iOS toast 的连续显示

1. 新的 toast 来了, 如果当前的 toast 还没有结束, 需要停止当前的延迟计时器, 重新开启一个新的计时器.
2. 新的 toast 来了, 会因为动画地显示而闪一下, 可以根据 toast 是否显示, 来确定是否需要添加动画.

   #pragma mark 隐藏视图
   - (void)hideSelfViewWithDelay{
       [self performSelector:@selector(hideSelfView) withObject:nil afterDelay:2];
   }
   
   - (void)cancelHideSelfViewWithDelay{
       [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(hideSelfView) object:nil];
   }
   
   
   /**
    更新 toast 提示, 重新延迟2秒显示
    */
   - (void)restartHideSelfViewWithDelay{
       [self cancelHideSelfViewWithDelay];
       [self hideSelfViewWithDelay];
   }
   
   
   - (void)hideSelfView{
       self.alpha = 1;
       [UIView animateWithDuration:0.25 animations:^{
           self.alpha = 0.1;
       } completion:^(BOOL finished) {
           self.hidden = YES;
       }];
   }
   
   
   /**
    如果已经显示, 就不添加动画
    */
   - (void)showSelfView{
       if (self.hidden == YES) {
           self.hidden = NO;
           self.alpha = 0.1;
           [UIView animateWithDuration:0.25 animations:^{
               self.alpha = 1;
           }];
       }
     
       [self restartHideSelfViewWithDelay];
   }
   
   - (void)dealloc{
       [self cancelHideSelfViewWithDelay];
   }