dispatch_sync may result in dead-lock

如下代碼會引發死鎖

 1  dispatch_block_t block = ^{
 2         for (int i = 0; i < 100; i++) {
 3             NSLog(@"dispatch_sync:%d", i);
 4         }
 5     };
 6     
 7     dispatch_queue_t queue = dispatch_queue_create("FIFO.QUEQE", 0);
 8     dispatch_async(queue, block);
 9 
10     NSLog(@"finish dispatch_asyn");
11     
12     dispatch_sync(dispatch_get_main_queue(), ^{
13         NSLog(@"sync");
14     });
15     
16     NSLog(@"finish dispatch_sync");

View Code

Apple的官方文檔中這麼描述：Calls to dispatch_sync() targeting the current queue will result in dead-lock.

爲何會發生死鎖呢？

當在主線程中執行第一個block時，它沒有將控制權返回給當前線程，即主線程；而後又嘗試在主線程中執行第二個block，此時主線程會一直等待queue完成，於是造成死鎖