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

Static properties initialized by a closure are run lazily and at most once, so this prints only once, in spite of being called twice:

/*
run like:

    swift once.swift
    swift once.swift run

to see both cases
*/
class Once {
    static let run: Void = {
        print("Behold! \(__FUNCTION__) runs!")
        return ()
    }()
}

if Process.arguments.indexOf("run") != nil {
    let _ = Once.run
    let _ = Once.run
    print("Called twice, but only printed \"Behold\" once, as desired.")
} else {
    print("Note how it's run lazily, so you won't see the \"Behold\" text now.")
}

Example runs:

~/W/WhenDoesStaticDefaultRun> swift once.swift
Note how it's run lazily, so you won't see the "Behold" text now.
~/W/WhenDoesStaticDefaultRun> swift once.swift run
Behold! Once runs!
Called twice, but only printed "Behold" once, as desired.

Answer 2

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")
    }
}

Answer 3

I think the best approach is to just construct resources lazily as needed. Swift makes this easy.

There are several options. As already mentioned, you can initialize a static property within a type using a closure.

However, the simplest option is to define a global variable (or constant) and initialize it with a closure then reference that variable anywhere the initialization code is required to have happened once:

let resourceInit : Void = {
  print("doing once...")
  // do something once
}()

Another option is to wrap the type within a function so it reads better when calling. For example:

func doOnce() {
    struct Resource {
        static var resourceInit : Void = {
            print("doing something once...")
        }()
    }

    let _ = Resource.resourceInit
}

You can do variations on this as needed. For example, instead of using the type internal to the function, you can use a private global and internal or public function as needed.

However, I think the best approach is just to determine what resources you need to initialize and create them lazily as global or static properties.