GCD

1 串行队列，异步执行------所有异步添加到串行队列上的任务，都会在串行队列所创建的那个线程中去执行。

    dispatch_queue_t serialQueue = dispatch_queue_create("com.zijie.serial", DISPATCH_QUEUE_SERIAL);

    NSThread *thread = [NSThread currentThread];
    NSLog(@"1-----%@", thread);

    dispatch_async(serialQueue, ^{
        NSLog(@"2----%@", [NSThread currentThread]);
    });
    
    dispatch_async(serialQueue, ^{
        NSLog(@"3----%@", [NSThread currentThread]);
    });
    
    dispatch_async(serialQueue, ^{
        NSLog(@"4----%@", [NSThread currentThread]);
    });
    
    dispatch_async(serialQueue, ^{
        NSLog(@"5----%@", [NSThread currentThread]);
    });

// 打印结果：
2017-02-24 11:09:40.454 YYGCDDemo[73122:8089151] 1-----<NSThread: 0x7a0536b0>{number = 1, name = main}
2017-02-24 11:09:40.455 YYGCDDemo[73122:8089185] 2----<NSThread: 0x79772a10>{number = 2, name = (null)}
2017-02-24 11:09:40.456 YYGCDDemo[73122:8089185] 3----<NSThread: 0x79772a10>{number = 2, name = (null)}
2017-02-24 11:09:40.456 YYGCDDemo[73122:8089185] 4----<NSThread: 0x79772a10>{number = 2, name = (null)}
2017-02-24 11:09:40.456 YYGCDDemo[73122:8089185] 5----<NSThread: 0x79772a10>{number = 2, name = (null)}

2 串行队列，同步执行---------都会在主线程执行

    dispatch_queue_t serialQueue = dispatch_queue_create("com.zijie.serial", DISPATCH_QUEUE_SERIAL);

    NSThread *thread = [NSThread currentThread];
    NSLog(@"1-----%@", thread);
    
    dispatch_sync(serialQueue, ^{
        NSLog(@"2----%@", [NSThread currentThread]);
    });
    
    dispatch_sync(serialQueue, ^{
        NSLog(@"3----%@", [NSThread currentThread]);
    });
    
    dispatch_sync(serialQueue, ^{
        NSLog(@"4----%@", [NSThread currentThread]);
    });
    
    dispatch_sync(serialQueue, ^{
        NSLog(@"5----%@", [NSThread currentThread]);
    });

// 打印结果：
2017-02-24 11:17:17.158 YYGCDDemo[73250:8105871] 1-----<NSThread: 0x79721be0>{number = 1, name = main}
2017-02-24 11:17:17.159 YYGCDDemo[73250:8105871] 2----<NSThread: 0x79721be0>{number = 1, name = main}
2017-02-24 11:17:17.159 YYGCDDemo[73250:8105871] 3----<NSThread: 0x79721be0>{number = 1, name = main}
2017-02-24 11:17:17.160 YYGCDDemo[73250:8105871] 4----<NSThread: 0x79721be0>{number = 1, name = main}
2017-02-24 11:17:17.160 YYGCDDemo[73250:8105871] 5----<NSThread: 0x79721be0>{number = 1, name = main}

3 并行队列，异步执行-------会启动多个线程并行执行，完成结果没有顺序

    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.zijie.concurrent", DISPATCH_QUEUE_CONCURRENT);
    
    NSThread *thread = [NSThread currentThread];
    NSLog(@"1-----%@", thread);  
    
    dispatch_async(concurrentQueue, ^{
        [@"string1" length];
        NSLog(@"2----%@", [NSThread currentThread]);
    });
    
    dispatch_async(concurrentQueue, ^{
        [@"string2" length];
        NSLog(@"3----%@", [NSThread currentThread]);
    });
    
    dispatch_async(concurrentQueue, ^{
        [@"string3" length];
        NSLog(@"4----%@", [NSThread currentThread]);
    });
    
    dispatch_async(concurrentQueue, ^{
        [@"string4" length];
        NSLog(@"5----%@", [NSThread currentThread]);
    });

// 打印结果：
2017-02-24 11:22:29.054 YYGCDDemo[73385:8117872] 1-----<NSThread: 0x7c12eb20>{number = 1, name = main}
2017-02-24 11:22:29.056 YYGCDDemo[73385:8118127] 5----<NSThread: 0x7be2bba0>{number = 5, name = (null)}
2017-02-24 11:22:29.056 YYGCDDemo[73385:8118016] 3----<NSThread: 0x7be2f4c0>{number = 3, name = (null)}
2017-02-24 11:22:29.056 YYGCDDemo[73385:8118010] 2----<NSThread: 0x7c0611e0>{number = 2, name = (null)}
2017-02-24 11:22:29.057 YYGCDDemo[73385:8118126] 4----<NSThread: 0x7c0616b0>{number = 4, name = (null)}

4 并发队列，同步执行-------都会在主线程执行-------没有开启新线程，任务逐个执行

    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.zijie.concurrent", DISPATCH_QUEUE_CONCURRENT);
    
    NSThread *thread = [NSThread currentThread];
    NSLog(@"1-----%@", thread);
    
    dispatch_sync(concurrentQueue, ^{
        NSLog(@"2----%@", [NSThread currentThread]);
    });
    
    dispatch_sync(concurrentQueue, ^{
        NSLog(@"3----%@", [NSThread currentThread]);
    });
    
    dispatch_sync(concurrentQueue, ^{
        NSLog(@"4----%@", [NSThread currentThread]);
    });
    
    dispatch_sync(concurrentQueue, ^{
        NSLog(@"5----%@", [NSThread currentThread]);
    });

// 打印结果：
2017-02-24 11:19:05.166 YYGCDDemo[73292:8110126] 1-----<NSThread: 0x7bf48c80>{number = 1, name = main}
2017-02-24 11:19:05.169 YYGCDDemo[73292:8110126] 2----<NSThread: 0x7bf48c80>{number = 1, name = main}
2017-02-24 11:19:05.169 YYGCDDemo[73292:8110126] 3----<NSThread: 0x7bf48c80>{number = 1, name = main}
2017-02-24 11:19:05.169 YYGCDDemo[73292:8110126] 4----<NSThread: 0x7bf48c80>{number = 1, name = main}
2017-02-24 11:19:05.170 YYGCDDemo[73292:8110126] 5----<NSThread: 0x7bf48c80>{number = 1, name = main}

5 计时器 dispatch_source_t

　　NSTimer受runloop的影响，由于runloop需要处理很多任务，导致NSTimer的精度降低，在日常开发中，如果我们需要对定时器的精度要求很高的话，可以考虑dispatch_source_t去实现 。dispatch_source_t精度很高，系统自动触发，系统级别的源。

　　注意：dispatch_source_t必须定义为全局变量，因为如果是局部变量，dispatch_source_create创建后，没有对象持有它，当代码执行结束后，就会释放。

暂停计时器：dispatch_suspend(_timer);

恢复计时器：dispatch_resume(_timer);

// 实现倒计时功能
void btnClick:(id)sender {

    __block NSInteger seconds = 59;
    
    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
　　
    _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, globalQueue);
    dispatch_source_set_timer(_timer, DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(timer, ^{
        NSLog(@"---------%d", seconds);
        seconds -= 1;
        
        if (seconds < 0) {
            dispatch_source_cancel(_timer);
        }        
    });
　　// 启动定时器
    dispatch_resume(_timer);
}