ScrollController should notify when its offset changes (#8768)

This change make it easier to track the position of the scroll view without
having to worry about the position object changing out from under the
controller.

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

@@ -5,11 +5,12 @@
 import 'dart:async';
 
 import 'package:flutter/animation.dart';
+import 'package:flutter/foundation.dart';
 import 'package:meta/meta.dart';
 
 import 'scroll_position.dart';
 
-class ScrollController {
+class ScrollController extends ChangeNotifier {
   ScrollController({
     this.initialScrollOffset: 0.0,
   }) {
@@ -101,6 +102,7 @@ class ScrollController {
   void attach(ScrollPosition position) {
     assert(!_positions.contains(position));
     _positions.add(position);
+    position.addListener(notifyListeners);
   }
 
   /// Unregister the given position with this controller.
@@ -109,9 +111,17 @@ class ScrollController {
   /// controller will not manipulate the given position.
   void detach(ScrollPosition position) {
     assert(_positions.contains(position));
+    position.removeListener(notifyListeners);
     _positions.remove(position);
   }
 
+  @override
+  void dispose() {
+    for (ScrollPosition position in _positions)
+      position.removeListener(notifyListeners);
+    super.dispose();
+  }
+
   static ScrollPosition createDefaultScrollPosition(ScrollPhysics physics, AbstractScrollState state, ScrollPosition oldPosition) {
     return new ScrollPosition(
       physics: physics,

packages/flutter/test/widgets/scroll_controller_test.dart

@@ -230,4 +230,33 @@ void main() {
     controller.animateTo(1.0, duration: const Duration(seconds: 1), curve: Curves.linear);
     await tester.pumpAndSettle();
   });
+
+  testWidgets('Scroll controllers notify when the position changes', (WidgetTester tester) async {
+    final ScrollController controller = new ScrollController();
+
+    final List<double> log = <double>[];
+
+    controller.addListener(() {
+      log.add(controller.offset);
+    });
+
+    await tester.pumpWidget(new ListView(
+      controller: controller,
+      children: kStates.map<Widget>((String state) {
+        return new Container(height: 200.0, child: new Text(state));
+      }).toList(),
+    ));
+
+    expect(log, isEmpty);
+
+    await tester.drag(find.byType(ListView), const Offset(0.0, -250.0));
+
+    expect(log, equals(<double>[ 250.0 ]));
+    log.clear();
+
+    controller.dispose();
+
+    await tester.drag(find.byType(ListView), const Offset(0.0, -130.0));
+    expect(log, isEmpty);
+  });
 }