Fix scrolling of Block.

Since our build function depends on scrollBehavior.isScrollable, any
time we update scrollBehavior we are implicitly updating our state. As
such, we must do so during a setState() call, or else we won't rebuild
and might not bother to listen to the scroll gestures.

This probably broke when we made Block not listen to gestures if it
wasn't overflowing.

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

@@ -277,17 +277,23 @@ class ScrollableViewport extends Scrollable {
   double _childSize = 0.0;
   void _handleViewportSizeChanged(Size newSize) {
     _viewportSize = scrollDirection == ScrollDirection.vertical ? newSize.height : newSize.width;
+    setState(() {
       _updateScrollBehaviour();
+    });
   }
   void _handleChildSizeChanged(Size newSize) {
     _childSize = scrollDirection == ScrollDirection.vertical ? newSize.height : newSize.width;
+    setState(() {
       _updateScrollBehaviour();
+    });
   }
   void _updateScrollBehaviour() {
+    // if you don't call this from build() or syncConstructorArguments(), you must call it from setState().
     scrollTo(scrollBehavior.updateExtents(
       contentExtent: _childSize,
       containerExtent: _viewportSize,
-      scrollOffset: scrollOffset));
+      scrollOffset: scrollOffset
+    ));
   }
 
   Widget buildContent() {
@@ -423,6 +429,7 @@ abstract class ScrollableWidgetList extends Scrollable {
   }
 
   void _updateScrollBehavior() {
+    // if you don't call this from build() or syncConstructorArguments(), you must call it from setState().
     double contentExtent = itemExtent * itemCount;
     if (padding != null)
       contentExtent += _leadingPadding + _trailingPadding;
@@ -636,16 +643,22 @@ class ScrollableMixedWidgetList extends Scrollable {
   OverscrollBehavior get scrollBehavior => super.scrollBehavior;
 
   void _handleSizeChanged(Size newSize) {
+    setState(() {
       scrollBy(scrollBehavior.updateExtents(
         containerExtent: newSize.height,
         scrollOffset: scrollOffset
       ));
+    });
   }
 
   void _handleLayoutChanged() {
-    double newScrollOffset = scrollBehavior.updateExtents(
+    double newScrollOffset;
+    setState(() {
+      newScrollOffset = scrollBehavior.updateExtents(
         contentExtent: layoutState.didReachLastChild ? layoutState.contentsSize : double.INFINITY,
-      scrollOffset: scrollOffset);
+        scrollOffset: scrollOffset
+      );
+    });
     if (_contentChanged) {
       _contentChanged = false;
       scrollTo(newScrollOffset);

packages/unit/test/widget/block_test.dart

+import 'package:quiver/testing/async.dart';
+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import '../engine/mock_events.dart';
+import 'widget_tester.dart';
+
+final Key blockKey = new Key('test');
+
+void main() {
+  test('Cannot scroll a non-overflowing block', () {
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
+      return new Block([
+        new Container(
+          height: 200.0, // less than 600, the height of the test area
+          child: new Text('Hello')
+        )
+      ],
+      key: blockKey);
+    });
+    tester.pumpFrameWithoutChange(); // for SizeObservers
+
+    Point middleOfContainer = tester.getCenter(tester.findText('Hello'));
+    Point target = tester.getCenter(tester.findWidget((widget) => widget.key == blockKey));
+    TestPointer pointer = new TestPointer();
+    tester.dispatchEvent(pointer.down(target), target);
+    tester.dispatchEvent(pointer.move(target + const Offset(0.0, -10.0)), target);
+
+    tester.pumpFrameWithoutChange(1.0);
+
+    expect(tester.getCenter(tester.findText('Hello')) == middleOfContainer, isTrue);
+
+    tester.dispatchEvent(pointer.up(), target);
+  });
+
+  test('Can scroll an overflowing block', () {
+    WidgetTester tester = new WidgetTester();
+
+    tester.pumpFrame(() {
+      return new Block([
+        new Container(
+          height: 2000.0, // more than 600, the height of the test area
+          child: new Text('Hello')
+        )
+      ],
+      key: blockKey);
+    });
+    tester.pumpFrameWithoutChange(); // for SizeObservers
+
+    Point middleOfContainer = tester.getCenter(tester.findText('Hello'));
+    Point target = tester.getCenter(tester.findWidget((widget) => widget.key == blockKey));
+    TestPointer pointer = new TestPointer();
+    tester.dispatchEvent(pointer.down(target), target);
+    tester.dispatchEvent(pointer.move(target + const Offset(0.0, -10.0)), target);
+
+    tester.pumpFrameWithoutChange(1.0);
+
+    expect(tester.getCenter(tester.findText('Hello')) == middleOfContainer, isFalse);
+
+    tester.dispatchEvent(pointer.up(), target);
+  });
+}