grand central dispatch - How do I execute code once and only once in Swift?

Question

The answers I've seen so far (1, 2, 3) recommend using GCD's dispatch_once thus:

var token: dispatch_once_t = 0
func test() {
    dispatch_once(&token) {
        print("This is printed only on the first call to test()")
    }
    print("This is printed for each call to test()")
}
test()

Output:

This is printed only on the first call to test()
This is printed for each call to test()

But wait a minute. token is a variable, so I could easily do this:

var token: dispatch_once_t = 0
func test() {
    dispatch_once(&token) {
        print("This is printed only on the first call to test()")
    }
    print("This is printed for each call to test()")
}
test()

token = 0

test()

Output:

This is printed only on the first call to test()
This is printed for each call to test()
This is printed only on the first call to test()
This is printed for each call to test()

So dispatch_once is of no use if we I can change the value of token! And turning token into a constant is not straightforward as it needs to of type UnsafeMutablePointer<dispatch_once_t>.

So should we give up on dispatch_once in Swift? Is there a safer way to execute code just once?

Answer 1

A man went to the doctor, and said "Doctor, it hurts when I stamp on my foot". The doctor replied, "So stop doing it".

If you deliberately alter your dispatch token, then yes - you'll be able to execute the code twice. But if you work around the logic designed to prevent multiple execution in any way, you'll be able to do it. dispatch_once is still the best method to ensure code is only executed once, as it handles all the (very) complex corner cases around initialisation and race conditions that a simple boolean won't cover.

If you're worried that someone might accidentally reset the token, you can wrap it up in a method and make it as obvious as it can be what the consequences are. Something like the following will scope the token to the method, and prevent anyone from changing it without serious effort:

func willRunOnce() -> () {
    struct TokenContainer {
        static var token : dispatch_once_t = 0
    }

    dispatch_once(&TokenContainer.token) {
        print("This is printed only on the first call")
    }
}