withObservableState API

a.js

function Compose() {
  this.initialState({
    characterCount: 0
  });
  
  this.after('initialize', function () {
    this.provideObservableResource('dockedCompose.state', this.state);
    this.attachChild(CharacterCount, this.$node);
  });
}

function CharacterCount() {
  this.initialState({
    characterCount: 0
  });
  
  this.after('initialize', function () {
    this.requestObservableResource('dockedCompose.state', {onlyKeys: ['characterCount']})
      .subscribe(this.mergeState);
    this.after('stateChanged', this.render);
  });
  this.render = function () {
    ...
  }
}

I like it. Presumably Compose has a withObservableState mixin?

To separate state and it's observable version, I think the line in compose should be:

this.provideObservableResource('dockedCompose.state', this.observableState);

Yeah, I initially only had observableState available via a resource, but there's no reason to have that limitation. observableState can live on the component and the component can expose it any way it wants. In this case through provideResource().

this.provideResource('dockedCompose.state', this.getObservableState);

getObservableState is a function that accepts some options, eg ({keys: ['characterCount', etc]}, and returns a customised instance of that components observableSate which means it can be called via requestResource() in the previous way.