big update to observe, template_binding, polymer

pkg/observe/lib/src/observer_transform.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.
+
+library observe.src.observer_transform;
+
+import 'package:observe/observe.dart';
+
+/// ObserverTransform is used to dynamically transform observed value(s).
+///
+///    var obj = new ObservableBox(10);
+///    var observer = new PathObserver(obj, 'value');
+///    var transform = new ObserverTransform(observer,
+///        (x) => x * 2, setValue: (x) => x ~/ 2);
+///
+///    // Open returns the current value of 20.
+///    transform.open((newValue) => print('new: $newValue'));
+///
+///    obj.value = 20; // prints 'new: 40' async
+///    new Future(() {
+///      transform.value = 4; // obj.value will be 2
+///    });
+///
+/// ObserverTransform can also be used to reduce a set of observed values to a
+/// single value:
+///
+///    var obj = new ObservableMap.from({'a': 1, 'b': 2, 'c': 3});
+///    var observer = new CompoundObserver()
+///      ..addPath(obj, 'a')
+///      ..addPath(obj, 'b')
+///      ..addPath(obj, 'c');
+///
+///    var transform = new ObserverTransform(observer,
+///        (values) => values.fold(0, (x, y) => x + y));
+///
+///    // Open returns the current value of 6.
+///    transform.open((newValue) => print('new: $newValue'));
+///
+///    obj['a'] = 2;
+///    obj['c'] = 10; // will print 'new 14' asynchronously
+///
+class ObserverTransform extends Bindable {
+  Bindable _bindable;
+  Function _getTransformer, _setTransformer;
+  Function _notifyCallback;
+  var _value;
+
+  ObserverTransform(Bindable bindable, computeValue(value), {setValue(value)})
+      : _bindable = bindable,
+        _getTransformer = computeValue,
+        _setTransformer = setValue;
+
+  open(callback(value)) {
+    _notifyCallback = callback;
+    _value = _getTransformer(_bindable.open(_observedCallback));
+    return _value;
+  }
+
+  _observedCallback(newValue) {
+    final value = _getTransformer(newValue);
+    if (value == _value) return null;
+    _value = value;
+    return _notifyCallback(value);
+  }
+
+  void close() {
+    if (_bindable != null) _bindable.close();
+    _bindable = null;
+    _getTransformer = null;
+    _setTransformer = null;
+    _notifyCallback = null;
+    _value = null;
+  }
+
+  get value => _value = _getTransformer(_bindable.value);
+
+  set value(newValue) {
+    if (_setTransformer != null) {
+      newValue = _setTransformer(newValue);
+    }
+    _bindable.value = newValue;
+  }
+}