在开发过程中经常会用到：几秒后 执行某个方法

1、NSRunLoop

[self performSelector:@selector(laterExecute) withObject:nil afterDelay:5.0f];

2、定时器

[NSTimer scheduledTimerWithTimeInterval:5.0f target:self selector:@selector(laterExecute) userInfo:nil repeats:NO];

3、GCD

    double delayInSeconds = 5.0;
    __block ViewController *selfBlcok = self;
    
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void) {
        [selfBlcok laterExecute];
    });