GCD那些事

發表於2016-08-19

文章目錄

一. GCD的介紹

GCD屬於系統級的執行緒管理，在Dispatch queue中執行需要執行的任務效能非常的高。GCD這塊已經開源，地址:http://libdispatch.macosforge.org。GCD中的FIFO佇列稱為dispatch queue，用來保證先進來的任務先得到執行。這裡我們從作用和場景同時分析，看一下到底在哪些地方會使用。單單隻介紹作用的話，可能

二. 常用的GCD

1. `dispatch_semaphore_t,dispatch_semaphore_wait,dispatch_semaphore_signal`

作用:

在多執行緒下控制多執行緒的併發數目。

建立訊號量，可以設定訊號量的資源數。0表示沒有資源，呼叫dispatch_semaphore_wait會立即等待。
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
等待訊號，可以設定超時引數。該函式返回0表示得到通知，非0表示超時。dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
通知訊號，如果等待執行緒被喚醒則返回非0，否則返回0。
dispatch_semaphore_signal(semaphore);

使用場景一:併發佇列

比如現在我每次想執行10個任務。休息兩秒。繼續執行10個任務。可以這麼寫.

-(void) testSemaphore{
dispatch_group_t group = dispatch_group_create();
dispatch_semaphore_t semaphore = dispatch_semaphore_create(10); //訊號總量是10
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
for (int i = 0; i 100; i++)
{
   dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);//訊號量-1
   dispatch_group_async(group, queue, ^{
       NSLog(@"%i",i);
       sleep(2);
       dispatch_semaphore_signal(semaphore);   //訊號量＋1
   });
}
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
}
```	
執行結果:  

```objc
2016-06-01 20:30:35.113 XXGCD[39559:306278] 7
2016-06-01 20:30:35.113 XXGCD[39559:306277] 6
2016-06-01 20:30:35.113 XXGCD[39559:306271] 0
2016-06-01 20:30:35.113 XXGCD[39559:306274] 3
2016-06-01 20:30:35.113 XXGCD[39559:306270] 1
2016-06-01 20:30:35.113 XXGCD[39559:306272] 2
2016-06-01 20:30:35.113 XXGCD[39559:306275] 4
2016-06-01 20:30:35.113 XXGCD[39559:306276] 5
2016-06-01 20:30:35.113 XXGCD[39559:306279] 8
2016-06-01 20:30:35.113 XXGCD[39559:306280] 9
	
2016-06-01 20:30:37.117 XXGCD[39559:306272] 11
2016-06-01 20:30:37.117 XXGCD[39559:306270] 12
2016-06-01 20:30:37.117 XXGCD[39559:306275] 10
2016-06-01 20:30:37.117 XXGCD[39559:306280] 13
2016-06-01 20:30:37.117 XXGCD[39559:306276] 15
2016-06-01 20:30:37.117 XXGCD[39559:306279] 14
2016-06-01 20:30:37.117 XXGCD[39559:306274] 16
2016-06-01 20:30:37.117 XXGCD[39559:306277] 17
2016-06-01 20:30:37.117 XXGCD[39559:306278] 18
2016-06-01 20:30:37.117 XXGCD[39559:306271] 19
...

-(void) testSemaphore{

dispatch_group_t group = dispatch_group_create();

dispatch_semaphore_t semaphore = dispatch_semaphore_create(10); //訊號總量是10

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

for (int i = 0; i 100; i++)

{

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);//訊號量-1

dispatch_group_async(group, queue, ^{

NSLog(@"%i",i);

sleep(2);

dispatch_semaphore_signal(semaphore); //訊號量＋1

});

}

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

}

```

執行結果:

