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Question

I have read a lot of material on stack overflow and I just cannot figure this one out:

I have been this line of code from an online source for hours and I just don't know why the closure that is being passed into a function is escaping, here is the code:

func composeFunction(functionA: @escaping (Int) -> String, functionB:  @escaping (String) -> String) -> ((Int) -> String) { 

    return {

        number in 

        functionB(functionA(number))
    }
}

From apple's documentation, closures are escaping when: 1) Asynchronous operation that runs on a background thread 2) The closure is interactive with properties outside of it's scope (using self )

But I don't see those are happening, many help will be appreciated!

Thanks!

Answer 1

your func composeFunction returns a ((Int) -> (String)) and that is exactly a closure. Of course now this means functionA and functionB are going to be escaping because we don't know when and where this closure will be called. Moreover, this is because it needs to know if it should keep the reference to the objects passed in / being manipulated. For example, if all your closure parameters had (() -> Void) instead and you also returned (() -> Void) then it would not need to be escaping.

Answer 2

{
    number in 
    functionB(functionA(number))
}

is a closure. Returning it causes both functionB and functionA to escape, because it becomes unknown at what point this closure will be called or deallocated.

This is different from just return functionB(functionA(number)) , which causes the immediate invocation of both of those functions, and causes them to never escape the composeFunction context.