Swift 3 :Closure use of non-escaping parameter may allow it to escape

Question

I have the following function where I have completion handler but I\'m getting this error:

Closure use of non-escaping parameter may allow it to escape
         


        

Answer 1

@escaping is infectious to all calling methods, and the compiler determines when you must include it.

Consider this example (which compiles):

dispatchSometime( { print("Oh yeah") })

func dispatchSometime(_ block: ()->()) {
    dispatchNow(block)
}

func dispatchNow(_ block: ()->()) {
    block()
}

This modified example, however, produces two errors of type non-escaping parameter may allow it to escape:

dispatchSometime( { print("Oh yeah") })

func dispatchSometime(_ block: ()->()) {
    dispatchLater(block)
}

func dispatchLater(_ block: ()->()) {
    DispatchQueue.main.async(execute: block)
}

The dispatch on main means the dispatchLater method needs @escaping, and once you've added that, the dispatchSometime method also requires @escaping for the example to compile.

dispatchSometime( { print("Oh yeah") })

func dispatchSometime(_ block: @escaping ()->()) {
    dispatchLater(block)
}

func dispatchLater(_ block: @escaping ()->()) {
    DispatchQueue.main.async(execute: block)
}

However, the take away is just:

• Keep adding @escaping up the call chain until the compiler stops complaining.
• The keyword doesn't change anything: it's a warning which says, essentially, "be careful to use weak with captured variables as they may be retained along with the block itself."

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Implications

The really fun case with this is where you have to adjust several methods to include the @escaping keyword, which gets the compiler to stop complaining. However, if those methods are actually conforming to a protocol, that protocol's methods must also get the @escaping keyword, which also infects all other protocol conformants. Fun!