Detached Tasks in Swift explained with code examples

Detached tasks allow you to create a new top-level task and disconnect from the current structured concurrency context. You could argue that using them results in unstructured concurrency since you’re disconnecting potentially relevant tasks.

While it sounds terrible to disconnect from structured concurrency, there are still examples of use cases in which you can benefit from detached tasks. However, it’s essential to be aware of the consequences to ensure you know what you’re doing. Before reading this article, I encourage you to read my articles on tasks and task groups.

What is a detached task?

A detached task runs a given operation asynchronously as part of a new top-level task.

Task.detached(priority: .background) {
    // Runs asynchronously
}

The code inside the closure will be executed asynchronously from the parent context.

The following code demonstrates this concept by using an async method to print out a value:

await asyncPrint("Operation one")
Task.detached(priority: .background) {
    // Runs asynchronously
    await self.asyncPrint("Operation two")
}
await asyncPrint("Operation three")

func asyncPrint(_ string: String) async {
    print(string)
}

// Prints:
// Operation one
// Operation three
// Operation two

In other words, by using a detached task, we stepped away from structured concurrency, and we’re no longer in control of the execution order.

Risks of using detached tasks

Tasks that run detached will create a new context to operate in. They won’t inherit the parent task’s priority and the task-local storage, and they won’t cancel if the parent task gets cancelled:

let outerTask = Task {
    /// This one will cancel.
    await longRunningAsyncOperation()

    /// This detached task won't cancel.
    Task.detached(priority: .background) {
        /// And, therefore, this task won't cancel either.
        await self.longRunningAsyncOperation()
    }
}
outerTask.cancel()

If you want the detached task to cancel seamlessly, you must hold a reference and cancel it manually. On top of that, they are not automatically canceled as soon as you release your reference. You would no longer have a way to cancel the task yourself while the task continues independently.

Lastly, since you’re executing code asynchronously, you’ll have to use ‘self’ to make capture semantics explicit explicitly:

Detached tasks require you to use explicit 'self'. — Detached tasks require you to use explicit ‘self’

Another indication of a risk that comes with disconnecting from the parent’s local storage is that you’re more likely to run into retain cycles.

When to use a detached task

Detached tasks should be your last resort. In many cases, you’ll be able to run tasks in parallel using task groups instead and benefit from parent-child relationships. The latter will allow you to cancel the parent task and all related child tasks automatically.

However, in some cases, you have operations that can run independently, don’t require a connection with the parent context, and are acceptable to succeed if the parent operation cancels. You don’t want to await the results or block the parent actor from executing other tasks. An example could be cleaning up a directory:

Task.detached(priority: .background) {
    await DirectoryCleaner.cleanup()
}

In this example, we don’t reference any local references using self. The cleanup code runs independently, can continue while the parent context cancels, and executes using a background priority.

Continuing your journey into Swift Concurrency

There are many more concurrency topics for you to explore, so why don’t you continue your journey?

Conclusion

It can be tempting to use detached tasks if you want to execute code asynchronously, but they should be your last resort. You can often solve the same using regular child tasks or task groups. It’s essential to be aware of the consequences when you do decide to use a detached task.

If you like to improve your Swift knowledge, even more, check out the Swift category page. Feel free to contact me or tweet to me on Twitter if you have any additional tips or feedback.

Thanks!

SwiftLee

A weekly blog about Swift, iOS and Xcode Tips and Tricks