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How to fix the error “async call in a function that does not support concurrency”

Paul Hudson    @twostraws   

Updated for Xcode 13.2

This error occurs when you’ve tried to call an async function from a synchronous function, which is not allowed in Swift – asynchronous functions must be able to suspend themselves and their callers, and synchronous functions simply don’t know how to do that.

If your asynchronous work needs to be waited for, you don’t have much of a choice but to mark your current code as also being async so that you can use await as normal. However, sometimes this can result in a bit of an “async infection” – you mark one function as being async, which means its caller needs to be async too, as does its caller, and so on, until you’ve turned one error into 50.

In this situation, you can create a dedicated Task to solve the problem. We’ll be covering this API in more detail later on, but here’s how it would look in your code:

func doAsyncWork() async {
    print("Doing async work")
}

func doRegularWork() {
    Task {
        await doAsyncWork()
    }
}

doRegularWork()

Download this as an Xcode project

Tasks like this one are created and run immediately. We aren’t waiting for the task to complete, so we shouldn’t use await when creating it.

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