CancellablePromiseKit

CancellablePromiseKit is an extension for PromiseKit. A Promise is an abstraction of an asynchonous operation that can succeed or fail. A CancellablePromise, provided by this library, extends this concept to represent tasks that can be cancelled/aborted.

Installation

CancellablePromiseKit is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod 'CancellablePromiseKit'

To run the unit tests, clone the repo, and run pod install from the Example directory first.

Making cancellable Promises

Creating a CancellablePromise is done similarly to creating a Promise:

func startTask() -> CancellablePromise<String> {
    let task = Task(…)
    return CancellablePromise<String> { resolver in
        task.completion = { (value, error) in
            resolver.resolve(value, error)
        }
        let cancel = {
            task.stop()
            print("Task was cancelled")
        }
        return cancel
    }
}
let runningTask = startTask()

This initializer is almost identical to the one of Promise. However, the block has to return a handler that will be executed when the CancellablePromise is cancelled. It has to perform the necessary steps to abort the underlying task.

Cancelling

A CancellablePromise has a cancel() function for stopping the task:

runningTask.cancel()

Calling cancel() will reject the promise with a CancellablePromiseError.cancelled. A catch handler in your promise chain will not be called unless you set the policy to .allErrors:

runningTask.catch(policy: .allErrors) { error in
    // Will be called with error being CancellablePromiseError.cancelled
}
runningTask.cancel()

race and when variants with autoCancel

The race function of PromiseKit allows you to wait until the first task of a list of tasks fulfills. You can use this task's result for further processing. The other tasks, however, continue executing, although their result will be ignored. If you're working with CancellablePromises, there is a special overloaded race(:, autoCancel:) function that offers you an additional parameter called autoCancel. If you set it to true, all other tasks in the list will be cancelled:

race([cancellablePromise1, cancellablePromise2, cancellablePromise3], autoCancel: true)

If cancellablePromise1 will fulfill, cancellablePromise2 and cancellablePromise3 will be cancelled automatically. So, if these represent large downloads for example, no further bandwidth will be wasted.

Similarly, there are when(resolved:, autoCancel:) and when(fulfilled:, autoCancel:) that cancel all other promises if one fails.

The race(:, autoCancel:), when(resolved:, autoCancel:) and when(fulfilled:, autoCancel:) return a CancellablePromise which itself can be cancelled. Cancelling that promise will cancel the passed promises only if autoCancel is true.

The autoCancel parameter is false by default so that leaving it out will produce the same behaviour as with the regular PromiseKit.

then

The overloaded then function allows you to chain CancellablePromises which creates another CancellablePromise:

let cancellableChain = cancellablePromise1.then {
    cancellablePromise2
}
cancellableChain.cancel()

Cancelling the chain will cancel all included pending promises, i.e. cancellablePromise1 and cancellablePromise2 in this example.

asPromise and asCancellable

If you want to use a Promise and a CancellablePromise in one expression (like when or race), you can convert between them. Every CancellablePromise provides asPromise(), and every Promise provides asCancellable(). Calling cancel() on the latter will cause a reject, but the underlying promise, that you called asCancellable() on, will continue beeing pending.

Other initializers of CancellablePromise

CancellablePromise is not a subclass of Promise, but a wrapper around it. You can create a new instance by passing a promise and a cancel block:

let existingPromise, existingResolver = Promise<String>.pending()
let cancelFunction = { existingResolver.reject(MyError()) }

let cancellablePromise = CancellablePromise<String>(using: existingPromise, cancel: cancelFunction)

In some cases, you're building your cancellable task using other promises. In that case, you can use the initializer that provides you with a cancelPromise. It is a Promise<Void> that nevers fulfills, but which will reject when cancel() is called. Putting it in a race with another promise allows you wait until that promise fulfills, unless the process is cancelled. The following example executes two tasks in parallel, followed by a third task. The whole process can be cancelled at any time:

let cancellablePromise = CancellablePromise<String>(wrapper: { cancelPromise in
    let task1: Promise<String> = Task()
    let task2: Promise<String> = Task()
    let tasks = when(fulfilled: task1, task2)
    return firstly {
        race(tasks.asVoid(), cancelPromise)
    }.then {
        let value1 = task1.value!
        let value2 = task2.value!
        let task3: Promise<String> = AnotherTask(value1, value2)
        return race(task3.asVoid(), cancelPromise).map {
            task3.value!
        }
    }
})

Calling cancellablePromise.cancel() in this example will cause cancelPromise to reject, which will cause all race calls and hereby the whole cancellablePromise to reject.

when variants for cancelPromise

As shown in the previous example, building a cancellable promise often involves waiting for another promise to finish, while also expecting a cancellation. Building this with race requires the promises to be converted to Promise<Void>, which makes getting the resolved value cumbersome. For that reason, there is the when(:, while:) overload.

Instead of:

let task: Promise<String> = ...
race(task.asVoid(), cancelPromise).then { 
    let value = tasks.value!
    // use value
}

you can write:

when(task, while: cancelPromise).then { value in
    // use value
}

The example above can be rewritten as:

let cancellablePromise = CancellablePromise<String>(wrapper: { cancelPromise in
    let task1: Promise<String> = Task()
    let task2: Promise<String> = Task()
    return firstly {
        let tasks = when(fulfilled: task1, task2)
        return when(tasks, while: cancelPromise)
    }.then { (value1, value2) in
        let task3: Promise<String> = AnotherTask(value1, value2)
        return when(task3, while: cancelPromise)
    }
})

In order to perform some actions when cancelPromise rejects, use catch:

let taskWithCancel = when(task, while: cancelPromise).then { value in
    // ...
}
taskWithCancel.catch(policy: .allErrors) { error in
    switch error {
    case CancellablePromiseError.cancelled:
        // do something to cancel the underlying task 
    default:
        // task failed
    }
}

TODO

• There should be overloads for done, get, catch etc. that return a CancellablePromise. At the moment, the implementations of Thenable take effect and return a regular Promise.
• There should be a firstly for CancellablePromise.
• There should be an asCancellable() on Guarantee that returns a CancellablePromise. There won't be a CancellableGuarantee because a task that offers to be cancelled cannot also claim to always fulfill.
• PromiseKit has many overloads for race and when that can receive arrays, varadic parameters, etc. CancellablePromiseKit could have those, too.
• There could be factory functions like pending, value, etc., like in PromiseKit

Author

Johannes Dörr, mail@johannesdoerr.de

License

CancellablePromiseKit is available under the MIT license. See the LICENSE file for more info.