GCD相当好用，但用不好就会死锁,始终要记着这样一句秘籍：

不要在串行队列放dispatch_sync、dispatch_apply

下面看几个例子

// 防死锁秘籍：不要在串行队列放dispatch_sync、dispatch_apply   // 死锁  主线程调用dispatch_apply    dispatch_async(dispatch_get_main_queue(), ^{        dispatch_apply(5, dispatch_get_main_queue(), ^(size_t i) {            NSLog(@"%ld",i);            NSLog(@"%d",55);        });      });    NSLog(@"%d",999);    //死锁 主线程调用dispatch_apply    dispatch_apply(5, dispatch_get_main_queue(), ^(size_t i) {        NSLog(@"%ld",i);        NSLog(@"%d",777);    });           //不死锁    dispatch_queue_t queue=  dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    dispatch_apply(5, queue, ^(size_t i) {        NSLog(@"%ld",i);         NSLog(@"%d",66);    });    NSLog(@"%d",88);    //不死锁    dispatch_async(queue, ^{        dispatch_apply(5, dispatch_get_main_queue(), ^(size_t i) {            NSLog(@"%ld",i);        });    });//    //不死锁    dispatch_async(queue,^{        dispatch_sync(dispatch_get_main_queue(), ^{            NSLog(@"test");        });    });    //死锁    dispatch_sync(queue,^{        dispatch_sync(dispatch_get_main_queue(), ^{            NSLog(@"test");        });    });