Swift 3+ GCD's dispatch_once

DispatchOnce.swift

extension DispatchQueue {
    private static var _onceTracker = [String]()
    
    func once(file: String = #file, function: String = #function, line: Int = #line, _ block:() -> Void) {
        let token = file + ":" + function + ":" + String(line)
        once(token: token, 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.
    ///
    /// - Parameters:
    ///   - token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID
    ///   - block: Block to execute once
    func once(token: String, _ block:() -> Void) {
        objc_sync_enter(self)
        defer { objc_sync_exit(self) }
        
        if DispatchQueue._onceTracker.contains(token) {
            return
        }
        
        DispatchQueue._onceTracker.append(token)
        block()
    }
}