GCD 2种并发队列，2种串行对象的创建

1.串行队列

// i.获得主队列

    dispatch_queue_t queue = dispatch_get_main_queue();
//       
      ii.自己创建的串行队列

     //串行队列+异步线程。开辟一条新的线程。上面的任务依次执行

    dispatch_queue_t queue=dispatch_queue_create("wangning", DISPATCH_QUEUE_SERIAL);

    dispatch_queue_t queue2=dispatch_queue_create("wangning2", NULL);
   
2. 并发队列

    

dispatch_queue_t queue3=dispatch_queue_create("wangning", DISPATCH_QUEUE_CONCURRENT);

// 1.获得全局的并发队列

    dispatch_queue_t queue =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)

#define DISPATCH_QUEUE_PRIORITY_HIGH 2

#define DISPATCH_QUEUE_PRIORITY_DEFAULT 0

#define DISPATCH_QUEUE_PRIORITY_LOW (-2)

#define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN

3.

异步函数用在主队列上不会开辟新的线程

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