定时任务

- 方法1：performSelector

```objc
// 1.5s后自动调用self的hideHUD方法
[self performSelector:@selector(hideHUD) withObject:nil afterDelay:1.5];
```
- 方法2：GCD

```objc
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
// 1.5s后自动执行这个block里面的代码
self.hud.alpha = 0.0;
});
```
- 方法3：NSTimer

```objc
// 1.5s后自动调用self的hideHUD方法
[NSTimer scheduledTimerWithTimeInterval:1.5 target:self selector:@selector(hideHUD) userInfo:nil repeats:NO];
// repeats如果为YES，意味着每隔1.5s都会调用一次self的hidHUD方法
```