dispatch_once after the Swift 3 GCD API changes

Question

What is the new syntax for dispatch_once in Swift after the changes made in language version 3? The old version was as follows.

var token: dispa         


        

Answer 1

Edit

@Frizlab's answer - this solution is not guaranteed to be thread-safe. An alternative should be used if this is crucial

Simple solution is

lazy var dispatchOnce : Void  = { // or anyName I choose

    self.title = "Hello Lazy Guy"

    return
}()

used like

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    _ = dispatchOnce
}

Answer 2

You can declare a top-level variable function like this:

private var doOnce: ()->() = {
    /* do some work only once per instance */
    return {}
}()

then call this anywhere:

doOnce()

Answer 3

You can still use it if you add a bridging header:

typedef dispatch_once_t mxcl_dispatch_once_t;
void mxcl_dispatch_once(mxcl_dispatch_once_t *predicate, dispatch_block_t block);

Then in a .m somewhere:

void mxcl_dispatch_once(mxcl_dispatch_once_t *predicate, dispatch_block_t block) {
    dispatch_once(predicate, block);
}

You should now be able to use mxcl_dispatch_once from Swift.

Mostly you should use what Apple suggest instead, but I had some legitimate uses where I needed to dispatch_once with a single token in two functions and there is not covered by what Apple provide instead.