Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/src/change_notifier.dart

Index: observatory_pub_packages/observe/src/change_notifier.dart
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--- observatory_pub_packages/observe/src/change_notifier.dart	(revision 0)
+++ observatory_pub_packages/observe/src/change_notifier.dart	(working copy)
@@ -0,0 +1,81 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library observe.src.change_notifier;
+
+import 'dart:async';
+import 'dart:collection' show UnmodifiableListView;
+import 'package:observe/observe.dart';
+import 'package:observe/src/observable.dart' show notifyPropertyChangeHelper;
+
+/// Mixin and base class for implementing an [Observable] object that performs
+/// its own change notifications, and does not need to be considered by
+/// [Observable.dirtyCheck].
+///
+/// When a field, property, or indexable item is changed, a derived class should
+/// call [notifyPropertyChange]. See that method for an example.
+abstract class ChangeNotifier implements Observable {
+  StreamController _changes;
+  List<ChangeRecord> _records;
+
+  Stream<List<ChangeRecord>> get changes {
+    if (_changes == null) {
+      _changes = new StreamController.broadcast(sync: true,
+          onListen: observed, onCancel: unobserved);
+    }
+    return _changes.stream;
+  }
+
+  // TODO(jmesserly): should these be public? They're useful lifecycle methods
+  // for subclasses. Ideally they'd be protected.
+  /// Override this method to be called when the [changes] are first observed.
+  void observed() {}
+
+  /// Override this method to be called when the [changes] are no longer being
+  /// observed.
+  void unobserved() {
+    // Free some memory
+    _changes = null;
+  }
+
+  bool deliverChanges() {
+    var records = _records;
+    _records = null;
+    if (hasObservers && records != null) {
+      _changes.add(new UnmodifiableListView<ChangeRecord>(records));
+      return true;
+    }
+    return false;
+  }
+
+  /// True if this object has any observers, and should call
+  /// [notifyPropertyChange] for changes.
+  bool get hasObservers => _changes != null && _changes.hasListener;
+
+  /// Notify that the field [name] of this object has been changed.
+  ///
+  /// The [oldValue] and [newValue] are also recorded. If the two values are
+  /// equal, no change will be recorded.
+  ///
+  /// For convenience this returns [newValue]. This makes it easy to use in a
+  /// setter:
+  ///
+  ///     var _myField;
+  ///     @reflectable get myField => _myField;
+  ///     @reflectable set myField(value) {
+  ///       _myField = notifyPropertyChange(#myField, _myField, value);
+  ///     }
+  notifyPropertyChange(Symbol field, Object oldValue, Object newValue)
+      => notifyPropertyChangeHelper(this, field, oldValue, newValue);
+
+  void notifyChange(ChangeRecord record) {
+    if (!hasObservers) return;
+
+    if (_records == null) {
+      _records = [];
+      scheduleMicrotask(deliverChanges);
+    }
+    _records.add(record);
+  }
+}