How to use async/await in synchronous Swift code with tasks

The async/await feature introduced in Swift 5.5 allows us to write asynchronous code in a shorter and safer way. For a recap on async/await, check out this quick guide on async/await in Swift.

In some situations, we need to call async/await methods or properties from synchronous context. How can we do that? Let's look at an example.

The following method uses the UNUserNotificationCenter's async/await methods to ask for authorization to send notifications, then fetches the settings and returns them.

func requestNotificationAuthorization() async throws -> UNNotificationSettings {
    let notificationCenter = UNUserNotificationCenter.current()
    try await notificationCenter.requestAuthorization(options: [.alert, .sound])
    let settings = await notificationCenter.notificationSettings()
    return settings
}

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We would like to call this method from our AppDelegate's method applicationDidFinishLaunching(_ application: UIApplication).

Let's try:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {

    let settings = try await requestNotificationAuthorization()
    return true
}

We get the compiler error async call in a function that does not support concurrency. Since the delegate method applicationDidFinishLaunching is not marked with async, we cannot just await a method.

Xcode suggests to add async to the function to resolve the error, but since it's not our function, we cannot just change it.

That's where tasks come in. A task is represented by the Task struct in Swift. When using a Task, we provide a closure that can contain synchronous or asynchronous code to perform.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {

    Task {
        do {
            let settings = try await requestNotificationAuthorization()
            // Handle settings
        } catch {
            // Handle error
        }
    }
    return true
}

We don’t need to manually start the task, it starts running immediately after creation. We also don't need to keep a reference to it - only if we need more control over the task like the possibility to cancel.

And that's basically it. Tasks provide us with an easy way to bridge async/await with other Swift code.

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