```objc

2016-06-01 20:30:35.113 XXGCD[39559:306278] 7

2016-06-01 20:30:35.113 XXGCD[39559:306277] 6

2016-06-01 20:30:35.113 XXGCD[39559:306271] 0

2016-06-01 20:30:35.113 XXGCD[39559:306274] 3

2016-06-01 20:30:35.113 XXGCD[39559:306270] 1

2016-06-01 20:30:35.113 XXGCD[39559:306272] 2

2016-06-01 20:30:35.113 XXGCD[39559:306275] 4

2016-06-01 20:30:35.113 XXGCD[39559:306276] 5

2016-06-01 20:30:35.113 XXGCD[39559:306279] 8

2016-06-01 20:30:35.113 XXGCD[39559:306280] 9

2016-06-01 20:30:37.117 XXGCD[39559:306272] 11

2016-06-01 20:30:37.117 XXGCD[39559:306270] 12

2016-06-01 20:30:37.117 XXGCD[39559:306275] 10

2016-06-01 20:30:37.117 XXGCD[39559:306280] 13

2016-06-01 20:30:37.117 XXGCD[39559:306276] 15

2016-06-01 20:30:37.117 XXGCD[39559:306279] 14

2016-06-01 20:30:37.117 XXGCD[39559:306274] 16

2016-06-01 20:30:37.117 XXGCD[39559:306277] 17

2016-06-01 20:30:37.117 XXGCD[39559:306278] 18

2016-06-01 20:30:37.117 XXGCD[39559:306271] 19

...

使用場景二:非同步佇列中做事，等待回撥後執行某件事

-(void) testAsynFinished{
    __block BOOL isok = NO;
    
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    
    XXEngine *engine = [[XXEngine alloc] init];
    [engine queryCompletion:^(BOOL isOpen) {//sleep了3秒
        isok = isOpen;
        dispatch_semaphore_signal(sema);
        NSLog(@"success!");
    } onError:^(BOOL isOpen, int errorCode) {
        isok = NO;
        dispatch_semaphore_signal(sema);
        NSLog(@"error!");
	
    }];
    NSLog(@"finished");
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
}

-(void) testAsynFinished{

__block BOOL isok = NO;

dispatch_semaphore_t sema = dispatch_semaphore_create(0);

XXEngine *engine = [[XXEngine alloc] init];

[engine queryCompletion:^(BOOL isOpen) {//sleep了3秒

isok = isOpen;

dispatch_semaphore_signal(sema);

NSLog(@"success!");

} onError:^(BOOL isOpen, int errorCode) {

isok = NO;

dispatch_semaphore_signal(sema);

NSLog(@"error!");

}];

NSLog(@"finished");

dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);

}

執行結果:

2016-06-02 15:01:06.168 XXGCD[35443:290673] success!
2016-06-02 15:01:06.168 XXGCD[35443:290673] finished

1 2	2016-06-02 15:01:06.168 XXGCD[35443:290673] success! 2016-06-02 15:01:06.168 XXGCD[35443:290673] finished

使用場景三:生產者，消費者

-(void) testProductAndConsumer{
 
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    
    dispatch_queue_t producerQueue = dispatch_queue_create("producer", DISPATCH_QUEUE_CONCURRENT);//生產者執行緒跑的佇列
    dispatch_queue_t consumerQueue = dispatch_queue_create("consumer", DISPATCH_QUEUE_CONCURRENT);//消費者執行緒跑的佇列
    
    __block int cakeNumber = 0;
    dispatch_async(producerQueue,  ^{ //生產者佇列
        while (1) {
            if (!dispatch_semaphore_signal(sem))
            {
                NSLog(@"Product:生產出了第%d個蛋糕",++cakeNumber);
                sleep(1); //wait for a while
                continue;
            }
        }
    });
    dispatch_async(consumerQueue,  ^{//消費者佇列
        while (1) {
            if (dispatch_semaphore_wait(sem, dispatch_time(DISPATCH_TIME_NOW, 0*NSEC_PER_SEC))){
                if(cakeNumber > 0){
                    NSLog(@"Consumer:拿到了第%d個蛋糕",cakeNumber--);
                }
                continue;
            }
        }
    });
}

-(void) testProductAndConsumer{

dispatch_semaphore_t sem = dispatch_semaphore_create(0);

dispatch_queue_t producerQueue = dispatch_queue_create("producer", DISPATCH_QUEUE_CONCURRENT);//生產者執行緒跑的佇列

dispatch_queue_t consumerQueue = dispatch_queue_create("consumer", DISPATCH_QUEUE_CONCURRENT);//消費者執行緒跑的佇列

__block int cakeNumber = 0;

dispatch_async(producerQueue, ^{ //生產者佇列

while (1) {

if (!dispatch_semaphore_signal(sem))

{

NSLog(@"Product:生產出了第%d個蛋糕",++cakeNumber);

sleep(1); //wait for a while

continue;

}

});

dispatch_async(consumerQueue, ^{//消費者佇列

while (1) {

if (dispatch_semaphore_wait(sem, dispatch_time(DISPATCH_TIME_NOW, 0*NSEC_PER_SEC))){

if(cakeNumber > 0){

NSLog(@"Consumer:拿到了第%d個蛋糕",cakeNumber--);

}

continue;

}

});

}

執行結果:

