Honey Honey - 5 months ago 13
Swift Question

How do closures capture values from previous calls?

typealias IntMaker = (Void)->Int

func makeCounter() ->IntMaker{
var n = 0 // Line A

func adder()->Integer{
n = n + 1
return n
}
return adder
}

let counter1 = makeCounter()

counter1() // returns 1
counter1() // returns 2
counter1() // returns 3


Isn't 'Line A' called each time we call
counter1()
? meaning that
var n = 0
should be called every time...

Why is that the counter returns different values? Shouldn't they always be returning '1' ?

Answer

You've called makeCounter() once. That creates your new closure, and assigns it to counter1. This closure closes over the mutable var n, and will remain captured as long as this closure exists.

Calling counter1() will execute it, but it retains the same captured n, and mutates it. This particular "adder" will ALWAYS capture this same n, so long as it exists..

To get the behavior you're suggesting, you need to make new closures which capture new instances of n:

let counter1 = makeCounter()

counter1() // returns 1
counter1() // returns 2
counter1() // returns 3

var counter2 = makeCounter()
counter2() // returns 1
counter2 = makeCounter()
counter2() // returns 1
counter2 = makeCounter()
counter2() // returns 1

Now both counter1 and counter2 each have their own separate instances of n.

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