Add a hasListeners to ChangeNotifier (#14946)

I found that some ValueListeners want to know when they should start
doing work (e.g. if the value comes from polling a network resource).

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

@@ -68,6 +68,27 @@ class ChangeNotifier extends Listenable {
     return true;
   }
 
+  /// Whether any listeners are currently registered.
+  ///
+  /// Clients should not depend on this value for their behavior, because having
+  /// one listener's logic change when another listener happens to start or stop
+  /// listening will lead to extremely hard-to-track bugs. Subclasses might use
+  /// this information to determine whether to do any work when there are no
+  /// listeners, however; for example, resuming a [Stream] when a listener is
+  /// added and pausing it when a listener is removed.
+  ///
+  /// Typically this is used by overriding [addListener], checking if
+  /// [hasListeners] is false before calling `super.addListener()`, and if so,
+  /// starting whatever work is needed to determine when to call
+  /// [notifyListeners]; and similarly, by overriding [removeListener], checking
+  /// if [hasListeners] is false after calling `super.removeListener()`, and if
+  /// so, stopping that same work.
+  @protected
+  bool get hasListeners {
+    assert(_debugAssertNotDisposed());
+    return _listeners.isNotEmpty;
+  }
+
   /// Register a closure to be called when the object changes.
   ///
   /// This method must not be called after [dispose] has been called.

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

@@ -8,14 +8,14 @@ import 'dart:collection';
 ///
 /// Consider using an [ObserverList] instead of a [List] when the number of
 /// [contains] calls dominates the number of [add] and [remove] calls.
+// TODO(ianh): Use DelegatingIterable, possibly moving it from the collection
+// package to foundation, or to dart:collection.
 class ObserverList<T> extends Iterable<T> {
   final List<T> _list = <T>[];
   bool _isDirty = false;
   HashSet<T> _set;
 
   /// Adds an item to the end of this list.
-  ///
-  /// The given item must not already be in the list.
   void add(T item) {
     _isDirty = true;
     _list.add(item);
@@ -23,6 +23,8 @@ class ObserverList<T> extends Iterable<T> {
 
   /// Removes an item from the list.
   ///
+  /// This is O(N) in the number of items in the list.
+  ///
   /// Returns whether the item was present in the list.
   bool remove(T item) {
     _isDirty = true;
@@ -49,4 +51,10 @@ class ObserverList<T> extends Iterable<T> {
 
   @override
   Iterator<T> get iterator => _list.iterator;
+
+  @override
+  bool get isEmpty => _list.isEmpty;
+
+  @override
+  bool get isNotEmpty => _list.isNotEmpty;
 }

packages/flutter/test/foundation/change_notifier_test.dart

@@ -235,4 +235,32 @@ void main() {
       "Listenable.merge([null, Instance of 'TestNotifier'])",
     );
   });
+
+  test('hasListeners', () {
+    final HasListenersTester<bool> notifier = new HasListenersTester<bool>(true);
+    expect(notifier.testHasListeners, isFalse);
+    void test1() { }
+    void test2() { }
+    notifier.addListener(test1);
+    expect(notifier.testHasListeners, isTrue);
+    notifier.addListener(test1);
+    expect(notifier.testHasListeners, isTrue);
+    notifier.removeListener(test1);
+    expect(notifier.testHasListeners, isTrue);
+    notifier.removeListener(test1);
+    expect(notifier.testHasListeners, isFalse);
+    notifier.addListener(test1);
+    expect(notifier.testHasListeners, isTrue);
+    notifier.addListener(test2);
+    expect(notifier.testHasListeners, isTrue);
+    notifier.removeListener(test1);
+    expect(notifier.testHasListeners, isTrue);
+    notifier.removeListener(test2);
+    expect(notifier.testHasListeners, isFalse);
+  });
+}
+
+class HasListenersTester<T> extends ValueNotifier<T> {
+  HasListenersTester(T value) : super(value);
+  bool get testHasListeners => hasListeners;
 }