PROPOSAL: Allow retention of bindings on a read if only values directly on the observable are read

[justinbmeyer]

I'd like to retain "active" behavior on getters and async getters that only read values directly on their observable. For example:

const SaltShaker = DefineMap.extend({
	saltCount: {
		value({listenTo, resolve}){
			var saltCount = resolve(0);
			listenTo("fill", ()=>{
				resolve(saltCount = 2);
			});
			listenTo("shake", ()=>{
				resolve(--saltCount);
			});
		}
	},
	fill: function(){
		this.dispatch("fill");
	},
	shake: function(){
		let hadSalt = this.saltCount;
		this.dispatch("shake");
		return hadSalt ? "salt" : null;
	},
	get empty() {
		return ! this.saltCount;
	}
});

As saltCount is only listening on an instance of saltShaker, we can leave it bound once someone reads this.saltCount.

The following should work:

const saltShaker = new SaltShaker();

// when read, we notice that the observation is only listening on `saltShaker`:
saltShaker.saltCount;

// Now that saltCount is active, everything works:
saltShaker.fill();  

console.log( saltShaker.shake() ) //-> "salt"
console.log( saltShaker.shake() ) //-> "salt"  
console.log( saltShaker.shake() ) //-> null   

console.log( saltShaker.empty )   //-> true

I think this would be REALLY nice for getters and async getters:

var VM = DefineMap.extend({
  todoId: "number",
  get todoPromise(){
    return Todo.get({id: this.todoId})
  },
  todo: {
    get(lastSet, resolve){
       todoPromise.then(resolve);
    }
  }
});

var vm = new VM({todoId: 5});

vm.todo //-> undefined
setTimeout(function(){
  vm.todo //-> {todo}
},10000)

Obviously, there would still be corner cases like in saltShaker above. But overall, I think these sorts of behaviors would act more like people suspect.

Thoughts?