (_wonArenaForPrimaryPointer && disposition == GestureDisposition.rejected) {
       if (onTapCancel != null)
         onTapCancel();
       _reset();
@@ -115,7 +115,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
     super.acceptGesture(pointer);
     if (pointer == primaryPointer) {
       _checkDown();
-      _wonArena = true;
+      _wonArenaForPrimaryPointer = true;
       _checkUp();
     }
   }
@@ -140,7 +140,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
   }
 
   void _checkUp() {
-    if (_wonArena && _finalPosition != null) {
+    if (_wonArenaForPrimaryPointer && _finalPosition != null) {
       resolve(GestureDisposition.accepted);
       if (onTapUp != null)
         onTapUp(new TapUpDetails(globalPosition: _finalPosition));
@@ -152,7 +152,7 @@ class TapGestureRecognizer extends PrimaryPointerGestureRecognizer {
 
   void _reset() {
     _sentTapDown = false;
-    _wonArena = false;
+    _wonArenaForPrimaryPointer = false;
     _finalPosition = null;
   }
 

packages/flutter/test/widget/draggable_test.dart

@@ -1200,6 +1200,29 @@ void main() {
     expect(find.text('Target'), findsOneWidget);
   });
 
+  testWidgets('Tap above long-press draggable works', (WidgetTester tester) async {
+    List<String> events = <String>[];
+
+    await tester.pumpWidget(new MaterialApp(
+      home: new Material(
+        child: new Center(
+          child: new GestureDetector(
+            onTap: () {
+              events.add('tap');
+            },
+            child: new LongPressDraggable<int>(
+              feedback: new Text('Feedback'),
+              child: new Text('X'),
+            ),
+          ),
+        ),
+      ),
+    ));
+
+    expect(events, isEmpty);
+    await tester.tap(find.text('X'));
+    expect(events, equals(<String>['tap']));
+  });
 }
 
 class DragTargetData { }