利用UIScrollView 使加到其上的图片(下拉放大)

//定义宏
#define WIDTH (self.view.frame.size.width)
#define HEITH 200

@interface RootViewController ()<UIScrollViewDelegate>

@property (nonatomic,retain) UIScrollView *scrollView;
@property (nonatomic,retain) UIImageView *imageView;

@end

@implementation RootViewController
// 创建UIScrollView
- (void)create
{
   _scrollView = [[UIScrollViewalloc] initWithFrame:CGRectMake(0,0, self.view.frame.size.width, self.view.frame.size.height)];
    //控制只能上下移动
   _scrollView.contentSize =CGSizeMake(0,1000);
    // 添加到view上
    [self.viewaddSubview:_scrollView];
    //设置背景色为亮灰色
    _scrollView.backgroundColor = [UIColorlightGrayColor];
    // 释放
    [_scrollView release];
    // 创建UIImageView
   _imageView = [[UIImageViewalloc] initWithFrame:CGRectMake(0,0, self.view.frame.size.width, 200)];
    // 给_imageView 一张图片
   _imageView.image = [UIImageimageNamed:@"lufei"];
    // 把_imageView加到_scrollView
    [_scrollView addSubview:_imageView];
    // 设置代理人
    _scrollView.delegate =self;
    [_imageView release];
}
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    // 偏移量y的变化
   CGFloat dy = scrollView.contentOffset.y;
   NSLog(@"%f", dy);
    // 判断拉倒方向
   if (dy < 0) {
       // 利用公式
       _imageView.frame =CGRectMake(-(-dy * (WIDTH /HEITH)) / 2, dy,WIDTH - dy * (WIDTH /HEITH), HEITH - dy);
    }
}
- (void)viewDidLoad {
    [superviewDidLoad];
    // Do any additional setup after loading the view.
    [selfcreate];
}
@end