GCD 简单使用（二）

系统默认有两个队列

dispatch_get_main_queue 串行

dispatch_get_global_queue 并行

dispatch_queue_t myQueue = dispatch_get_main_queue();

并发 分别有三中权限

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGT,0);

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW,0);

dispatch_once  保证在应用生命周期中让代码永远只运行一次  主要用于写单例 

static dispatch_once_t onceToken;

dispatch_once(&onceToken,^{});

dispatch_after 等几秒钟执行

double afterSeconds = 2.0;

dispatch_time_t afterTime = dispatch_time(DISPATCH_TIME_NOW,afterSeconds * NSEC_PER_SEC);

dispatch_after(afterTime,dispatch_get_main_queue(),^{});  

group 和 notify Dispatch Group机制允许我们监听一组任务是否完成：

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFUILT,0);

dispatch_group_t group = dispatch_group_creat();

dispatch_group_async(group,queue,^{});

dispatch_group_async(group,queue,^{});

dispatch_group_notify(group,dispatch_get_main_queue(),^{});//都运行完后运行。

wait  等待一段时间看是否结束

diapatch_group_wait(group,DISPATCH_TIME_FOREVER);

dispatch_barrier_async 在他之前的任务运行完后它才运行 在它之后的任务必须等他运行完后才能运行。