原创Blog，转载请注明出处
本文阅读的过程中，如有概念不懂，请参照前专栏中之前的文章，如果还有疑惑，请留言。
这是我关于GCD专栏的地址
http://blog.csdn.net/column/details/swift-gcd.html
func dispatch_after(_ when: dispatch_time_t,
_ queue: dispatch_queue_t!,
_ block: dispatch_block_t!)参数when 过了多久执行的时间间隔 queue 提交到的队列 block 执行的任务
func hwcDelay(delay:Double, closure:()->()) {
dispatch_after(
dispatch_time(
DISPATCH_TIME_NOW,
Int64(delay * Double(NSEC_PER_SEC))
),
dispatch_get_main_queue(), closure)
}class ViewController: UIViewController{
func hwcDelay(delay:Double, closure:()->()) {
dispatch_after(
dispatch_time(
DISPATCH_TIME_NOW,
Int64(delay * Double(NSEC_PER_SEC))
),
dispatch_get_main_queue(), closure)
}
override func viewDidLoad(){
super.viewDidLoad()
hwcDelay(1.0){
var alertview = UIAlertView(title:"Dispatch_after",message:"Message",delegate:self,cancelButtonTitle:"OK")
alertview.show()
}
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
} func dispatch_apply(_ iterations: UInt,
_ queue: dispatch_queue_t!,
_ block: ((UInt) -> Void)!)参数iterations 执行的次数 queue 提交到的队列 block 执行的任务
class ViewController: UIViewController{
var hwcarray = ["hello","hwc","hellohwc"]
override func viewDidLoad(){
super.viewDidLoad()
dispatch_apply(3,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)){
(index:UInt) -> () in
var expObject = self.hwcarray[Int(index)] as NSString
NSLog("%d",expObject.length)
}
NSLog("Dispatch_after is over")
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
} 可以看到，输出是3 5 8 dispatch_after is over由于这样会阻塞主线程，而下文又与dispatch_apply的执行结果无关，所以可以在异步队列中掉dispatch_apply,然后执行完成后进行通知
class ViewController: UIViewController{
var hwcarray = ["hello","hwc","hellohwc"]
override func viewDidLoad(){
super.viewDidLoad()
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)){
dispatch_apply(3,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)){
(index:UInt) -> () in
var expObject = self.hwcarray[Int(index)] as NSString
NSLog("%d",expObject.length)
}
NSLog("Dispatch_after in global queue is over")
}
NSLog("Dispatch_after in main queue is over")
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
}这样输出为8 Dispatch_after in main queue is over 3 5 Dispatch_after in global queue is over可以看到，相对主队列(主线程)是异步的，在global队列中是并行执行的
class hwcSingleton {
var testVariable:Int!
func print(){
testVariable = testVariable + 1
println(testVariable)
}
class var sharedObject: hwcSingleton {
struct StaticStruct {
static var predicate : dispatch_once_t = 0
static var instance : hwcSingleton? = nil
}
dispatch_once(&StaticStruct.predicate) {
StaticStruct.instance = hwcSingleton()
StaticStruct.instance?.testVariable = 10
}
return StaticStruct.instance!
}
}完整详解GCD系列（二）dispatch_after;dispatch_apply;dispatch_once
原文：http://blog.csdn.net/hello_hwc/article/details/41204027