AnyTask

A small Swift Package introducing an AnyTask type, providing type-erasure for Swift Tasks, which makes it very easy to store in a collection since the generics are removed.

Additionally, an AnyTask will cancel itself when it is being destroyed, relieving you of the need to manually cancel any pending tasks when a collection of AnyTasks is destroyed. You can also opt out of this behaviour if you prefer.

Finally, an AnyTask can also be configured to fail an assertion in debug mode if a cancellation attempt occurs when the task is already cancelled.

Why is this useful

It’s easy to forget to cancel a Swift Task, which will continue to run even if nothing keeps a reference to it. Unless you are creating a “fire and forget” task, you probably want to make sure that it is cancelled when it is no longer needed (i.e. when an owning class is destroyed).

This means that you have to manually store a Task somewhere so that it can be cancelled at a later date, except it’s quite difficult to store different types of Tasks in a collection because they can be specialized with different types:

let taskA = Task<Bool, Never> { // code to return a Bool }
let taskB = Task<Void, Error> { // code that doesn't return a value but can throw }
let tasks = [taskA, taskB] // This code won't compile

This means you need a different collection for every posible specialized Task type, or just a different property for every Task type, and then make sure that you remember to cancel them all properly in deinit.

AnyTask type-erases Tasks, which means that you can just store every task you create in a single collection (or set) and it will also automatically cancel all the tasks in the collection when the collection is destroyed, because AnyTask cancels itself when it is deinitted:

var tasks: [AnyTask] = []
Task<Bool, Never> {}.store(in: &tasks)
Task<Void, Error> {}.store(in: &tasks)

// When `tasks` is destroyed, the `AnyTask`s will automatically cancel the underlying type-erased `Task`s. 

Usage

Erasing a Task

You can create an AnyTask manually from a Swift Task but it is simpler to use a convenience function on Task to store in a collection of AnyTasks:

var tasks: [AnyTask] = []
Task {
    // Async task code
}.store(in: &tasks)

You can also do the same thing with a set of AnyTasks:

var tasks: Set<AnyTask> = []
Task {
    // Async task code
}.store(in: &tasks)

If you want to you can also make a call to erase a task explicitly:

let task: AnyTask = Task {
    // Async task code
}.erased()

Or you can also create an AnyTask manually from a Task:

let task = Task {
    // Async task code
}
let anyTask = AnyTask(task)

Setting options

Wherever you are able to erase a Task to AnyTask you are able to override the default options that the AnyTask is configured with.

Task {}.store(in: &tasks, options: [.assertOnOverCancellation])

Task {}.erased(options: [])

AnyTask(Task {}, options: [])

You can see a full list of available Options in code.

Cancelling a task

You can still manually check if an AnyTask is cancelled by checking the isCancelled property:

var tasks: [AnyTask] = []
let task: AnyTask = Task {}.store(in: &tasks)
task.isCancelled

You can also explicitly cancel a task by calling cancel():

var tasks: [AnyTask] = []
let task: AnyTask = Task {}.store(in: &tasks)
task.cancel()

GitHub

View Github