Add ValueNotifier (#8463)

It's common to have a ChangeNotifier that wraps a single value. This class
makes that easy by providing a generic implementation.

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

@@ -37,6 +37,10 @@ abstract class Listenable {
 /// [ChangeNotifier] is optimised for small numbers (one or two) of listeners.
 /// It is O(N) for adding and removing listeners and O(N²) for dispatching
 /// notifications (where N is the number of listeners).
+///
+/// See also:
+///
+///  * [ValueNotifier], which is a [ChangeNotifier] that wraps a single value.
 class ChangeNotifier extends Listenable {
   ObserverList<VoidCallback> _listeners = new ObserverList<VoidCallback>();
 
@@ -155,3 +159,23 @@ class _MergingListenable extends ChangeNotifier {
     super.dispose();
   }
 }
+
+/// A [ChangeNotifier] that holds a single value.
+///
+/// When [value] is replaced, this class notifies its listeners.
+class ValueNotifier<T> extends ChangeNotifier {
+  /// Creates a [ChangeNotifier] that wraps this value.
+  ValueNotifier(this._value);
+
+  /// The current value stored in this notifier.
+  ///
+  /// When the value is replaced, this class notifies its listeners.
+  T get value => _value;
+  T _value;
+  set value(T newValue) {
+    if (_value == newValue)
+      return;
+    _value = newValue;
+    notifyListeners();
+  }
+}

packages/flutter/test/foundation/change_notifier_test.dart

@@ -190,4 +190,20 @@ void main() {
     expect(() { source.dispose(); }, throwsFlutterError);
     expect(() { source.notify(); }, throwsFlutterError);
   });
+
+  test('Value notifier', () {
+    final ValueNotifier<double> notifier = new ValueNotifier<double>(2.0);
+
+    final List<double> log = <double>[];
+    final VoidCallback listener = () { log.add(notifier.value); };
+
+    notifier.addListener(listener);
+    notifier.value = 3.0;
+
+    expect(log, equals(<double>[ 3.0 ]));
+    log.clear();
+
+    notifier.value = 3.0;
+    expect(log, isEmpty);
+  });
 }