多线程之GCD的使用

几个主要的运用方法：

//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

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
});