delay() 的实现到处都是，下面是一个 Swift 3 版本的，适用于macOS：

public typealias Task = (_ cancel : Bool) -> Void

extension CommonUtil {
    
    public static func delay(_ after : TimeInterval, task : @escaping ()->()) ->  Task? {
        return delay(after.dispatchTimeInterval, task: task)
    }
    
    /** 异步延迟调用一个闭包. Samples:
     * ```swift
     * _ = CommonUtil.delay(.milliseconds(200), task: {
     *     Swift.print("delay 200ms.")
     * })
     * _ = CommonUtil.delay(.microseconds(1_000_000), task: {
     *     Swift.print("delay 1s.")
     * })
     * _ = CommonUtil.delay(.nanoseconds(1_500_000_000), task: {
     *     Swift.print("delay 1.5s.")
     * })
     * ```
     */
    public static func delay(_ after : DispatchTimeInterval, task : @escaping ()->()) ->  Task? {
        
        func dispatch_later(block:@escaping ()->()) {
            DispatchQueue.main.asyncAfter(deadline: .now() + after) {
                block()
            }
        }
        
        let closure = task
        var result: Task? = nil
        
        let delayedClosure: Task? = {
            cancel in
            if (cancel == false) {
                DispatchQueue.main.async() {
                    closure()
                }
            }
            //closure = nil
            result = nil as Task?
        }
        
        result = delayedClosure
        
        dispatch_later {
            if let delayedClosure = result {
                delayedClosure(false)
            }
        }
        
        return result;
    }
    
    public static func cancel(task:Task?) {
        task?(true)
    }
}