第十一篇、UITableView headerview下拉放大

核心代码：

-(void)createTableViewHeaderView{

    _tableViewHeaderView = [[UIView alloc] initWithFrame:(CGRectMake(, , ScreenWidth, _imageHeight))];
    _headerBackView = [[UIImageView alloc] init];

//    背景图
    _headerBackView.frame = CGRectMake(, , ScreenWidth, _imageHeight);
    _headerBackView.image = [UIImage imageNamed:@"bj1@2x.jpg"];

    [_tableViewHeaderView addSubview:_headerBackView];
    _photoImageView = [[UIImageView alloc] initWithFrame:CGRectMake((ScreenWidth - )/,  ,  ,  )];
    [self.tableViewHeaderView addSubview:self.photoImageView];
    _photoImageView.layer.cornerRadius =  ;
    _photoImageView.layer.masksToBounds = YES;

    _photoImageView.image = [UIImage imageNamed:@"2.jpg"];

    _userNameLabel = [[UILabel alloc] initWithFrame:CGRectMake(, _photoImageView.frame.origin.y + _photoImageView.frame.size.height +  , ScreenWidth,  )];
    _userNameLabel.font = [UIFont fontWithName:@"iconfont" size: ];
    _userNameLabel.text = @"纳兰性德";
    _userNameLabel.textAlignment = ;
    _userNameLabel.font = [UIFont systemFontOfSize:  ];
    _userNameLabel.textColor = [UIColor whiteColor];
    [_tableViewHeaderView addSubview:self.userNameLabel];

    _introduceLabel = [[UILabel alloc] initWithFrame:CGRectMake((ScreenWidth -  )/, _userNameLabel.frame.origin.y + _userNameLabel.frame.size.height +  ,  ,  )];
    _introduceLabel.alpha = .;
    _introduceLabel.text = @"人生若只如初见，何事秋风悲画扇";
    _introduceLabel.textAlignment = ;
    _introduceLabel.font = [UIFont systemFontOfSize: ];
    _introduceLabel.textColor = _userNameLabel.textColor;
    [_tableViewHeaderView addSubview:self.introduceLabel];

    self.tableView.tableHeaderView = _tableViewHeaderView;

}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{

    CGFloat width = self.view.frame.size.width; // 图片宽度
    CGFloat yOffset = scrollView.contentOffset.y;  // 偏移的y值
    if (yOffset < ) {
        CGFloat totalOffset = _imageHeight + ABS(yOffset);
        CGFloat f = totalOffset / _imageHeight;
        self.headerBackView.frame =  CGRectMake(- (width * f - width) / , yOffset, width * f, totalOffset); //拉伸后的图片的frame应该是同比例缩放。
    }

}