Remove need for once()

[asolove]

tl;dr; autoCancelPrevious should not allow its observer to be replaced after it is cancelled. There is a problem upstream with the propertyObserver that allows this to happen, which I can't diagnose. We may want to hold off this merge and fix there.

I spent some time tracing down sources of multiple calls of functions passed to once(). I discovered that all of the ones caused by the current test suite boil down to replacing parent objects that have property observers on them.

var test = Bindings.defineBindings({
  a: []
}, {
  "b": {"<-": "a.0"}
}
test.a[0] = 2; // no problem
test.a = [] // kaboom!

The property change observer on ".0" has its cancel callback called, and then later in the same call-stack its replace callback is called. Disallowing this in autoCancelPrevious removed all cases of functions passed to once being called more than once.

[asolove]

This is the only substantive bit.

[kriskowal]

Ah hah!

[kriskowal]

The test.a[0] = 2 line isn’t going to trigger a property change dispatch unless it’s changed to test.a.set(0, 2). I’ll check whether this is germane before merging.

[kriskowal]

Consider this a send-off, @asolove. I followed up and also removed all use of autoCancelPrevious. I used memwatch and wrote out a scenario to convince myself that the previous observers were canceled without the extra decorator. One side effect is that observe functions do not necessarily return anything, but vigilance in all cases where a cancel function might or might not exist solves the problem. Another side effect is that the equality binder specs had to change, but near as I can tell, the new behavior is even more logical, though for my life I have no idea why it changed.

If the Montage tree controller breaks, I would look there.

[asolove]

🍷 What a sendoff!