列表界面—上拉，发布按钮消失，下拉，按钮出现

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//MARK: 创建发布商品的btn
-(void)creatPublishGoodsBtn{
    _addButton =[UIButton buttonWithType:UIButtonTypeCustom];
    _addButton.frame =CGRectMake((MainScreenWidth -40)/2, MainScreenHeight -50, 40, 40);
    [_addButton setBackgroundImage:[UIImage imageNamed:@"postNew"] forState:UIControlStateNormal];
    [_addButton addTarget:self action:@selector(postNewTieziInPindao) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:_addButton];
}

//MARK: <scrollView delegate>
- (void) scrollViewDidScroll:(UIScrollView *)scrollView{
    if (scrollView != _tableView) {
        return;
    }
    if (scrollView.contentOffset.y > 0 && (_contentY - scrollView.contentOffset.y < 0)) {
        if (_addButton.hidden == NO) {
            _addButton.hidden = YES;
        }
    }else{
        if (_addButton.hidden == YES) {
            _addButton.hidden = NO;
        }
    }
    _contentY = scrollView.contentOffset.y;
    
}