多用GCD，少用performSelect系列方法

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];

　　});