Merge pull request flutter#1400 from HansMuller/land_snap_scrolling_u…

…nit_test

Land snap_scrolling_test.dart

sky/unit/test/widget/snap_scrolling_test.dart

@@ -0,0 +1,87 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:quiver/testing/async.dart';
+import 'package:sky/src/fn3.dart';
+import 'package:test/test.dart';
+
+import '../fn3/widget_tester.dart';
+
+const double itemExtent = 200.0;
+ScrollDirection scrollDirection = ScrollDirection.vertical;
+GlobalKey scrollableListKey;
+
+Widget buildItem(BuildContext context, int item) {
+  return new Container(
+    key: new ValueKey<int>(item),
+    width: itemExtent,
+    height: itemExtent,
+    child: new Text(item.toString())
+  );
+}
+
+double snapOffsetCallback(double offset) {
+  return (offset / itemExtent).floor() * itemExtent;
+}
+
+Widget buildScrollableList() {
+  scrollableListKey = new GlobalKey();
+  return new Container(
+    height: itemExtent * 2.0,
+    child: new ScrollableList<int>(
+      key: scrollableListKey,
+      snapOffsetCallback: snapOffsetCallback,
+      scrollDirection: scrollDirection,
+      items: [0, 1, 2, 3, 4, 5, 7, 8, 9],
+      itemBuilder: buildItem,
+      itemExtent: itemExtent
+    )
+  );
+}
+
+ScrollableState get scrollableState => scrollableListKey.currentState;
+
+double get scrollOffset =>  scrollableState.scrollOffset;
+void set scrollOffset(double value) {
+  scrollableState.scrollTo(value);
+}
+
+void fling(double velocity) {
+  Offset velocityOffset = scrollDirection == ScrollDirection.vertical
+    ? new Offset(0.0, velocity)
+    : new Offset(velocity, 0.0);
+  scrollableState.fling(velocityOffset);
+}
+
+void main() {
+  test('ScrollableList snap scrolling, fling(-800)', () {
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(new Center(child: buildScrollableList()));
+    expect(scrollOffset, 0.0);
+
+    new FakeAsync().run((async) {
+      fling(-800.0);
+      tester.pumpFrameWithoutChange(); // Start the scheduler at 0.0
+      tester.pumpFrameWithoutChange(1000.0);
+      async.elapse(new Duration(seconds: 1));
+      expect(scrollOffset, closeTo(200.0, 1.0));
+    });
+  });
+
+  test('ScrollableList snap scrolling, fling(-2000)', () {
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(new Center(child: buildScrollableList()));
+    expect(scrollOffset, 0.0);
+
+    new FakeAsync().run((async) {
+      fling(-2000.0);
+      tester.pumpFrameWithoutChange(); // Start the scheduler at 0.0
+      tester.pumpFrameWithoutChange(1000.0);
+      async.elapse(new Duration(seconds: 1));
+      expect(scrollOffset, closeTo(400.0, 1.0));
+    });
+  });
+}