Task Cancellation

[saniul]

I was wondering, what is the intended approach to task cancellation?

BFTask only exposes methods for task consumption and continuation, while the cancellation is implemented through BFTaskCompletionSource (which actually just calls internal cancellation methods of BFTask).
There is no way to cancel a BFTask without having a reference to its completion source.

Does this mean that a task producer should keep a reference to all its long-running cancellable task completion sources and expose an interface for identifying and cancelling a specific task?

[darrinm]

I've been wondering the same thing. Thanks for asking.

[bklimt]

It's generally bad design to keep track of the BFTaskCompletionSource for cancellation. For one, that introduces too many opportunities for the source to have its result or error set when you didn't want them to. But a bigger concern is that as you compose tasks together, you will often have one concept of a "cancelable operation" that's composed of several tasks chained together. For example, querying for a new object online might have multiple async subtasks for retrying or writing the results to a local file or database. And some high-level tasks may even share subtasks. You don't want cancellation one of the high-level tasks to cancel all of them.

So a better model is to create a "cancellation token" at the top level, and pass that to each async function that you want to be part of the same "cancelable operation". Then, in your continuation blocks, you can check whether the cancellation token has been cancelled and bail out early by returning a [BFTask cancelledTask].

We are likely to add some concept like this to Bolts at some point in the future. In the meantime, you can use an NSOperation simply as an implementation of an object that is thread-safe and has cancel and isCancelled method. :-) For example:

- (void)doSomethingComplicatedAsync:(NSOperation *)cancellationToken {
    [[self doSomethingAsync:cancellationToken] continueWithBlock:^{
        if (cancellationToken.isCancelled) {
            return [BFTask cancelledTask];
        }
        // Do something that takes a while.
        return result;
    }];
}

// Somewhere else.
NSOperation *cancellationToken = [[NSOperation alloc] init];
[obj doSomethingComplicatedAsync:cancellationToken];

// When you get bored...
[cancellationToken cancel];

[saniul]

Cool, thanks for advice!

I guess It might be a good idea to propose the cancellation token approach in the README?

[bklimt]

Yeah, I think so. Feel free to open a pull request. ;-)

Or we will add it later.

[saniul]

Already on it.