implement dirty checking for @observable objects

pkg/observe/lib/src/observable.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;
+
+/**
+ * Use `@observable` to make a field automatically observable.
+ */
+const Object observable = const _ObservableAnnotation();
+
+/**
+ * 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. However if you implement change notification yourself, you
+ * should also implement [ChangeNotifier], so [dirtyCheck] knows to skip the
+ * object.
+ *
+ * You can use [ObservableBase] or [ObservableMixin] to implement this.
+ */
+abstract class Observable {
+  /**
+   * The stream of change records to this object. Records will be delivered
+   * asynchronously.
+   *
+   * [deliverChanges] can be called to force synchronousdelivery.

[Siggi Cherem (dart-lang), 2013/07/22 21:47:22]

add space to split words: synchronous delivery

[Jennifer Messerly, 2013/07/22 22:38:39]

Done.

+   */
+  Stream<List<ChangeRecord>> get changes;
+
+  /**
+   * Synchronously deliver pending [changes]. Returns true if any records were
+   * delivered, otherwise false.
+   */
+  // 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
+  //
+  // The rationale for that, and for async delivery in general, is the principal
+  // that you shouldn't run code (observers) when it doesn't expect to be run.
+  // If you do that, you risk violating invariants that the code assumes.
+  //
+  // For this reason, we need to match the ES Harmony version. The way we can do
+  // this in Dart is to add a method on StreamSubscription (possibly by
+  // subclassing Stream* types) that immediately delivers records for only
+  // that subscription. Alternatively, we could consider using something other
+  // than Stream to deliver the multicast change records, and provide an
+  // Observable->Stream adapter.
+  bool deliverChanges();
+
+  /**
+   * Performs dirty checking of objects that inherit from [ObservableMixin].
+   * This scans all observed objects using mirrors and determines if any fields
+   * have changed. If they have, it delivers the changes for the object.
+   */
+  static void dirtyCheck() => dirtyCheckObservables();
+}
+
+/**
+ * Base class implementing [Observable].
+ *
+ * When a field, property, or indexable item is changed, the change record
+ * will be sent to [changes].
+ */
+typedef ObservableBase = Object with ObservableMixin;
+
+/**
+ * Mixin for implementing [Observable] objects.
+ *
+ * When a field, property, or indexable item is changed, the change record
+ * will be sent to [changes].
+ */
+abstract class ObservableMixin implements Observable {
+  StreamController _changes;
+  InstanceMirror _mirror;
+
+  Map<Symbol, Object> _values;
+
+  Stream<List<ChangeRecord>> get changes {
+    if (_changes == null) {
+      _changes = new StreamController.broadcast(sync: true,
+          onListen: _observed, onCancel: _unobserved);
+    }
+    return _changes.stream;
+  }
+
+  /**
+   * True if this object has any observers, and should call
+   * [notifyPropertyChange] for changes.
+   */
+  bool get hasObservers => _changes != null && _changes.hasListener;
+
+  void _observed() {
+    // Register this object for dirty checking purposes.
+    registerObservable(this);
+
+    var mirror = reflect(this);
+    var values = new Map<Symbol, Object>();
+
+    // TODO(jmesserly): this should consider the superclass. Unfortunately
+    // that is not possible right now because of:
+    // http://code.google.com/p/dart/issues/detail?id=9434
+    for (var field in mirror.type.variables.values) {
+      if (field.isFinal || field.isStatic || field.isPrivate) continue;
+
+      for (var meta in field.metadata) {
+        if (identical(observable, meta.reflectee)) {
+          var name = field.simpleName;
+          try {
+            values[name] = mirror.getField(name).reflectee;
+          } catch (e, s) {
+            new Completer().completeError(e, s);

[Siggi Cherem (dart-lang), 2013/07/22 21:47:22]

is the idea that this exception gets attached to the current zone, but keep
going until this loop finishes?

[Jennifer Messerly, 2013/07/22 22:38:39]

yeah, essentially if we end up running user code (e.g. a getter) it shouldn't
cause us to get in an inconsistent state.

BUT -- since we won't reach this spot with getters -- I'm not sure it's possible
for this to fail. I'll removed the try+catch and add a comment that we don't
expect this to execute user code.

+          }
+          break;
+        }
+      }
+    }
+
+    _mirror = mirror;
+    _values = values;
+  }
+
+  /** Release data associated with observation. */
+  void _unobserved() {
+    // Note: we don't need to explicitly unregister from the dirty check list.
+    // This will happen automatically at the next call to dirtyCheck.
+    if (_values != null) {
+      _mirror = null;
+      _values = null;
+    }
+  }
+
+  bool deliverChanges() {
+    if (_values == null || !hasObservers) return false;
+
+    List changes = null;
+    _values.forEach((name, oldValue) {
+      var newValue;
+      try {
+        newValue = _mirror.getField(name).reflectee;
+      } catch (e, s) {
+        new Completer().completeError(e, s);
+        return;
+      }
+      if (!identical(oldValue, newValue)) {
+        if (changes == null) changes = <PropertyChangeRecord>[];
+        changes.add(new PropertyChangeRecord(name));
+        _values[name] = newValue;
+      }
+    });
+
+    if (changes == null) return false;
+
+    // TODO(jmesserly): make "changes" immutable
+    _changes.add(changes);
+    return true;
+  }
+}
+
+/**
+ * The type of the `@observable` annotation.
+ *
+ * Library private because you should be able to use the [observable] field
+ * to get the one and only instance. We could make it public though, if anyone
+ * needs it for some reason.
+ */
+class _ObservableAnnotation {
+  const _ObservableAnnotation();
+}