implement dirty checking for @observable objects

pkg/observe/lib/src/change_notifier.dart

+// 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.
+
+part of observe;
+
+/**
+ * Interface representing an [Observable] object that performs its own change
+ * notifications, and does not need to be considered by [Observable.dirtyCheck].
+ */
+abstract class ChangeNotifier extends Observable {
+  /**
+   * Notify observers of a change.
+   *
+   * For most objects [ChangeNotifierMixin.notifyPropertyChange] is more
+   * convenient, but collections sometimes deliver other types of changes such
+   * as a [ListChangeRecord].
+   */
+  void notifyChange(ChangeRecord record);
+}
+
+/**
+ * Base class implementing [ChangeNotifier].
+ *
+ * When a field, property, or indexable item is changed, a derived class should
+ * call [notifyPropertyChange]. See that method for an example.
+ */
+typedef ChangeNotifierBase = Object with ChangeNotifierMixin;
+
+/**
+ * Mixin for implementing [ChangeNotifier] objects.
+ *
+ * When a field, property, or indexable item is changed, a derived class should
+ * call [notifyPropertyChange]. See that method for an example.
+ */
+abstract class ChangeNotifierMixin implements ChangeNotifier {
+  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() {}
+
+  bool deliverChanges() {
+    var records = _records;
+    _records = null;
+    if (hasObservers && records != null) {
+      // TODO(jmesserly): make "records" immutable
+      _changes.add(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
+   * identical, no change will be recorded.
+   *
+   * For convenience this returns [newValue]. This makes it easy to use in a
+   * setter:
+   *
+   *     var _myField;
+   *     get myField => _myField;
+   *     set myField(value) {
+   *       _myField = notifyPropertyChange(
+   *           const Symbol('myField'), _myField, value);
+   *     }
+   */
+  // TODO(jmesserly): should this be == instead of identical, to prevent
+  // spurious loops?
+  notifyPropertyChange(Symbol field, Object oldValue, Object newValue) {
+    if (hasObservers && !identical(oldValue, newValue)) {
+      notifyChange(new PropertyChangeRecord(field));
+    }
+    return newValue;
+  }
+
+  void notifyChange(ChangeRecord record) {
+    if (!hasObservers) return;
+
+    if (_records == null) {
+      _records = [];
+      runAsync(deliverChanges);
+    }
+    _records.add(record);
+  }
+}