Support changing the dismissable threshold (#8007)

* Support changing the dismissable threshold for any direction for dismissable widget.

* Fixing review comments.

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

@@ -79,7 +79,8 @@ class Dismissable extends StatefulWidget {
     this.onResize,
     this.onDismissed,
     this.direction: DismissDirection.horizontal,
-    this.resizeDuration: const Duration(milliseconds: 300)
+    this.resizeDuration: const Duration(milliseconds: 300),
+    this.dismissThresholds: const <DismissDirection, double>{},
   }) : super(key: key) {
     assert(key != null);
     assert(secondaryBackground != null ? background != null : true);
@@ -113,6 +114,14 @@ class Dismissable extends StatefulWidget {
   /// immediately after the the widget is dismissed.
   final Duration resizeDuration;
 
+  /// The offset threshold the item has to be dragged in order to be considered dismissed.
+  ///
+  /// Represented as a fraction, e.g. if it is 0.4, then the item has to be dragged at least
+  /// 40% towards one direction to be considered dismissed. Clients can define different
+  /// thresholds for each dismiss direction. This allows for use cases where item can be
+  /// dismissed to end but not to start.
+  final Map<DismissDirection, double> dismissThresholds;
+
   @override
   _DismissableState createState() => new _DismissableState();
 }
@@ -195,6 +204,10 @@ class _DismissableState extends State<Dismissable> with TickerProviderStateMixin
     return _dragExtent > 0 ? DismissDirection.down : DismissDirection.up;
   }
 
+  double get _dismissThreshold {
+    return config.dismissThresholds[_dismissDirection] ?? _kDismissThreshold;
+  }
+
   bool get _isActive {
     return _dragUnderway || _moveController.isAnimating;
   }
@@ -262,6 +275,9 @@ class _DismissableState extends State<Dismissable> with TickerProviderStateMixin
   }
 
   bool _isFlingGesture(Velocity velocity) {
+    // Cannot fling an item if it cannot be dismissed by drag.
+    if (_dismissThreshold >= 1.0)
+      return false;
     final double vx = velocity.pixelsPerSecond.dx;
     final double vy = velocity.pixelsPerSecond.dy;
     if (_directionIsXAxis) {
@@ -299,7 +315,7 @@ class _DismissableState extends State<Dismissable> with TickerProviderStateMixin
       final double flingVelocity = _directionIsXAxis ? details.velocity.pixelsPerSecond.dx : details.velocity.pixelsPerSecond.dy;
       _dragExtent = flingVelocity.sign;
       _moveController.fling(velocity: flingVelocity.abs() * _kFlingVelocityScale);
-    } else if (_moveController.value > _kDismissThreshold) {
+    } else if (_moveController.value > _dismissThreshold) {
       _moveController.forward();
     } else {
       _moveController.reverse();

packages/flutter/test/widgets/dismissable_test.dart

@@ -23,30 +23,37 @@ void handleOnDismissed(DismissDirection direction, int item) {
   dismissedItems.add(item);
 }
 
-Widget buildDismissableItem(int item) {
+Widget buildTest({ double startToEndThreshold }) {
+  Widget buildDismissableItem(int item) {
     return new Dismissable(
         key: new ValueKey<int>(item),
         direction: dismissDirection,
-    onDismissed: (DismissDirection direction) { handleOnDismissed(direction, item); },
-    onResize: () { handleOnResize(item); },
+        onDismissed: (DismissDirection direction) {
+          handleOnDismissed(direction, item);
+        },
+        onResize: () {
+          handleOnResize(item);
+        },
         background: background,
+        dismissThresholds: startToEndThreshold == null
+            ? <DismissDirection, double>{}
+            : <DismissDirection, double>{DismissDirection.startToEnd: startToEndThreshold},
         child: new Container(
             width: itemExtent,
             height: itemExtent,
             child: new Text(item.toString())
         )
     );
-}
+  }
 
-Widget widgetBuilder() {
   return new Container(
       padding: const EdgeInsets.all(10.0),
       child: new ScrollableList(
           scrollDirection: scrollDirection,
           itemExtent: itemExtent,
-      children: <int>[0, 1, 2, 3, 4].where(
-        (int i) => !dismissedItems.contains(i)
-      ).map(buildDismissableItem)
+          children: <int>[0, 1, 2, 3, 4]
+              .where((int i) => !dismissedItems.contains(i))
+              .map(buildDismissableItem)
       )
   );
 }
@@ -58,7 +65,7 @@ Future<Null> dismissElement(WidgetTester tester, Finder finder, { DismissDirecti
 
   Point downLocation;
   Point upLocation;
-  switch(gestureDirection) {
+  switch (gestureDirection) {
     case DismissDirection.endToStart:
       // getTopRight() returns a point that's just beyond itemWidget's right
       // edge and outside the Dismissable event listener's bounds.
@@ -99,11 +106,11 @@ Future<Null> dismissItem(WidgetTester tester, int item, { DismissDirection gestu
 
   await dismissElement(tester, itemFinder, gestureDirection: gestureDirection);
 
