ios之gcd浅析

A。普通的GCD异步运行与主线程更新写法：

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
        // long-running task  
        dispatch_async(dispatch_get_main_queue(), ^{  
            // update UI  
        });  
    }); 

DISPATCH_QUEUE_PRIORITY_DEFAULT  这个有3个参数，这里是使用默认的，其他两个参数分别是DISPATCH_QUEUE_PRIOPITY_HIGH 和  DISPATCH_QUEUE_PRIOPITY_LOW

B。任务串行的GCD

    NSDate *da = [NSDate date];  
    NSString *daStr = [da description];  
    const char *queueName = [daStr UTF8String];  
    dispatch_queue_t myQueue = dispatch_queue_create(queueName, DISPATCH_QUEUE_SERIAL);  
      
    dispatch_async(myQueue, ^{  
        [NSThread sleepForTimeInterval:6];  
        NSLog(@"[NSThread sleepForTimeInterval:6];");  
    });  
      
    dispatch_async(myQueue, ^{  
        [NSThread sleepForTimeInterval:3];  
        NSLog(@"[NSThread sleepForTimeInterval:3];");  
    });  
      
    dispatch_async(myQueue, ^{  
        [NSThread sleepForTimeInterval:1];  
        NSLog(@"[NSThread sleepForTimeInterval:1];");  
    });  
      
    dispatch_release(myQueue);  

C。任务并行的GCD

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
      
    dispatch_async(myQueue, ^{  
        [NSThread sleepForTimeInterval:6];  
        NSLog(@"[NSThread sleepForTimeInterval:6];");  
    });  
      
    dispatch_async(myQueue, ^{  
        [NSThread sleepForTimeInterval:3];  
        NSLog(@"[NSThread sleepForTimeInterval:3];");  
    });  
      
    dispatch_async(myQueue, ^{  
        [NSThread sleepForTimeInterval:1];  
        NSLog(@"[NSThread sleepForTimeInterval:1];");  
    });  
      
    dispatch_release(myQueue);  

D。实现监听一组任务是否完成，完成后得到通知执行其他的操作GCD

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:6];  
        NSLog(@"group1 [NSThread sleepForTimeInterval:6];");  
    });  
    dispatch_group_async(group, queue, ^{  
        [NSThread sleepForTimeInterval:3];  
        NSLog(@"group2 [NSThread sleepForTimeInterval:3];");  
    });  
    dispatch_group_async(group, queue, ^{  
        [NSThread sleepForTimeInterval:1];  
        NSLog(@"group3 [NSThread sleepForTimeInterval:1];");  
    });  
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{  
        NSLog(@"main thread.");  
    });  
    dispatch_release(group);

E。前面的任务执行结束后它才执行，而且它后面的任务等它执行完成之后才会执行GCD

dispatch_queue_t queue = dispatch_queue_create("gcdtest.rongfzh.yc", DISPATCH_QUEUE_CONCURRENT);  
      
    dispatch_async(queue, ^{  
        [NSThread sleepForTimeInterval:3];  
        NSLog(@"dispatch_async1");  
    });  
    dispatch_async(queue, ^{  
        [NSThread sleepForTimeInterval:1];  
        NSLog(@"dispatch_async2");  
    });  
    dispatch_barrier_async(queue, ^{  
        NSLog(@"dispatch_barrier_async");  
        [NSThread sleepForTimeInterval:0.5];  
          
    });  
    dispatch_async(queue, ^{  
        [NSThread sleepForTimeInterval:1];  
        NSLog(@"dispatch_async3");  
    });  

F。延时执行GCD

double delayInSeconds = 2.0;  
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);  
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){  
        // code to be executed on the main queue after delay  
    });  

G。保证整个应用程序生命周期中某段代码只被执行一次GCD

static dispatch_once_t onceToken;  
    dispatch_once(&onceToken, ^{  
        // code to be executed once  
    });  

H。执行某个代码片段N次GCD

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); 
dispatch_apply(5, queue, ^(size_t index) {

// 执行5次

});

 
I。通过dispatch_queue_create函数创建的queue,其优先级的线程和采用默认优先级的Global dispatch queue的线程是相同的，那么如何改变通过dispatch_queue_create创建queue的优先级呢？可以通过dispatch_set_target_queue这个函数该实现。

dispatch_queue_t myQueue = dispatch_queue_create("org.itjoy.gcd", NULL);
    dispatch_queue_t globalHightQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    dispatch_set_target_queue(myQueue, globalHighQueue);