Adds callback for Long Press Up (#22822)

* add onLongPressUp geasture which is fired when a long-press ends

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

@@ -11,29 +11,49 @@ import 'recognizer.dart';
 /// same location for a long period of time.
 typedef GestureLongPressCallback = void Function();
 
+/// Signature for when a pointer stops contacting the screen after a long press gesture was detected.
+typedef GestureLongPressUpCallback = void Function();
+
 /// Recognizes when the user has pressed down at the same location for a long
 /// period of time.
 class LongPressGestureRecognizer extends PrimaryPointerGestureRecognizer {
   /// Creates a long-press gesture recognizer.
   ///
   /// Consider assigning the [onLongPress] callback after creating this object.
-  LongPressGestureRecognizer({ Object debugOwner }) : super(deadline: kLongPressTimeout, debugOwner: debugOwner);
+  LongPressGestureRecognizer({ Object debugOwner })
+      : super(deadline: kLongPressTimeout, debugOwner: debugOwner);
+
+  bool _longPressAccepted = false;
 
-  /// Called when a long-press is recognized.
+  /// Called when a long press gesture has been recognized.
   GestureLongPressCallback onLongPress;
 
+  /// Called when the pointer stops contacting the screen after the long-press gesture has been recognized.
+  GestureLongPressUpCallback onLongPressUp;
+
   @override
   void didExceedDeadline() {
     resolve(GestureDisposition.accepted);
-    if (onLongPress != null)
+    _longPressAccepted = true;
+    if (onLongPress != null) {
       invokeCallback<void>('onLongPress', onLongPress);
     }
+  }
 
   @override
   void handlePrimaryPointer(PointerEvent event) {
-    if (event is PointerUpEvent)
+    if (event is PointerUpEvent) {
+      if (_longPressAccepted == true && onLongPressUp != null) {
+        _longPressAccepted = false;
+        invokeCallback<void>('onLongPressUp', onLongPressUp);
+      } else {
         resolve(GestureDisposition.rejected);
       }
+    } else if (event is PointerDownEvent || event is PointerCancelEvent) {
+      // the first touch, initialize the  flag with false
+      _longPressAccepted = false;
+    }
+  }
 
   @override
   String get debugDescription => 'long press';

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

@@ -150,6 +150,7 @@ class GestureDetector extends StatelessWidget {
     this.onTapCancel,
     this.onDoubleTap,
     this.onLongPress,
+    this.onLongPressUp,
     this.onVerticalDragDown,
     this.onVerticalDragStart,
     this.onVerticalDragUpdate,
@@ -242,6 +243,9 @@ class GestureDetector extends StatelessWidget {
   /// a long period of time.
   final GestureLongPressCallback onLongPress;
 
+  /// A pointer that has triggered a long-press has stopped contacting the screen.
+  final GestureLongPressUpCallback onLongPressUp;
+
   /// A pointer has contacted the screen and might begin to move vertically.
   final GestureDragDownCallback onVerticalDragDown;
 
@@ -348,12 +352,13 @@ class GestureDetector extends StatelessWidget {
       );
     }
 
-    if (onLongPress != null) {
+    if (onLongPress != null || onLongPressUp !=null) {
       gestures[LongPressGestureRecognizer] = GestureRecognizerFactoryWithHandlers<LongPressGestureRecognizer>(
         () => LongPressGestureRecognizer(debugOwner: this),
         (LongPressGestureRecognizer instance) {
           instance
-            ..onLongPress = onLongPress;
+            ..onLongPress = onLongPress
+            ..onLongPressUp = onLongPressUp;
         },
       );
     }

packages/flutter/test/gestures/long_press_test.dart

@@ -137,4 +137,25 @@ void main() {
     drag.dispose();
   });
 
+  testGesture('Should recognize long press up', (GestureTester tester) {
+    final LongPressGestureRecognizer longPress = LongPressGestureRecognizer();
+
+    bool longPressUpRecognized = false;
+    longPress.onLongPressUp = () {
+      longPressUpRecognized = true;
+    };
+
+    longPress.addPointer(down);
+    tester.closeArena(5);
+    expect(longPressUpRecognized, isFalse);
+    tester.route(down); // kLongPressTimeout = 500;
+    expect(longPressUpRecognized, isFalse);
+    tester.async.elapse(const Duration(milliseconds: 300));
+    expect(longPressUpRecognized, isFalse);
+    tester.async.elapse(const Duration(milliseconds: 700));
+    tester.route(up);
+    expect(longPressUpRecognized, isTrue);
+
+    longPress.dispose();
+  });
 }

packages/flutter/test/widgets/gesture_detector_test.dart

@@ -314,4 +314,33 @@ void main() {
     expect(tap, 0);
     expect(longPress, 1);
   });
+
+  testWidgets('Long Press Up Callback called after long press', (WidgetTester tester) async {
+    int longPressUp = 0;
+
+    await tester.pumpWidget(
+      Container(
+        alignment: Alignment.topLeft,
+        child: Container(
+          alignment: Alignment.center,
+          height: 100.0,
+          color: const Color(0xFF00FF00),
+          child: GestureDetector(
+            onLongPressUp: () {
+              longPressUp += 1;
+            },
+          ),
+        ),
+      ),
+    );
+
+    Future<Null> longPress(Duration timeout) async {
+      final TestGesture gesture = await tester.startGesture(const Offset(400.0, 50.0));
+      await tester.pump(timeout);
+      await gesture.up();
+    }
+
+    await longPress(kLongPressTimeout + Duration(seconds: 1)); // To make sure the time for long press has occured
+    expect(longPressUp, 1);
+  });
 }