feat($q): Add optional canceler and promise.cancel method

[dbinit]

Add an optional argument to $q.defer to provide a canceling function.
This exposes a promise.cancel method that can be used to call the
canceling function and reject the promise.

See existing implementations in Dojo Toolkit and Promised-IO library
for reference.

This will make it easier to implement things like #1159.

[caiotoon]

Excellent! I believe this is a very important feature. With a optional canceler from a promise, we would be able to let all the logic regarding the canceling in the same place as the starting.

+1

[IgorMinar]

How is this going to work with promise chains, joins and forks? This is the hard part about this feature.

Let's say that you have this scenario:

var promiseA = $http.get('/foo');
promiseB1 = promiseA.then(doSomething);
promiseB2 = promiseA.then(doSomethingElse);
promiseB2.cancel();

what happens to promiseA and promiseB1?

[IgorMinar]

one of the guiding principles of promise paradigm is that the communication is one way from the resolver to the promise consumer. your cancel makes the communication bidirectional which introduces these problems that I illustrated above.

[dbinit]

Thanks for taking a look Igor.

I've added more tests and, as a result, cleaned up the implementation a bit.

In the scenario you mentioned (I've added a test), all derived promises would be canceled. In the case of something like $q.all(), it does not implement cancellation, so it would not be available on the resulting promise.

Dojo has a decent description.

[dbinit]

Closing this as it's unlikely to go through... and I think I prefer #2529 approach.

[akarelas]

pitty this was cancelled

[dbinit]

@akarelas, #2529 was merged instead.

[blabno]

Is http the only "promise" you are using?
What if my promise resolves on $window.setTimeout and it has nothing to do with $http? Or what if $http is wrapped with several layers of abstraction?