2016-06-02 15:04:18.785 XXGCD[35593:294211] Product:生產出了第1個蛋糕
2016-06-02 15:04:18.785 XXGCD[35593:294209] Consumer:拿到了第1個蛋糕
2016-06-02 15:04:19.790 XXGCD[35593:294209] Consumer:拿到了第1個蛋糕
2016-06-02 15:04:19.790 XXGCD[35593:294211] Product:生產出了第1個蛋糕

2016-06-02 15:04:18.785 XXGCD[35593:294211] Product:生產出了第1個蛋糕

2016-06-02 15:04:18.785 XXGCD[35593:294209] Consumer:拿到了第1個蛋糕

2016-06-02 15:04:19.790 XXGCD[35593:294209] Consumer:拿到了第1個蛋糕

2016-06-02 15:04:19.790 XXGCD[35593:294211] Product:生產出了第1個蛋糕

2. `dispatch_group_t,dispatch_group_notify`

用來阻塞一個執行緒，直到一個或多個任務完成執行。有時候你必須等待任務完成的結果，然後才能繼續後面的處理

使用場景一:併發佇列

-(void) testGCDGroup{
    dispatch_group_t group = dispatch_group_create();
    for(int i = 0; i 10; i++)
    {
        dispatch_group_enter(group);
        [[XXEngine instance] doAsyncWorkWithCompletionBlock:^{
            dispatch_group_leave(group);
        }];
    }
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"testGCDGroup1 更新UI操作");
}

-(void) testGCDGroup{

dispatch_group_t group = dispatch_group_create();

for(int i = 0; i 10; i++)

{

dispatch_group_enter(group);

[[XXEngine instance] doAsyncWorkWithCompletionBlock:^{

dispatch_group_leave(group);

}];

}

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

NSLog(@"testGCDGroup1 更新UI操作");

}

等價於:

-(void) testGCDGroup2{
    dispatch_group_t group = dispatch_group_create();
    for(int i = 0; i 10; i++)
    {
        dispatch_group_enter(group);
        [[XXEngine instance] doAsyncWorkWithCompletionBlock:^{
            dispatch_group_leave(group);
        }];
    }
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"testGCDGroup2 更新UI操作");
    });
}
```  

執行結果:  

```objc
2016-06-02 15:29:54.392 XXGCD[36197:313551] test
2016-06-02 15:29:54.392 XXGCD[36197:313569] test
2016-06-02 15:29:54.393 XXGCD[36197:313572] test
2016-06-02 15:29:54.393 XXGCD[36197:313571] test
2016-06-02 15:29:54.392 XXGCD[36197:313558] test
2016-06-02 15:29:54.392 XXGCD[36197:313568] test
2016-06-02 15:29:54.392 XXGCD[36197:313563] test
2016-06-02 15:29:54.393 XXGCD[36197:313570] test
2016-06-02 15:29:54.393 XXGCD[36197:313573] test
2016-06-02 15:29:54.393 XXGCD[36197:313574] test
2016-06-02 15:29:56.398 XXGCD[36197:313513] testGCDGroup1 更新UI操作

-(void) testGCDGroup2{

dispatch_group_t group = dispatch_group_create();

for(int i = 0; i 10; i++)

{

dispatch_group_enter(group);

[[XXEngine instance] doAsyncWorkWithCompletionBlock:^{

dispatch_group_leave(group);

}];

}

dispatch_group_notify(group, dispatch_get_main_queue(), ^{

NSLog(@"testGCDGroup2 更新UI操作");

});

}

```

執行結果:

```objc

2016-06-02 15:29:54.392 XXGCD[36197:313551] test

2016-06-02 15:29:54.392 XXGCD[36197:313569] test

2016-06-02 15:29:54.393 XXGCD[36197:313572] test

2016-06-02 15:29:54.393 XXGCD[36197:313571] test

2016-06-02 15:29:54.392 XXGCD[36197:313558] test

2016-06-02 15:29:54.392 XXGCD[36197:313568] test

2016-06-02 15:29:54.392 XXGCD[36197:313563] test

2016-06-02 15:29:54.393 XXGCD[36197:313570] test

2016-06-02 15:29:54.393 XXGCD[36197:313573] test

2016-06-02 15:29:54.393 XXGCD[36197:313574] test

2016-06-02 15:29:56.398 XXGCD[36197:313513] testGCDGroup1 更新UI操作

使用場景二:多工執行完成，重新整理UI

需要在主執行緒中執行，比如操作GUI，那麼我們只要將main queue而非全域性佇列傳給dispatch_group_notify函式就行了。

-(void) testGCDGroup3{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue, ^{
        [NSThread sleepForTimeInterval:1];
        NSLog(@"group1");
    });
    dispatch_group_async(group, queue, ^{
        [NSThread sleepForTimeInterval:2];
        NSLog(@"group2");
    });
    dispatch_group_async(group, queue, ^{
        [NSThread sleepForTimeInterval:3];
        NSLog(@"group3");
    });
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"testGCDGroup3 更新UI操作");
    });
}

