Swift-3: closure with escaping and non-escaping behaviour together

Question

I have encountered a situation when a closure may be called inside the function f1 (in which the closure is passed to) or it may be passed to some other function f2 .

So now I wonder how should I define escaping behaviour of this closure. I mean should I put @escaping or not?

Sample function:

func f1(_ completionHandler: ()->()){
    if someFlag == true{
        completionHandler()
        return
    }
    f2(completionHandler)
}

func f2(_ completionHandler: ()->()){
    // some other magic
}

Sorry if there are some syntactical errors, (typed the methods here), my question is, what should be type attribute of completionHandler for function f1 ?

Answer 1

The rule for when you need @escaping is simple – if a closure function argument can escape the lifetime of the function call, it needs to be marked as @escaping (the compiler simply won't let you compile it otherwise).

In your example code, completionHandler is not marked as @escaping in f2 – therefore it cannot escape the lifetime of f2 . Therefore it cannot possibly escape the lifetime of f1 either, thus you don't need to mark f1 's completionHandler as @escaping .

If however, f2 's completionHandler could escape the lifetime of f2 , then you would have to mark both f2 and f1 's parameters as @escaping as it could escape the lifetime of both calls.