Inout parameter in async callback does not work as expected

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Sad to say, modifying inout parameter in async-callback is meaningless.

From the official document:

Parameters can provide default values to simplify function calls and can be passed as in-out parameters, which modify a passed variable once the function has completed its execution.

…

An in-out parameter has a value that is passed in to the function, is modified by the function, and is passed back out of the function to replace the original value.

Semantically, in-out parameter is not “call-by-reference”, but “call-by-copy-restore”.

In your case, counter is write-backed only when getOneUserApiData() returns, not in dataTaskWithRequest() callback.

Here is what happened in your code

  1. at getOneUserApiData() call, the value of counter 0 copied to c1
  2. the closure captures c1
  3. call dataTaskWithRequest()
  4. getOneUserApiData returns, and the value of – unmodified – c1 is write-backed to counter
  5. repeat 1-4 procedure for c2, c3, c4 …
  6. … fetching from the Internet …
  7. callback is called and c1 is incremented.
  8. callback is called and c2 is incremented.
  9. callback is called and c3 is incremented.
  10. callback is called and c4 is incremented.
  11. …

As a result counter is unmodified 🙁

Detailed explaination

Normally, in-out parameter is passed by reference, but it’s just a result of compiler optimization. When closure captures inout parameter, “pass-by-reference” is not safe, because the compiler cannot guarantee the lifetime of the original value. For example, consider the following code:

func foo() -> () -> Void {
    var i = 0
    return bar(&i)
}

func bar(inout x:Int) -> () -> Void {
    return {
        x++
        return
    }
}

let closure = foo()
closure()

In this code, var i is freed when foo() returns. If x is a reference to i, x++ causes access violation. To prevent such race condition, Swift adopts “call-by-copy-restore” strategy here.

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