iOS——系统提供的dispatch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 excuted once

});

// 延迟2秒执行：

double delayInSeconds = 2.0;

dispatch_time_y popTime = dispatch_time（DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);

dispatch_after(popTime, dispatch_get_main_queue(), ^(void){

　　// code to be excuted on the main queue after delay

});