How can I listen for changes to properties on objects inside a list, with polymer-dart?

Question

I have a list of objects (for example, people), and I dynamically add and remove from the list. I want to run a query across the list when a certain property changes on any item in the list.

For example, I want to know if any object in the list has its "signedAgreement" property changed. I don't want to manually attached listeners to each object, I just want to ask the list. How can I do this?

My code:

library my_element;

import 'package:polymer/polymer.dart';
import 'dart:html';
import 'models.dart';

@CustomTag("my-element")
class MyElement extends PolymerElement with ObservableMixin {
  final List people = toObservable([]); // observe adds/removes to the list
  final Person newPerson = new Person();

  // How can I know when to re-evaluate signedCount?

  int get signedCount => people.where((Person p) => p.signedAgreement).length;

  void save(Event e, var detail, Node target) {
    people.add(new Person.from(newPerson));
    newPerson.blank();
  }
}

And my model object looks like:

library models;

import 'package:polymer/polymer.dart';

class Person extends Object with ObservableMixin {
  @observable String name;
  @observable bool signedAgreement = false;

  Person();

  Person.from(Person other) {
    name = other.name;
    signedAgreement = other.signedAgreement;
  }

  blank() {
    name = '';
    signedAgreement = false;
  }
}

Answer 1

Enter: ListPathObserver!

Add this constructor:

  MyElement() {
    ListPathObserver observer = new ListPathObserver(people, 'signedAgreement');
    observer.changes.listen((_) => notifyProperty(this, const Symbol('signedCount')));
  }

Here, observer will fire when any person in people has its signedAgreement property changed.

Then, in the callback, we notify the observer system that it should go look at signedCount.