系统提供的dispatch方法

为了方便地使用GCD，苹果提供了一些方法方便我们将block放在主线程或后台线程执行，或者延后执行。

//后台执行
dispatch_async(dispatch_get_global_queue(0, 0), ^{
  // something
});
//主线程执行
dispatch_async(dispatch_get_main_queue(), ^{
  // something
});
//一次性执行
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
 // code to be executed once
});
// 延迟2秒执行
double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
 // code to be executed on the main queue after delay
});

自定义dispatch_queue_t可以用dispatch_queue_create,如：

dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);
dispatch_async(urls_queue, ^{
 // your code
});
dispatch_release(urls_queue);

GCD的高级用法，让后台2个线程并行执行，然后等2个线程都结束后，再汇总执行结果。这个可以使用dispatch_group,dispatch_group_async,dispatch_group_notify来实现：

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
  // 并行执行的线程一
});
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
  // 并行执行的线程二
});
dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{
  // 汇总结果
});