-  await tester.pumpWidget(widgetBuilder()); // start the slide
-  await tester.pumpWidget(widgetBuilder(), const Duration(seconds: 1)); // finish the slide and start shrinking...
-  await tester.pumpWidget(widgetBuilder()); // first frame of shrinking animation
-  await tester.pumpWidget(widgetBuilder(), const Duration(seconds: 1)); // finish the shrinking and call the callback...
-  await tester.pumpWidget(widgetBuilder()); // rebuild after the callback removes the entry
+  await tester.pumpWidget(buildTest()); // start the slide
+  await tester.pumpWidget(buildTest(), const Duration(seconds: 1)); // finish the slide and start shrinking...
+  await tester.pumpWidget(buildTest()); // first frame of shrinking animation
+  await tester.pumpWidget(buildTest(), const Duration(seconds: 1)); // finish the shrinking and call the callback...
+  await tester.pumpWidget(buildTest()); // rebuild after the callback removes the entry
 }
 
 class Test1215DismissableWidget extends StatelessWidget {
@@ -133,7 +140,7 @@ void main() {
     scrollDirection = Axis.vertical;
     dismissDirection = DismissDirection.horizontal;
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     expect(dismissedItems, isEmpty);
 
     await dismissItem(tester, 0, gestureDirection: DismissDirection.startToEnd);
@@ -151,7 +158,7 @@ void main() {
     scrollDirection = Axis.horizontal;
     dismissDirection = DismissDirection.vertical;
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     expect(dismissedItems, isEmpty);
 
     await dismissItem(tester, 0, gestureDirection: DismissDirection.up);
@@ -169,7 +176,7 @@ void main() {
     scrollDirection = Axis.vertical;
     dismissDirection = DismissDirection.endToStart;
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     expect(dismissedItems, isEmpty);
 
     await dismissItem(tester, 0, gestureDirection: DismissDirection.startToEnd);
@@ -187,7 +194,7 @@ void main() {
     scrollDirection = Axis.vertical;
     dismissDirection = DismissDirection.startToEnd;
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     expect(dismissedItems, isEmpty);
 
     await dismissItem(tester, 0, gestureDirection: DismissDirection.endToStart);
@@ -203,7 +210,7 @@ void main() {
     scrollDirection = Axis.horizontal;
     dismissDirection = DismissDirection.up;
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     expect(dismissedItems, isEmpty);
 
     await dismissItem(tester, 0, gestureDirection: DismissDirection.down);
@@ -219,7 +226,7 @@ void main() {
     scrollDirection = Axis.horizontal;
     dismissDirection = DismissDirection.down;
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     expect(dismissedItems, isEmpty);
 
     await dismissItem(tester, 0, gestureDirection: DismissDirection.up);
@@ -231,6 +238,22 @@ void main() {
     expect(dismissedItems, equals(<int>[0]));
   });
 
+  testWidgets('drag-left has no effect on dismissable with a high dismiss threshold', (WidgetTester tester) async {
+    scrollDirection = Axis.vertical;
+    dismissDirection = DismissDirection.horizontal;
+
+    await tester.pumpWidget(buildTest(startToEndThreshold: 1.0));
+    expect(dismissedItems, isEmpty);
+
+    await dismissItem(tester, 0, gestureDirection: DismissDirection.startToEnd);
+    expect(find.text('0'), findsOneWidget);
+    expect(dismissedItems, isEmpty);
+
+    await dismissItem(tester, 0, gestureDirection: DismissDirection.endToStart);
+    expect(find.text('0'), findsNothing);
+    expect(dismissedItems, equals(<int>[0]));
+  });
+
   // This is a regression test for an fn2 bug where dragging a card caused an
   // assert "'!_disqualifiedFromEverAppearingAgain' is not true". The old URL
   // was https://github.com/domokit/sky_engine/issues/1068 but that issue is 404
@@ -241,22 +264,22 @@ void main() {
     scrollDirection = Axis.horizontal;
     dismissDirection = DismissDirection.down;
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     Point location = tester.getTopLeft(find.text('0'));
     Offset offset = const Offset(0.0, 5.0);
     TestGesture gesture = await tester.startGesture(location, pointer: 5);
     await gesture.moveBy(offset);
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     await gesture.moveBy(offset);
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     await gesture.moveBy(offset);
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     await gesture.moveBy(offset);
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     await gesture.up();
   });
 
-  // This one is for a case where dssmissing a widget above a previously
+  // This one is for a case where dismissing a widget above a previously
   // dismissed widget threw an exception, which was documented at the
   // now-obsolete URL https://github.com/flutter/engine/issues/1215 (the URL
   // died in the migration to the new repo). Don't copy this test; it doesn't
@@ -294,7 +317,7 @@ void main() {
     dismissDirection = DismissDirection.horizontal;
     background = new Text('background');
 
-    await tester.pumpWidget(widgetBuilder());
+    await tester.pumpWidget(buildTest());
     expect(dismissedItems, isEmpty);
 
     Finder itemFinder = find.text('0');