Swift-DispatchQueue.once

// https://stackoverflow.com/a/39983813/235297
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 + ":" + String(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) }


        if _onceTracker.contains(token) {
            return
        }

        _onceTracker.append(token)
        block()
    }
}