-(void) testGCDGroup3{

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, queue, ^{

[NSThread sleepForTimeInterval:1];

NSLog(@"group1");

});

dispatch_group_async(group, queue, ^{

[NSThread sleepForTimeInterval:2];

NSLog(@"group2");

});

dispatch_group_async(group, queue, ^{

[NSThread sleepForTimeInterval:3];

NSLog(@"group3");

});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{

NSLog(@"testGCDGroup3 更新UI操作");

});

}

執行結果:

2016-06-02 15:58:29.041 XXGCD[36675:337871] group1
2016-06-02 15:58:30.041 XXGCD[36675:337880] group2
2016-06-02 15:58:31.043 XXGCD[36675:337884] group3
2016-06-02 15:58:31.043 XXGCD[36675:337834] testGCDGroup3 更新UI操作

2016-06-02 15:58:29.041 XXGCD[36675:337871] group1

2016-06-02 15:58:30.041 XXGCD[36675:337880] group2

2016-06-02 15:58:31.043 XXGCD[36675:337884] group3

2016-06-02 15:58:31.043 XXGCD[36675:337834] testGCDGroup3 更新UI操作

3. `dispatch_apply`

是同步函式,會阻塞當前執行緒直到所有迴圈迭代執行完成。當提交到併發queue時,迴圈迭代的執行順序是不確定的

static int i = 0;
-(void) testGCDApply{
    dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_apply(10, globalQ, ^(size_t index) {
        // 執行10次
        [self printTest];
    });
}
-(void) printTest{
    NSLog(@"%d",++i);
}

static int i = 0;

-(void) testGCDApply{

dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_apply(10, globalQ, ^(size_t index) {

// 執行10次

[self printTest];

});

}

-(void) printTest{

NSLog(@"%d",++i);

}

執行結果:

2016-06-02 16:06:50.799 XXGCD[37133:347348] 6
2016-06-02 16:06:50.799 XXGCD[37133:347329] 2
2016-06-02 16:06:50.799 XXGCD[37133:347346] 8
2016-06-02 16:06:50.799 XXGCD[37133:347336] 1
2016-06-02 16:06:50.799 XXGCD[37133:347340] 3
2016-06-02 16:06:50.799 XXGCD[37133:347296] 4
2016-06-02 16:06:50.799 XXGCD[37133:347347] 7
2016-06-02 16:06:50.799 XXGCD[37133:347349] 5
2016-06-02 16:06:50.799 XXGCD[37133:347329] 9
2016-06-02 16:06:50.799 XXGCD[37133:347348] 10

2016-06-02 16:06:50.799 XXGCD[37133:347348] 6

2016-06-02 16:06:50.799 XXGCD[37133:347329] 2

2016-06-02 16:06:50.799 XXGCD[37133:347346] 8

2016-06-02 16:06:50.799 XXGCD[37133:347336] 1

2016-06-02 16:06:50.799 XXGCD[37133:347340] 3

2016-06-02 16:06:50.799 XXGCD[37133:347296] 4

2016-06-02 16:06:50.799 XXGCD[37133:347347] 7

2016-06-02 16:06:50.799 XXGCD[37133:347349] 5

2016-06-02 16:06:50.799 XXGCD[37133:347329] 9

2016-06-02 16:06:50.799 XXGCD[37133:347348] 10

4. `dispatch_source_set_timer,dispatch_suspend,dispatch_resume,dispatch_source_cancel`

`dispatch_suspend`:暫停`dispatch_source_t` 
`dispatch_resume`:恢復`dispatch_source_t`
`dispatch_cancel`:取消`dispatch_source_t`

`dispatch_suspend`:暫停`dispatch_source_t`

`dispatch_resume`:恢復`dispatch_source_t`

`dispatch_cancel`:取消`dispatch_source_t`

眾所周知，定時器有NSTimer，但是NSTimer有如下弊端：

必須保證有一個活躍的runloop，子執行緒的runloop是預設關閉的。這時如果不手動啟用runloop，performSelector和scheduledTimerWithTimeInterval的呼叫將是無效的
NSTimer的建立與撤銷必須在同一個執行緒操作、performSelector的建立與撤銷必須在同一個執行緒操作。
記憶體管理有潛在洩露的風險

_timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(doSometing) userInfo:nil repeats:YES];

1	_timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(doSometing) userInfo:nil repeats:YES];

如果此時不呼叫

[_timer invalidate];

1	[_timer invalidate];

是無法停止的。還有持有self，造成物件無法釋放。
所以在此我還是比較推薦用dispatch的timer,無須考慮這些事情

@interface XXGCDTimer()
{
    dispatch_source_t _timer;
}
@end
	
@implementation XXGCDTimer
-(void) startGCDTimer{
    NSTimeInterval period = 1.0; //設定時間間隔
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
     _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), period * NSEC_PER_SEC, 0); //每秒執行
    dispatch_source_set_event_handler(_timer, ^{
        //在這裡執行事件
        NSLog(@"每秒執行test");
    });
    
    dispatch_resume(_timer);
}
	
-(void) pauseTimer{
    if(_timer){
        dispatch_suspend(_timer);
    }
}
	
-(void) resumeTimer{
    if(_timer){
        dispatch_resume(_timer);
    }
}
	
-(void) stopTimer{
    if(_timer){
        dispatch_source_cancel(_timer);
        _timer = nil;
    }
}
@end

@interface XXGCDTimer()

{

dispatch_source_t _timer;

}

@end

@implementation XXGCDTimer

-(void) startGCDTimer{

NSTimeInterval period = 1.0; //設定時間間隔

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

_timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), period * NSEC_PER_SEC, 0); //每秒執行

dispatch_source_set_event_handler(_timer, ^{

//在這裡執行事件

NSLog(@"每秒執行test");

});

dispatch_resume(_timer);

}

-(void) pauseTimer{

if(_timer){

dispatch_suspend(_timer);

}

-(void) resumeTimer{

if(_timer){

dispatch_resume(_timer);

}

-(void) stopTimer{

if(_timer){

dispatch_source_cancel(_timer);

_timer = nil;

}

@end

5. `dispatch_barrier_async`

這裡之前的文章有非常詳細的解釋以及demo在文章後面。可以參考 iOS非同步讀寫總結之NSDictionary，這裡就不去詳細說明了。注意一下DISPATCH_QUEUE_CONCURRENT(並行佇列)和DISPATCH_QUEUE_SERIAL(序列佇列)即可

6. `dispatch_sync`和`dispatch_async`

這裡一個是同步，一個是非同步。只要注意一下死鎖的問題就好了。

錯誤場景1: （死鎖）結果無法輸出(呼叫場景限於主執行緒呼叫主執行緒)

-(void) startSYNC{
    dispatch_sync(dispatch_get_main_queue(), ^{
        NSLog(@"3");
    });
}

-(void) startSYNC{

dispatch_sync(dispatch_get_main_queue(), ^{

NSLog(@"3");

});

}

原因:
1:dispatch_sync在等待block語句執行完成，而block語句需要在主執行緒裡執行，所以dispatch_sync如果在主執行緒呼叫就會造成死鎖
2:dispatch_sync是同步的，本身就會阻塞當前執行緒，也即主執行緒。而又往主執行緒裡塞進去一個block，所以就會發生死鎖。

錯誤場景2: (死鎖)

- (void)testSYNC2 {
    dispatch_queue_t aSerialDispatchQueue =
    dispatch_queue_create("I.am.an.iOS.developer", DISPATCH_QUEUE_SERIAL);
    
    dispatch_sync(aSerialDispatchQueue, ^{
        // block 1
        NSLog(@"Good Night, Benjamin.");
        dispatch_sync(aSerialDispatchQueue, ^{
            // block 2
            NSLog(@"Good Night, Daisy.");
        });
    });
}  
```	
原因:  
簡單來說，在dispatch_sync巢狀使用時要注意：不能在一個巢狀中使用同一個serial dispatch queue，因為會發生死鎖；

##### 正確用法:  

```objc	
dispatch_async(dispatch_get_main_queue(), ^{
   //async 非同步佇列，dispatch_async 函式會立即返回, block會在後臺非同步執行。
   NSLog(@"2");//不會造成死鎖；
});

