Extract a Listenable base class from Animation and ChangeNotifier (#5889)

Having this base class lets classes like CustomPainter and DataTableSource be
more agnostic as to what's generating the repaints.

packages/flutter/lib/foundation.dart

@@ -14,6 +14,7 @@ export 'src/foundation/basic_types.dart';
 export 'src/foundation/binding.dart';
 export 'src/foundation/change_notifier.dart';
 export 'src/foundation/licenses.dart';
+export 'src/foundation/listenable.dart';
 export 'src/foundation/platform.dart';
 export 'src/foundation/print.dart';
 export 'src/foundation/synchronous_future.dart';

packages/flutter/lib/src/animation/animation.dart

@@ -4,6 +4,8 @@
 
 import 'dart:ui' show VoidCallback;
 
+import 'package:flutter/foundation.dart';
+
 /// The status of an animation
 enum AnimationStatus {
   /// The animation is stopped at the beginning
@@ -36,7 +38,7 @@ typedef void AnimationStatusListener(AnimationStatus status);
 ///
 /// To create a new animation that you can run forward and backward, consider
 /// using [AnimationController].
-abstract class Animation<T> {
+abstract class Animation<T> extends Listenable {
   /// Abstract const constructor. This constructor enables subclasses to provide
   /// const constructors so that they can be used in const expressions.
   const Animation();
@@ -46,11 +48,13 @@ abstract class Animation<T> {
   /// Calls the listener every time the value of the animation changes.
   ///
   /// Listeners can be removed with [removeListener].
+  @override
   void addListener(VoidCallback listener);
 
   /// Stop calling the listener every time the value of the animation changes.
   ///
   /// Listeners can be added with [addListener].
+  @override
   void removeListener(VoidCallback listener);
 
   /// Calls listener every time the status of the animation changes.

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -6,13 +6,15 @@ import 'package:meta/meta.dart';
 
 import 'assertions.dart';
 import 'basic_types.dart';
+import 'listenable.dart';
 
-/// Abstract class that can be extended or mixed in that provides
-/// a change notification API using [VoidCallback] for notifications.
-abstract class ChangeNotifier {
+/// A class that can be extended or mixed in that provides a change notification
+/// API using [VoidCallback] for notifications.
+class ChangeNotifier extends Listenable {
   List<VoidCallback> _listeners;
 
   /// Register a closure to be called when the object changes.
+  @override
   void addListener(VoidCallback listener) {
     _listeners ??= <VoidCallback>[];
     _listeners.add(listener);
@@ -20,6 +22,7 @@ abstract class ChangeNotifier {
 
   /// Remove a previously registered closure from the list of closures that are
   /// notified when the object changes.
+  @override
   void removeListener(VoidCallback listener) {
     _listeners?.remove(listener);
   }

packages/flutter/lib/src/foundation/listenable.dart

+// Copyright 2016 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 'basic_types.dart';
+
+/// An object that maintains a list of listeners.
+abstract class Listenable {
+  /// Abstract const constructor. This constructor enables subclasses to provide
+  /// const constructors so that they can be used in const expressions.
+  const Listenable();
+
+  /// Register a closure to be called when the object notifies its listeners.
+  void addListener(VoidCallback listener);
+
+  /// Remove a previously registered closure from the list of closures that the
+  /// object notifies.
+  void removeListener(VoidCallback listener);
+}

packages/flutter/lib/src/material/data_table_source.dart

@@ -59,4 +59,4 @@ abstract class DataTableSource extends ChangeNotifier {
   ///
   /// If the selected row count changes, call [notifyListeners].
   int get selectedRowCount;
-}
+}
\ No newline at end of file

packages/flutter/lib/src/material/time_picker.dart

@@ -426,6 +426,12 @@ class _DialState extends State<_Dial> {
       _animateTo(_getThetaForTime(config.selectedTime));
   }
 
+  @override
+  void dispose() {
+    _thetaController.dispose();
+    super.dispose();
+  }
+
   Tween<double> _thetaTween;
   Animation<double> _theta;
   AnimationController _thetaController;

packages/flutter/lib/src/rendering/flow.dart

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/animation.dart';
+import 'package:flutter/foundation.dart';
 import 'package:vector_math/vector_math_64.dart';
 
 import 'box.dart';
@@ -54,10 +54,10 @@ abstract class FlowPaintingContext {
 ///  * [Flow]
 ///  * [RenderFlow]
 abstract class FlowDelegate {
-  /// The flow will repaint whenever the [repaint] animation ticks.
-  const FlowDelegate({ Animation<dynamic> repaint }) : _repaint = repaint;
+  /// The flow will repaint whenever [repaint] notifies its listeners.
+  const FlowDelegate({ Listenable repaint }) : _repaint = repaint;
 
-  final Animation<dynamic> _repaint;
+  final Listenable _repaint;
 
   /// Override to control the size of the container for the children.
   ///

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -4,7 +4,7 @@
 
 import 'dart:ui' as ui show ImageFilter;
 
-import 'package:flutter/animation.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:meta/meta.dart';
 import 'package:vector_math/vector_math_64.dart';
@@ -1490,10 +1490,10 @@ class RenderFractionalTranslation extends RenderProxyBox {
 abstract class CustomPainter {
   /// Creates a custom painter.
   ///
-  /// The painter will repaint whenever the [repaint] animation ticks.
-  const CustomPainter({ Animation<dynamic> repaint }) : _repaint = repaint;
+  /// The painter will repaint whenever [repaint] notifies its listeners.
+  const CustomPainter({ Listenable repaint }) : _repaint = repaint;
 
-  final Animation<dynamic> _repaint;
+  final Listenable _repaint;
 
   /// Called whenever the object needs to paint. The given [Canvas] has its
   /// coordinate space configured such that the origin is at the top left of the