例如，要延后执行某项任务，可以有下面两种实现方式，而我们应该优先考虑第二种：
// Using performSelect: withObject: afterDelay:
[self performSelector: @selector(dosomething)
withObject:nil
waitUntilDone:NO];
// Using dispatch_after
dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW,(int64_t)(5.0 * NSEC_PER_SEC));
dispatch_after(time, dispatch_get_main_queue(),^(void){
[self dosomething];
});
想把任务放在主线程中执行，也可以有下面两种方式，而我们还是应该优先选后者：
// Using performSelectOnMainThread: withObject: waitUntilDone:
[self performSelectOnMainThread:@selector:(dosomething) withObject:nil waitUntilDone:NO];
//Using dispatch_async
//(or if waitUntilDone is YES, then dispatch_sync)
dispatch_async (dispatch_get_main_queue(),^{
[self doSomething];
});