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主线程上调用异步

- (void)syncOnMainThread
{
    NSLog(@"task2:%@",[NSThread currentThread]);
   
    dispatch_queue_t queue = dispatch_get_main_queue();
    
    dispatch_async(queue, ^{
        NSLog(@"task1:%@",[NSThread currentThread]);
    });
    
    NSLog(@"task3:%@",[NSThread currentThread]);
}

死锁：使用同步函数在主队列执行，会造成线程死锁情况

- (void)testDeadLock
{
    NSLog(@"task2:%@",[NSThread currentThread]);
    
    dispatch_queue_t queue = dispatch_get_main_queue();
    
    dispatch_sync(queue, ^{
        NSLog(@"task1:%@",[NSThread currentThread]);
    });
    
    NSLog(@"task3:%@",[NSThread currentThread]);
}

同步执行串行队列:不会开启新线程，按照顺序执行

- (void)syncChuanxing
{
    dispatch_queue_t queue = dispatch_queue_create("gcd.queue", NULL);
    //添加任务到队列中执行任务
    dispatch_sync(queue, ^{
        NSLog(@"task1:%@",[NSThread curren

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原文地址：http://www.cnblogs.com/chongdongren/p/4718517.html