1.1、GCD全局队列的四个优先级1.2、几种本文使用到的GCD类型1.3、dispatch_async/dispatch_async_f 1.4、dispatch_sync/dispatch_sync_f
2.1、dispatch_after2.2、dispatch_apply 2.3、dispatch_once
3.dispatch_group
4.dispatch_semaphore (信号量)
5.多线程系列
6.GCD死锁
