dispatch_once after the Swift 3 GCD API changes

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.<name> 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

From the doc:

Dispatch
The free function dispatch_once is no longer available in Swift. In Swift, you can use lazily initialized globals or static properties and get the same thread-safety and called-once guarantees as dispatch_once provided. Example:

let myGlobal: () = { … global contains initialization in a call to a closure … }()
_ = myGlobal  // using myGlobal will invoke the initialization code only the first time it is used.

While using lazy initialized globals can make sense for some one time initialization, it doesn't make sense for other types. It makes a lot of sense to use lazy initialized globals for things like singletons, it doesn't make a lot of sense for things like guarding a swizzle setup.

Here is a Swift 3 style implementation of dispatch_once:

public extension DispatchQueue {

    private static var _onceTracker = [String]()

    /**
     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.<name> or a GUID
     - parameter block: Block to execute once
     */
    public class func once(token: String, block:@noescape(Void)->Void) {
        objc_sync_enter(self); defer { objc_sync_exit(self) }

        if _onceTracker.contains(token) {
            return
        }

        _onceTracker.append(token)
        block()
    }
}

Here is an example usage:

DispatchQueue.once(token: "com.vectorform.test") {
    print( "Do This Once!" )
}

or using a UUID

private let _onceToken = NSUUID().uuidString

DispatchQueue.once(token: _onceToken) {
    print( "Do This Once!" )
}

As we are currently in a time of transition from swift 2 to 3, here is an example swift 2 implementation:

public class Dispatch
{
    private static var _onceTokenTracker = [String]()

    /**
     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.<name> or a GUID
     - parameter block: Block to execute once
     */
    public class func once(token token: String, @noescape block:dispatch_block_t) {
        objc_sync_enter(self); defer { objc_sync_exit(self) }

        if _onceTokenTracker.contains(token) {
            return
        }

        _onceTokenTracker.append(token)
        block()
    }

}