延时的三种方式

1.NSThread中的方法

        // 2.0s以后会自动调用self的hidHUD方法
        [self performSelector:@selector(hideHUD) withObject:nil afterDelay:1.5];

2.GCD中的延时

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        self.hud.alpha = 0.0;
    });

3.定时器延时

[NSTimer scheduledTimerWithTimeInterval:1.5 target:self selector:@selector(hideHUD) userInfo:nil repeats:NO];