原理：在ScrollView上面使用捏合手势时，ScrollView会给代理发送一条消息，询问要缩放的是哪个控件，即
-(UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
代理方法，我们在这个代理方法里面返回要缩放的视图即可

@interface ViewController ()<UIScrollViewDelegate>//签代理
{
    UIScrollView *scrolleView;
    UIImageView *imgV;
}

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    //初始化滚动视图
    scrolleView = [[UIScrollView alloc]initWithFrame:self.view.bounds];
    [self.view addSubview:scrolleView];

    //初始化imageview，设置图片
    imgV = [[UIImageView alloc]init];
    imgV.image = [UIImage imageNamed:@"高圆圆"];
    imgV.frame = CGRectMake(0, 0, imgV.image.size.width, imgV.image.size.height);
    [scrolleView addSubview:imgV];

    //设置代理,设置最大缩放和虽小缩放
    scrolleView.delegate = self;
    scrolleView.maximumZoomScale = 5;
    scrolleView.minimumZoomScale = 1;

    //设置UIScrollView的滚动范围和图片的真实尺寸一致
    scrolleView.contentSize = imgV.image.size;

}
//代理方法，告诉ScrollView要缩放的是哪个视图
-(UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    return imgV;
}

效果：

原图片.png

放大.png

缩小.png