Can swift exit out of the root closure?

Question

In Swift, if I'm inside of a closure, that is itself inside of another function, is there a way to exit out of the function itself?

Here's an example of what this might look like using closures from the GCDKit library.

func test() {
    GCDQueue.Default.async {
        print("Print me!")
        return //Is there a statement that does this?
    }.notify(.Main) {
        print("Never print me.")
    }
}

score 6 · Accepted Answer · answered May 29 '16 at 22:09

6

No there is not. Closures run in self-contained environments. For all you know, by the time that closure is executed, the thread on which test() was called is no longer executing the test() method.

answered May 29 '16 at 22:09

Adam H.

681
3
8

score 2 · Answer 2 · answered May 29 '16 at 22:32

Let's consider a simpler version that doesn't include any third-party libraries, extra queues, or other complexity. We'll just create a closure and immediately execute it.

func dothing(andPrint shouldPrint: Bool) {
    let closure = {
        guard shouldPrint else { return }
        print("I printed!")
    }

    closure()
    print("did it return?")
}

dothing(andPrint: false) // Prints "did it return?"

The return here exits the closure, not dothing. Since closure could be passed to some other function, or stored in a property and executed at some later time (possibly on a different queue as in your example), there's no way for the return to exit anything beyond itself. Consider if we were to refactor the creation of the closure into its own function:

func fetchClosure(andPrint shouldPrint: Bool) -> () -> Void {
    return {
        guard shouldPrint else { return }
        print("I printed!")
    }
}

func dothing(andPrint shouldPrint: Bool) {
    let closure = fetchClosure(andPrint: shouldPrint)

    closure()
    print("did it return?")
}

dothing(andPrint: false) // Prints "did it return?"

It shouldn't be surprising this has the same behavior (otherwise this wouldn't be a simple refactor). Now imagine how it would (or even could) work if return behaved any other way.

Your example is just a much more complicated version of the same thing. return exits the closure.

Can swift exit out of the root closure?

2 Answers2