死锁

往当前串行队列 同步添加任务，会造成死锁

performSelector: withObject: afterDelay:

在子线程上执行这个方法，这个方法本质向runloop中添加timer,要做线程保活

performSelector: onThread: withObject: waitUntilDone:

子线程上执行方法，子线程要做线程保活

队列组

队列组的应用：任务1，任务2异步执行；任务1，任务2完成后，w执行任务

   dispatch_group_t group = dispatch_group_create();
   dispatch_queue_t queue = dispatch_queue_create("com.bien.abc", DISPATCH_QUEUE_CONCURRENT);
   
   dispatch_group_async(group, queue, ^{
       //task1
   });
   
   dispatch_group_async(group, queue, ^{
       //task2
   });

   
   
   dispatch_group_notify(group, queue, ^{
//        task3
   });