- (void)testSYNC2 {

dispatch_queue_t aSerialDispatchQueue =

dispatch_queue_create("I.am.an.iOS.developer", DISPATCH_QUEUE_SERIAL);

dispatch_sync(aSerialDispatchQueue, ^{

// block 1

NSLog(@"Good Night, Benjamin.");

dispatch_sync(aSerialDispatchQueue, ^{

// block 2

NSLog(@"Good Night, Daisy.");

});

}

```

原因:

簡單來說，在dispatch_sync巢狀使用時要注意：不能在一個巢狀中使用同一個serial dispatch queue，因為會發生死鎖；

##### 正確用法:

```objc

dispatch_async(dispatch_get_main_queue(), ^{

//async 非同步佇列，dispatch_async 函式會立即返回, block會在後臺非同步執行。

NSLog(@"2");//不會造成死鎖；

});

7. `dispatch_get_global_queue`

-(void) test{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 耗時的操作
        dispatch_async(dispatch_get_main_queue(), ^{
            // 更新介面
            NSLog(@"update UI");
        });
    });
}

-(void) test{

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

// 耗時的操作

dispatch_async(dispatch_get_main_queue(), ^{

// 更新介面

NSLog(@"update UI");

});

}

全域性併發佇列的優先順序

#define DISPATCH_QUEUE_PRIORITY_HIGH 2 // 高
#define DISPATCH_QUEUE_PRIORITY_DEFAULT 0 // 預設（中）
#define DISPATCH_QUEUE_PRIORITY_LOW (-2) // 低
#define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN // 後臺

#define DISPATCH_QUEUE_PRIORITY_HIGH 2 // 高

#define DISPATCH_QUEUE_PRIORITY_DEFAULT 0 // 預設（中）

#define DISPATCH_QUEUE_PRIORITY_LOW (-2) // 低

#define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN // 後臺

注意:
在實際修改bug的時候發現，global全域性佇列並不會反覆建立新的執行緒，在系統啟動後底層會維護一個執行緒池的概念，一旦用Global的佇列結束，會快取一段時間。因為在測試的時候可以列印出Thread ID。在多執行緒情況下發現一些ThreadID是一樣的，由此得出結論。

8. `dispatch_once`

// 使用dispatch_once函式能保證某段程式碼在程式執行過程中只被執行1次
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    // 只執行1次的程式碼(這裡面預設是執行緒安全的)
});

// 使用dispatch_once函式能保證某段程式碼在程式執行過程中只被執行1次

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

// 只執行1次的程式碼(這裡面預設是執行緒安全的)

});

9. `dispatch_after`

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    // 2秒後非同步執行這裡的程式碼...
});

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

// 2秒後非同步執行這裡的程式碼...

});

三. 總結

這裡是對GCD做總結的同時，突出了一些使用場景。因為在開發過程中，我們不僅僅知道這東西是啥，更要知道什麼時候靈活運用，這裡提供了一些解決思路。關於Block相關的方法可以參考Block那些事.最後歡迎加入QQ群：237305299。一起探討iOS技術問題。

四. Demo下載

Demo

五. 參考連結

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IT風雲15年的那些人那些事
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GCD那些事

一. GCD的介紹

二. 常用的GCD

1. dispatch_semaphore_t,dispatch_semaphore_wait,dispatch_semaphore_signal

作用:

使用場景一:併發佇列

使用場景二:非同步佇列中做事，等待回撥後執行某件事

使用場景三:生產者，消費者

2. dispatch_group_t,dispatch_group_notify

使用場景一:併發佇列

使用場景二:多工執行完成，重新整理UI

3. dispatch_apply

4. dispatch_source_set_timer,dispatch_suspend,dispatch_resume,dispatch_source_cancel

5. dispatch_barrier_async

6. dispatch_sync和dispatch_async

錯誤場景1: （死鎖）結果無法輸出(呼叫場景限於主執行緒呼叫主執行緒)

錯誤場景2: (死鎖)

7. dispatch_get_global_queue

8. dispatch_once

9. dispatch_after

三. 總結

四. Demo下載

五. 參考連結

相關文章

1. `dispatch_semaphore_t,dispatch_semaphore_wait,dispatch_semaphore_signal`

2. `dispatch_group_t,dispatch_group_notify`

3. `dispatch_apply`

4. `dispatch_source_set_timer,dispatch_suspend,dispatch_resume,dispatch_source_cancel`

5. `dispatch_barrier_async`

6. `dispatch_sync`和`dispatch_async`

7. `dispatch_get_global_queue`

8. `dispatch_once`

9. `dispatch_after`