implement dirty checking for @observable objects

pkg/observe/lib/src/change_record.dart

Index: pkg/observe/lib/src/change_record.dart
diff --git a/pkg/observe/lib/src/change_record.dart b/pkg/observe/lib/src/change_record.dart
new file mode 100644
index 0000000000000000000000000000000000000000..652e89e071fbc6b7667ce9a740dc3c8c3bff3348
--- /dev/null
+++ b/pkg/observe/lib/src/change_record.dart
@@ -0,0 +1,57 @@
+// 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;
+
+/** 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>';
+}