AsyncDemo.swift

func doAsyncAction(completionHandler:(callback:AnyObject)->Void){
    
    let delay = Int64(NSEC_PER_MSEC*100)
    let afterTime = dispatch_time(DISPATCH_TIME_NOW, delay)
    let queue = dispatch_get_global_queue(0, 0)
    //等待100毫秒(0.1秒)後再執行
    dispatch_after(afterTime, queue) { () -> Void in
        completionHandler(callback: "Hello world")
    }
    
}

 /*以上先不用看，先假設有一個程序 doAsyncAction 會delay 100毫秒(0.1秒)後才會執行*/

print("1")
//執行doAsyncAction
doAsyncAction { (callback) -> Void in
    print("2")
    print("callback:\(callback)")
    print("3")
}
print("4")

 /* 印出來的順序還會是 1, 2, 3, 4嗎？*/