实现tablewViewHeader的图片放大效果

效果图：

headerImageViewZoom.gif

其实实现比较简单，只要把要放大的ImageView的Y值和height根据拖动的Y值来改变就有这的效果了

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    CGFloat offsetY = scrollView.contentOffset.y;
    NSLog(@"%f",offsetY);
    
    //多少距离开始放大
    CGFloat zoomHeight = 100;
    //设置下拉图片放大
    if (offsetY < - zoomHeight) {
        CGRect frame = self.headerView.backgroundView.frame;
        NSLog(@"前frame:%@",NSStringFromCGRect(frame));
        frame.origin.y = offsetY;
        frame.size.height = - offsetY;
        NSLog(@"后frame:%@",NSStringFromCGRect(frame));
        self.headerView.backgroundView.frame = frame;
    }
}