GCD 用法查询

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

// 合并汇总结果
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
});