完整详解GCD系列（三）dispatch_group

http://blog.csdn.net/hello_hwc/article/details/41409135

一、dispatch_group
把一组任务提交到队列中，这些队列可以不相关，然后坚挺这组任务完成的事件。
几个用到的函数
1、dispatch_group_create创建一个调度任务组

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func dispatch_group_create() -> dispatch_group_t!

2、dispatch_group_async 把一个任务异步提交到任务组里

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func dispatch_group_async(_ group: dispatch_group_t!,

_ queue: dispatch_queue_t!,

_ block: dispatch_block_t!)

参数： group 提交到的任务组，这个任务组的对象会一直持续到任务组执行完毕
queue 提交到的队列，任务组里不同任务的队列可以不同
block 提交的任务

3、dispatch_group_enter/dispatch_group_leave

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func dispatch_group_enter(_ group: dispatch_group_t!)

func dispatch_group_leave(_ group: dispatch_group_t!)

这两个方法显示的讲任务组中的任务未执行完毕的任务数目加减1，这种方式用在不使用dispatch_group_async来提交任务，注意：这两个函数要配合使用，有enter要有leave，这样才能保证功能完整实现。也可以用这对函数来让一个闭包关联多个Group

4、dispatch_group_notify 用来监听任务组事件的执行完毕

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func dispatch_group_notify(_ group: dispatch_group_t!,

_ queue: dispatch_queue_t!,

_ block: dispatch_block_t!)

参数： group监听的任务组
queue 执行完毕的这个闭包所在的队列
block 执行完毕所响应的任务
5、dispatch_group_wait 设置等待时间，在等待时间结束后，如果还没有执行完任务组，则返回。返回0代表执行成功，非0则执行失败

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long dispatch_group_wait ( dispatch_group_t group, dispatch_time_t timeout );

二、完整的代码解析,模拟提交三个下载任务

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class ViewController: UIViewController{

override func viewDidLoad(){

super.viewDidLoad()

var hwcGroup = dispatch_group_create()//创建group

var globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)//全局队列，这个队列为并行队列

var userCreateQueue = dispatch_queue_create("com.test.helloHwc",DISPATCH_QUEUE_SERIAL)//创建一个用户队列，这个队列为串行队列

downLoadTask1(group: hwcGroup,queue: globalQueueDefault)

downLoadTask2(group: hwcGroup,queue: userCreateQueue)

downLoadTask3(group: hwcGroup,queue: userCreateQueue)

//letresult = dispatch_group_wait(hwcGroup,DISPATCH_TIME_FOREVER)

dispatch_group_notify(hwcGroup,dispatch_get_main_queue()){

NSLog("Group tasks are done")

}

println("Now viewDidLoad is done")

}

func downLoadTask1(#group:dispatch_group_t,queue:dispatch_queue_t){

dispatch_group_async(group,queue){

sleep(3)

NSLog("Task1 is done")

}

}

func downLoadTask2(#group:dispatch_group_t,queue:dispatch_queue_t){

dispatch_group_async(group,queue){

sleep(3)

NSLog("Task2 is done")

}

}

func downLoadTask3(#group:dispatch_group_t,queue:dispatch_queue_t){

dispatch_group_async(group,queue){

sleep(3)

NSLog("Task3 is done")

}

}

override func didReceiveMemoryWarning(){

super.didReceiveMemoryWarning()

}

}

可以看到输出为

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Now viewDidLoad is done

Task2 is done

Task1 is done

Task3 is done

Group task is done

这里task1提交到全局队列中，task2和task3提交到用户穿件的串行队列中，所以task1和task2同时输出，task3在task2结束两秒后输出。

如果把注释那行取消，会等待一段时间，再返回，读者可以自己去试验下

三、关于如何使用dispatch_group_enter/dispatch_group_leave如何使用

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<pre name="code" class="plain">class ViewController: UIViewController{

override func viewDidLoad(){

super.viewDidLoad()

var hwcGroup = dispatch_group_create()//创建group

var globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)//全局队列，这个队列为并行队列

for index:UInt32 in 1...3{

dispatch_group_enter(hwcGroup)//提交了一个任务，任务数目加1

manualDownLoad(index){

println("Task\(index) is done")

dispatch_group_leave(hwcGroup)//完成一个任务，任务数目减1

}

}

let result = dispatch_group_wait(hwcGroup,DISPATCH_TIME_FOREVER)//等待直到完成

dispatch_group_notify(hwcGroup,dispatch_get_main_queue()){

println("Group tasks are done")

}

println("Now viewDidLoad is done")

}

func manualDownLoad(num:UInt32,block:()->()){

println("Downloading task\(num)")

sleep(num)

block()

}

override func didReceiveMemoryWarning(){

super.didReceiveMemoryWarning()

}

}

输出

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Downloading task1

Task1 is done

Downloading task2

Task2 is done

Downloading task3

Task3 is done

Now viewDidLoad is done

Group is done

可以看到，同步进行了下载，这里的任务是串行的，实际消耗时间9s。当然，也可以把下载的任务提交到异步队列中

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class ViewController: UIViewController{

override func viewDidLoad(){

super.viewDidLoad()

var hwcGroup = dispatch_group_create()//创建group

var globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)//全局队列，这个队列为并行队列

for index:UInt32 in 1...3{

dispatch_group_enter(hwcGroup)//提交了一个任务，任务数目加1

manualDownLoad(index,queue:globalQueueDefault){

NSLog("Task\(index) is done")

dispatch_group_leave(hwcGroup)//完成一个任务，任务数目减1

}

}

dispatch_group_notify(hwcGroup,dispatch_get_main_queue()){

NSLog("Group tasks are done")

}

NSLog("Now viewDidLoad is done")

}

func manualDownLoad(num:UInt32,queue:dispatch_queue_t, block:()->()){

dispatch_async(queue){

NSLog("Downloading task\(num)")

sleep(num)

block()

}

}

override func didReceiveMemoryWarning(){

super.didReceiveMemoryWarning()

}

}

这样的话，输出是

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Downloading task1

Downloading task2

Downloading task3

Now viewDidLoad is done

Task1 is done

Task2 is done

Task3 is done

由于三个模拟的下载任务是并行的，所以实际消耗时间3s