同步任务的作用

同步任务，可以让其他异步执行的任务，依赖某一个同步任务

例如：在用户登录之后，再异步下载文件！

- (void)gcdDemo1 {    dispatch_queue_t queue = dispatch_queue_create("com.itheima.queue", DISPATCH_QUEUE_CONCURRENT);    dispatch_sync(queue, ^{        NSLog(@"登录 %@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"下载 A %@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"下载 B %@", [NSThread currentThread]);    });}

代码改造，让登录也在异步执行

- (void)gcdDemo2 {    dispatch_queue_t queue = dispatch_queue_create("com.itheima.queue", DISPATCH_QUEUE_CONCURRENT);    void (^task)() = ^{        dispatch_sync(queue, ^{            NSLog(@"登录 %@", [NSThread currentThread]);        });        dispatch_async(queue, ^{            NSLog(@"下载 A %@", [NSThread currentThread]);        });        dispatch_async(queue, ^{            NSLog(@"下载 B %@", [NSThread currentThread]);        });    };    dispatch_async(queue, task);}

主队列调度同步队列不死锁

- (void)gcdDemo3 {    dispatch_queue_t queue = dispatch_queue_create("com.itheima.queue", DISPATCH_QUEUE_CONCURRENT);    void (^task)() = ^ {        dispatch_sync(dispatch_get_main_queue(), ^{            NSLog(@"死？");        });    };    dispatch_async(queue, task);}