GCD一些用法

很多细节处理上需要用到多线程，so,GCD的用处便来了。下面是一些较简单常用的用法：

//  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（这个用处较为广泛）：

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

});