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

Expanding on Tod Cunningham's answer above, I've added another method which makes the token automatically from file, function, and line.

public extension DispatchQueue {
    private static var _onceTracker = [String]()

    public class func once(file: String = #file,
                           function: String = #function,
                           line: Int = #line,
                           block: () -> Void) {
        let token = "\(file):\(function):\(line)"
        once(token: token, block: block)
    }

    /**
     Executes a block of code, associated with a unique token, only once.  The code is thread safe and will
     only execute the code once even in the presence of multithreaded calls.

     - parameter token: A unique reverse DNS style name such as com.vectorform. or a GUID
     - parameter block: Block to execute once
     */
    public class func once(token: String,
                           block: () -> Void) {
        objc_sync_enter(self)
        defer { objc_sync_exit(self) }

        guard !_onceTracker.contains(token) else { return }

        _onceTracker.append(token)
        block()
    }
}

So it can be simpler to call:

DispatchQueue.once {
    setupUI()
}

and you can still specify a token if you wish:

DispatchQueue.once(token: "com.hostname.project") {
    setupUI()
}

I suppose you could get a collision if you have the same file in two modules. Too bad there isn't #module