当scrollView向下滚动时，按钮会向下移动消失，等scrollview向上滚动时，按钮则又会出现。

Simulator Screen Shot - iPhone X - 2018-08-09 at 09.45.03.png

//实现scrollView代理
- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset{
    //竖直滑动时 判断是上滑还是下滑
    
//    NSLog(@"----------%g", velocity.y);
    
    if(velocity.y > 0){
        //上滑
//        NSLog(@"上滑");
        [UIView animateWithDuration:0.25 animations:^{
            
            CGRect frame = self.topicBtn.frame;
            frame.origin.y = XJ_ScreenHeight;
            self.topicBtn.frame  = frame;
        }];
    }else if (velocity.y < -1){
        //下滑
//        NSLog(@"下滑");
        [UIView animateWithDuration:0.25 animations:^{
            
            CGRect frame = self.topicBtn.frame;
            frame.origin.y = XJ_ScreenHeight - XJ_TabbarHeight - 35 - frame.size.height;
            self.topicBtn.frame  = frame;
        }];

    }
//    
//    //水平滑动时 判断是右滑还是左滑
//    if(velocity.x>0){
//        //右滑
//        NSLog(@"右滑");
//    }else{
//        //左滑
//        NSLog(@"左滑");
//    }
}
//点击屏幕顶部触发的方法
- (void)scrollViewDidScrollToTop:(UIScrollView *)scrollView {
    
    [UIView animateWithDuration:0.25 animations:^{
        
        CGRect frame = self.topicBtn.frame;
        frame.origin.y = XJ_ScreenHeight - XJ_TabbarHeight - 35 - frame.size.height;
        self.topicBtn.frame  = frame;
    }];

}