How to subscribe to change of an observable field

问题:

until lately I could use bindProperty like shown below or in this question, but that has changed with 0.8.0 and I don't know how to change my code to get the old behaviour (doSomething() gets called):

<polymer-element name="my-login" attributes="model">
  <template>
    <template if="{{"model.isLoggedIn}}">
      ...
    </template>
  </template>
  <script type= ... ></script>
</polymer-element>

.

@CustomTag("my-login")
class MyLogin extends PolymerElement with ObservableMixin {
  LoginModel model;

  @override
  inserted() {

  void doSomething() {
   ...
  }

logoutChangeSubscription = bindProperty(model, #isLoggedIn, () => doSomething());

  }
}

class Model extends Object with ObservableMixin {
  @observable bool isLoggedIn = false;
}

回答1:

With Polymer.dart 0.8 or greater, you can also use this convenience form:

isLoggedInChanged(oldValue) {
  doSomething();
}

Notice how you can create a method inside your PolymerElement that uses a name of yourFieldName*Changed

There's also onPropertyChange as defined here: http://api.dartlang.org/docs/bleeding_edge/observe.html#onPropertyChange

From the docs:

class MyModel extends ObservableBase {
  StreamSubscription _sub;
  MyOtherModel _otherModel;

  MyModel() {
    ...
    _sub = onPropertyChange(_otherModel, const Symbol('value'),
        () => notifyProperty(this, const Symbol('prop'));
  }

  String get prop => _otherModel.value;
  set prop(String value) { _otherModel.value = value; }
}

回答2:

Polymer.dart >= 1.0.0

@Property(observer: 'doSomething') bool isLoggedIn;

@reflectable
void doSomething(bool newValue, bool oldValue) => ...

or

@Observe('isLoggedIn') 
void doSomething(event, detail) => ...

Polymer.dart < 1.0.0

Ok, found it

new PathObserver(model, "isLoggedIn").changes.listen((e) => doSomething());

回答3:

It seems that the syntax has changed a little bit. The syntax of the solution proposed by Günter now seems to be:

new PathObserver(model, "isLoggedIn").open((e) => doSomething());