Implement Model-Driven-Views spec for Dart

sdk/lib/mdv_observe_impl/mdv_observe_impl.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.
+
+// This library itself is undocumented and not supported for end use.
+// Because dart:html must use some of this functionality, it has to be available
+// via a dart:* library. The public APIs are reexported via package:mdv_observe.
+// Generally we try to keep this library minimal, with utility types and
+// functions in the package.
+library dart.mdv_observe_impl;
+
+import 'dart:async';
+import 'dart:collection';
+
+/**
+ * Interface representing an observable object. This is used by data in
+ * model-view architectures to notify interested parties of [changes].
+ *
+ * This object does not require any specific technique to implement
+ * observability.
+ *
+ * You can use [ObservableMixin] as a base class or mixin to implement this.
+ */
+abstract class Observable {
+  /**
+   * The stream of change records to this object.
+   *
+   * Changes should be delivered in asynchronous batches by calling
+   * [queueChangeRecords].
+   *
+   * [deliverChangeRecords] can be called to force delivery.
+   */
+  Stream<List<ChangeRecord>> get changes;
+
+  // TODO(jmesserly): remove these ASAP.
+  /**
+   * *Warning*: this method is temporary until dart2js supports mirrors.
+   * Gets the value of a field or index. This should return null if it was
+   * not found.
+   */
+  getValueWorkaround(key);
+
+  /**
+   * *Warning*: this method is temporary until dart2js supports mirrors.
+   * Sets the value of a field or index. This should have no effect if the field
+   * was not found.
+   */
+  void setValueWorkaround(key, Object value);
+}
+
+/**
+ * Base class implementing [Observable].
+ *
+ * When a field, property, or indexable item is changed, a derived class should
+ * call [notifyPropertyChange]. See that method for an example.
+ */
+typedef ObservableBase = Object with ObservableMixin;
+
+/**
+ * Mixin for implementing [Observable] objects.
+ *
+ * When a field, property, or indexable item is changed, a derived class should
+ * call [notifyPropertyChange]. See that method for an example.
+ */
+abstract class ObservableMixin implements Observable {
+  StreamController<List<ChangeRecord>> _observers;
+  Stream<List<ChangeRecord>> _stream;
+  List<ChangeRecord> _changes;
+
+  Stream<List<ChangeRecord>> get changes {
+    if (_observers == null) {
+      _observers = new StreamController<List<ChangeRecord>>();
+      _stream = _observers.stream.asBroadcastStream();
+    }
+    return _stream;
+  }
+
+  void _deliverChanges() {
+    var changes = _changes;
+    _changes = null;
+    if (hasObservers && changes != null) {
+      // TODO(jmesserly): make "changes" immutable
+      _observers.add(changes);
+    }
+  }
+
+  /**
+   * True if this object has any observers, and should call
+   * [notifyPropertyChange] for changes.
+   */
+  bool get hasObservers => _observers != null && _observers.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;
+  }
+
+  /**
+   * Notify observers of a change. For most objects [notifyPropertyChange] is
+   * more convenient, but collections sometimes deliver other types of changes
+   * such as a [ListChangeRecord].
+   */
+  void notifyChange(ChangeRecord record) {
+    if (!hasObservers) return;
+
+    if (_changes == null) {
+      _changes = [];
+      queueChangeRecords(_deliverChanges);
+    }
+    _changes.add(record);
+  }
+}
+
+
+/** Records a change to an [Observable]. */
+abstract class ChangeRecord {
+  /** True if the change affected the given item, otherwise false. */
+  bool change(key);
+}
+
+/** A change record to a field of an observable object. */
+class PropertyChangeRecord extends ChangeRecord {
+  /** The field that was changed. */
+  final Symbol field;
+
+  PropertyChangeRecord(this.field);
+
+  bool changes(key) => key is Symbol && field == key;
+
+  String toString() => '#<PropertyChangeRecord $field>';
+}
+
+/** A change record for an observable list. */
+class ListChangeRecord extends ChangeRecord {
+  /** The starting index of the change. */
+  final int index;
+
+  /** The number of items removed. */
+  final int removedCount;
+
+  /** The number of items added. */
+  final int addedCount;
+
+  ListChangeRecord(this.index, {this.removedCount: 0, this.addedCount: 0}) {
+    if (addedCount == 0 && removedCount == 0) {
+      throw new ArgumentError('added and removed counts should not both be '
+          'zero. Use 1 if this was a single item update.');
+    }
+  }
+
+  /** Returns true if the provided index was changed by this operation. */
+  bool changes(key) {
+    // If key isn't an int, or before the index, then it wasn't changed.
+    if (key is! int || key < index) return false;
+
+    // If this was a shift operation, anything after index is changed.
+    if (addedCount != removedCount) return true;
+
+    // Otherwise, anything in the update range was changed.
+    return key < index + addedCount;
+  }
+
+  String toString() => '#<ListChangeRecord index: $index, '
+      'removed: $removedCount, addedCount: $addedCount>';
+}
+
+/**
+ * Synchronously deliver [Observable.changes] for all observables.
+ * If new changes are added as a result of delivery, this will keep running
+ * until all pending change records are delivered.
+ */
+// TODO(jmesserly): this is a bit different from the ES Harmony version, which
+// allows delivery of changes to a particular observer:
+// http://wiki.ecmascript.org/doku.php?id=harmony:observe#object.deliverchangerecords
+// However the binding system needs delivery of everything, along the lines of:
+// https://github.com/toolkitchen/mdv/blob/stable/src/model.js#L19
+// https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js#L590
+// TODO(jmesserly): in the future, we can use this to trigger dirty checking.
+void deliverChangeRecords() {
+  if (_deliverCallbacks == null) return;
+
+  while (!_deliverCallbacks.isEmpty) {
+    var deliver = _deliverCallbacks.removeFirst();
+
+    try {
+      deliver();
+    } catch (e, s) {
+      // Schedule the error to be top-leveled later.
+      new Completer().completeError(e, s);
+    }
+  }
+
+  // Null it out, so [queueChangeRecords] will reschedule this method.
+  _deliverCallbacks = null;
+}
+
+/** Queues an action to happen during the [deliverChangeRecords] timeslice. */
+void queueChangeRecords(void deliverChanges()) {
+  if (_deliverCallbacks == null) {
+    _deliverCallbacks = new Queue<Function>();
+    runAsync(deliverChangeRecords);
+  }
+  _deliverCallbacks.add(deliverChanges);
+}
+
+Queue _deliverCallbacks;