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=

[Siggi Cherem (dart-lang), 2014/02/03 22:52:48]

remove "="

[Jennifer Messerly, 2014/02/04 00:33:06]

Done.

+/// 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 _getValue, _setValue;
+  Function _notifyCallback;
+  var _value;
+
+  ObserverTransform(Bindable bindable, computeValue(value), {setValue(value)})
+      : _bindable = bindable, _getValue = computeValue, _setValue = setValue;
+
+  open(callback(value)) {
+    _notifyCallback = callback;
+    _value = _getValue(_bindable.open(_observedCallback));
+    return _value;
+  }
+
+  _observedCallback(value) {
+    value = _getValue(value);

[Siggi Cherem (dart-lang), 2014/02/03 22:52:48]

Consider introducing a new variable here:

var transformedValue = _getValue(value); // or newValue/updatedValue/etc

(it's a little confusing here that we are reusing the parameter variable for
something else.)

[Jennifer Messerly, 2014/02/04 00:33:06]

sure

+    if (value == _value) return null;
+    _value = value;
+    return _notifyCallback(value);
+  }
+
+  void close() {
+    if (_bindable != null) _bindable.close();
+    _bindable = null;
+    _getValue = null;
+    _setValue = null;
+    _notifyCallback = null;
+    _value = null;
+  }
+
+  get value {

[Siggi Cherem (dart-lang), 2014/02/03 22:52:48]

not sure if I like this suggestion, but here it goes:

   get value => _value = _getValue(_bindable.value);

only 4 occurrences of the word value in a single line :)

[Jennifer Messerly, 2014/02/04 00:33:06]

Done.

+    _value = _getValue(_bindable.value);
+    return _value;
+  }
+
+  set value(newValue) {
+    if (_setValue != null) {
+      newValue = _setValue(newValue);
+    }

[Siggi Cherem (dart-lang), 2014/02/03 22:52:48]

mmm, I was expecting that if _setValue == null, then no assignment would happen
below

[Jennifer Messerly, 2014/02/04 00:33:06]

we still want to pass on the value, just we don't need to transform it.
I renamed stuff, maybe it helps :)

+    _bindable.value = newValue;
+  }
+}