Discuss
  • Object.observe and deliverChangeRecords

Object.observe and deliverChangeRecords

# Jeremy Darling (11 years ago)

I maintain a Shim of Object.observe ( jdarling/Object.observe) that has been gaining in popularity since Chrome implemented the functionality in their public release. It had some usage before but it has really started picking up since.

That being beside the point, someone asked us about implementing the update to deliverChangeRecords ( jdarling/Object.observe#13). It seems that at some time deliverChangeRecords moved from the ObjectNotifier to Object itself.

Per the spec at harmony:observe_public_api

Object.deliverChangeRecords

A new function Object.deliverChangeRecords(callback) is added, which behaves as follows:

  1. If IsCallable(callback) is not true, throw a TypeError exception.
  2. Call [[DeliverChangeRecords]] with arguments: callback repeatedly until it returns false (no records were pending for delivery and callback no invoked).
  3. Return.

This seems very grey and not well defined at all. One would assume that per the above the following code would function calling changeHandler2 for all change records (and maybe, its unclear from the above, changeHandler1 as well).

var obj = {}; var changeHandler1 = function(changes) { console.log('1)', changes); };

var changeHandler2 = function(changes) { console.log('2)', changes); };

Object.observe(obj, changeHandler); obj.test = 5;

while(Object.deliverChangeRecords(changeHandler2)){ console.log('got a change') }

So we went to Chrome 36 and looked at how it handled the above code. To our surprise changeHandler1 got called with the change but changeHandler2 did not get called, also 'got a change' was never logged to the console.

This lead us to assume that the handler must be one that was already registered with Object.observe. So we tried the following code instead:

var obj = {}; var changeHandler = function(changes) { console.log('1)', changes); };

Object.observe(obj, changeHandler); obj.test = 5;

while(Object.deliverChangeRecords(changeHandler)){ console.log('got a change') }

When running the above changeHandler seems to be called via the normal Object.observe pipe and still 'got a change' is not recorded in the console. Basically making Object.deliverChangeRecords appear to be a noop that actually ignores the callback that has been passed into it.

So, after all the above, here is my question; What exactly is Object.deliverChangeRecords supposed to do? As a follow on to that, can someone post a test scenario that actually covers its proposed actions so we can implement it?

Seems to me, to add it in the way Chrome as so far we could just noop it and go on, but that just doesn't feel right in the grand scheme of things.

Thanks,

# Erik Arvidsson (11 years ago)

I think you are confusing the internal spec function from the API function deliverChangeRecords.

Object.deliverChangeRecords always returns undefined. It is the internal [[DeliverChangeRecords]] that gets called until the queue of change records is exhausted.

The point of Object.deliverChangeRecords is to get a synchronous delivery to the function that is listening to the mutations.

jsbin.com/qazipicipume/1/edit?js,console

HTH

# Jeremy Darling (11 years ago)

Reading your explanation and rereading the change notes at harmony:observe (need to learn to look there more) I see how this change came to be.

Time for a minor hack to get it in and then some major rewriting to support it properly along with other changes like the Synthetic Change Records.

Thanks for the clearup,