Asynchronous virtual properties cause extra recomputes

[dylanrtt]

In the following example, I create an async virtual foo property, and a bar property that depends on it. If you bind to bar, it computes once when foo is undefined, and then recomputes twice after foo is set (3 total times). It should only recompute/log one time after foo is set (2 total times).

var VM = can.Map.extend({
  define: {
    foo: {
      get(lastVal, setVal) {
        console.log('getting foo');
        setTimeout(() => {
          console.log('setting foo');
          setVal(5);
        }, 10);
      }
    },
    bar: {
      get() {
        var foo = this.attr('foo');
        if (foo) {
          console.log(`getting bar... foo = ${foo}`);
          return foo * 2;
        }
      }
    }
  }
});

Using stache binding:
http://jsbin.com/dagozulelu/edit?html,js,console,output

If it matters, in my use case, bar is also an asynchronous virtual property that makes a network request once foo is set.

As a workaround, I am setting foo in my component's inserted event for now.

[justinbmeyer]

Certainly a bug.

Seems like there are 2 problems. First, when setVal is called, it's not dispatching in a batch.

But the more important problem is likely when this.attr('foo') is being called ... it's actually setting up a binding on two things:

1. the "foo" property, which is correct.
2. the internal compute of the "foo" property, which is incorrect. This is happening here: https://github.com/bitovi/canjs/blob/minor/map/map.js#L259

The solution is probably to change:

https://github.com/bitovi/canjs/blob/minor/map/map.js#L259

To something like:

can.__notObserve(function(){
  computedAttr.compute();
})

But, I would avoid re-creating the function each time and have a helper like:

var readButDontObserveCompute = can.__notObserve(function(compute){
  return compute()
})

And then call:

return readButDontObserveCompute(computedAttr.compute);

[wclr]

I experience I think a like problems with async define.get, I tried solution above, it doesn't seem to work.

[justinbmeyer]

If you can submit a test, I'll get this in 2.3.8 asap.

Sent from my iPhone

On Dec 26, 2015, at 9:20 AM, Alex notifications@github.com wrote:

I experience I think a like problems with computes, I tried solution above, it doesn't seem to work.

—
Reply to this email directly or view it on GitHub.