ios - dispatch_sync 中的死锁

Question

以下代码中的死锁。为什么？

dispatch_queue_t queue = dispatch_get_main_queue(); 
dispatch_async(queue, ^{
    dispatch_sync(queue, ^{
        NSLog(@"Hello?");
    }); 
});

Answer 1

这是因为 dispatch_sync 会阻塞主线程线程，直到该 block 运行完毕。

所以它会永远等待，因为 block 应该运行的线程(主线程)被阻塞了。

该代码可以简化为

dispatch_sync(dispatch_get_main_queue(), ^{
    NSLog(@"Hello?");
}); 

摘自Concurrency Programming Guide的最后一段

Do not call the dispatch_sync function from a task that is executing on the same queue that you pass to your function call. Doing so will deadlock the queue. If you need to dispatch to the current queue, do so asynchronously using the dispatch_async function."

长话短说

该代码尝试在被阻塞的线程上运行代码块。