Notify Observable-Watchers programmatically in Dart

Asked By: Anonymous

Once again, a Dart/Polymer related question.

I wanted to use the JavaScript library, but since it’s not available in Dart I’ve written Wrapper classes which store a JsObject and delegate all calls from Dart to the corresponding JavaScript object. Basically it’s like a proxy.

Guess what, it works pretty great.

However, my observables don’t. To understand this, you have to take a look at the following structure of one of my “proxy”-classes.

class ParseObject extends Observable {

   JsObject _jsDelegate = new JsObject(context['Parse']['ParseObject']);

   void set(String key, dynamic value) {
      _jsDelegate.callMethod('set', [key, jsify(value)];

   dynamic get(String key) {
      return dartify(_jsDelegate.callMethod('get', [key]));


The HTML code of my Polymer Element looks like this:

<div>Name: {{project.get('name')}}</div>

Since the data binding is only evaluate in case the parameter of the method changed, it will never be updated and thus even though the name is changed, the old one will stay in place.

The solution I came up with is to store all the values the user is setting in the ParseObject#set(String, dynamic) method into a Map which is observable. This works but I think it’s quiete dirty since I have to make sure that both Maps, the one in Dart and the one in the ParseObject’s JavaScript representation equal.

Thus I am looking for a better solution and I think of some kind of method to tell Polymer to reevaluate it’s data bindings.

Does such a method exist or are there any other possibilities to address this problem?


Answered By: Anonymous

Extending observable by itself does nothing yet.
You need to annotate the getters with @observable (and if you are not using Polymer, you also need to add the observable transformer to pubspec.yaml). You can’t make functions observable (this works in Polymer elements but not in Observable model classes. For more details about observable see for example Implement an Observer pattern in Dart or Dart: @observable of getters

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