// 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
});