GCD
异步刷新屏幕在主队列中执行
dispatch_async(dispatch_get_main_queue(), ^{
//更新屏幕代码
});
dispatch_sync（）锁死
NSLog(@"1");
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"2"); //阻塞，造成锁死
});
NSLog(@"3");
在当前线程中调用dispatch_sync()回造成锁死
dispatch_sync()会做两件事情
- 将block加入到queue中
- 阻塞当前线程，等待block返回。
了解了dispatch_sync()做的事情就很明白为什么会被锁死，dispatch_sync()在调用立即阻塞当前线程等待block执行完毕，而block又发现当前线程被阻塞就等着，所以就造成了锁死。
下面是搜到的英文解释。
dispatch_sync does two things:
- queue a block
- blocks the current thread until the block has finished running
Given that the main thread is a serial queue (which means it uses only one thread), the following statement:
dispatch_sync(dispatch_get_main_queue(), ^(){/.../});
will cause the following events:
- dispatch_sync queues the block in the main queue.
- dispatch_sync blocks the thread of the main queue until the block finishes executing.
- dispatch_sync waits forever because the thread where the block is supposed to run is blocked.
The key to understanding this is that dispatch_sync does not execute blocks, it only queues them. Execution will happen on a future iteration of the run loop.
