Dismissible RTL (#13137)

Fix the dismissible demo in the gallery (make it actuall update when you pick something from its menu; give it a better affordance for resetting once you've dismissed everything).

Improve some docs.

Fix various flinging bugs with dismissible. Add tests for those cases.

Add a feature to flutter_test to support a drag-then-fling gesture (used by the flinging tests).

examples/flutter_gallery/lib/demo/material/leave_behind_demo.dart

@@ -60,20 +60,22 @@ class LeaveBehindDemoState extends State<LeaveBehindDemo> {
   }
 
   void handleDemoAction(LeaveBehindDemoAction action) {
-    switch (action) {
-      case LeaveBehindDemoAction.reset:
-        initListItems();
-        break;
-      case LeaveBehindDemoAction.horizontalSwipe:
-        _dismissDirection = DismissDirection.horizontal;
-        break;
-      case LeaveBehindDemoAction.leftSwipe:
-        _dismissDirection = DismissDirection.endToStart;
-        break;
-      case LeaveBehindDemoAction.rightSwipe:
-        _dismissDirection = DismissDirection.startToEnd;
-        break;
-    }
+    setState(() {
+      switch (action) {
+        case LeaveBehindDemoAction.reset:
+          initListItems();
+          break;
+        case LeaveBehindDemoAction.horizontalSwipe:
+          _dismissDirection = DismissDirection.horizontal;
+          break;
+        case LeaveBehindDemoAction.leftSwipe:
+          _dismissDirection = DismissDirection.endToStart;
+          break;
+        case LeaveBehindDemoAction.rightSwipe:
+          _dismissDirection = DismissDirection.startToEnd;
+          break;
+      }
+    });
   }
 
   void handleUndo(LeaveBehindItem item) {
@@ -161,9 +163,16 @@ class LeaveBehindDemoState extends State<LeaveBehindDemo> {
           )
         ]
       ),
-      body: new ListView(
-        children: leaveBehindItems.map(buildItem).toList()
-      )
+      body: leaveBehindItems.isEmpty
+          ? new Center(
+              child: new RaisedButton(
+                onPressed: () => handleDemoAction(LeaveBehindDemoAction.reset),
+                child: const Text('Reset the list'),
+              ),
+            )
+          : new ListView(
+             children: leaveBehindItems.map(buildItem).toList()
+            ),
     );
   }
 }

packages/flutter_test/lib/src/controller.dart

@@ -299,11 +299,28 @@ class WidgetController {
   ///
   /// A fling is essentially a drag that ends at a particular speed. If you
   /// just want to drag and end without a fling, use [drag].
+  ///
+  /// The `initialOffset` argument, if non-zero, causes the pointer to first
+  /// apply that offset, then pump a delay of `initialOffsetDelay`. This can be
+  /// used to simulate a drag followed by a fling, including dragging in the
+  /// opposite direction of the fling (e.g. dragging 200 pixels to the right,
+  /// then fling to the left over 200 pixels, ending at the exact point that the
+  /// drag started).
   Future<Null> fling(Finder finder, Offset offset, double speed, {
     int pointer,
     Duration frameInterval: const Duration(milliseconds: 16),
+    Offset initialOffset: Offset.zero,
+    Duration initialOffsetDelay: const Duration(seconds: 1),
   }) {
-    return flingFrom(getCenter(finder), offset, speed, pointer: pointer, frameInterval: frameInterval);
+    return flingFrom(
+      getCenter(finder),
+      offset,
+      speed,
+      pointer: pointer,
+      frameInterval: frameInterval,
+      initialOffset: initialOffset,
+      initialOffsetDelay: initialOffsetDelay,
+    );
   }
 
   /// Attempts a fling gesture starting from the given location, moving the
@@ -324,7 +341,19 @@ class WidgetController {
   ///
   /// A fling is essentially a drag that ends at a particular speed. If you
   /// just want to drag and end without a fling, use [dragFrom].
-  Future<Null> flingFrom(Offset startLocation, Offset offset, double speed, { int pointer, Duration frameInterval: const Duration(milliseconds: 16) }) {
+  ///
+  /// The `initialOffset` argument, if non-zero, causes the pointer to first
+  /// apply that offset, then pump a delay of `initialOffsetDelay`. This can be
+  /// used to simulate a drag followed by a fling, including dragging in the
+  /// opposite direction of the fling (e.g. dragging 200 pixels to the right,
+  /// then fling to the left over 200 pixels, ending at the exact point that the
+  /// drag started).
+  Future<Null> flingFrom(Offset startLocation, Offset offset, double speed, {
+    int pointer,
+    Duration frameInterval: const Duration(milliseconds: 16),
+    Offset initialOffset: Offset.zero,
+    Duration initialOffsetDelay: const Duration(seconds: 1),
+  }) {
     assert(offset.distance > 0.0);
     assert(speed > 0.0); // speed is pixels/second
     return TestAsyncUtils.guard(() async {
@@ -335,8 +364,13 @@ class WidgetController {
       double timeStamp = 0.0;
       double lastTimeStamp = timeStamp;
       await sendEventToBinding(testPointer.down(startLocation, timeStamp: new Duration(milliseconds: timeStamp.round())), result);
+      if (initialOffset.distance > 0.0) {
+        await sendEventToBinding(testPointer.move(startLocation + initialOffset, timeStamp: new Duration(milliseconds: timeStamp.round())), result);
+        timeStamp += initialOffsetDelay.inMilliseconds;
+        await pump(initialOffsetDelay);
+      }
       for (int i = 0; i <= kMoveCount; i += 1) {
-        final Offset location = startLocation + Offset.lerp(Offset.zero, offset, i / kMoveCount);
+        final Offset location = startLocation + initialOffset + Offset.lerp(Offset.zero, offset, i / kMoveCount);
         await sendEventToBinding(testPointer.move(location, timeStamp: new Duration(milliseconds: timeStamp.round())), result);
         timeStamp += timeStampDelta;
         if (timeStamp - lastTimeStamp > frameInterval.inMilliseconds) {