ViewController.h

@interface ViewController : UIViewController<UIScrollViewDelegate>
{
    UIScrollView *_scrollView;
}

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    //创建滚动视图
    _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, 320, 480)];
    _scrollView.delegate = self;
    
    //设置最大的方法倍数
    _scrollView.maximumZoomScale = 2;
    //设置最小缩小倍数
    _scrollView.minimumZoomScale = .5;
    
    [self.view addSubview:_scrollView];
    
    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 320, 480)];
    imageView.tag = 101;
    imageView.userInteractionEnabled = YES;
    imageView.image = [UIImage imageNamed:@"0.JPG"];
    [_scrollView addSubview:imageView];
    
    //添加手势
    UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(tapAction)];
    tap.numberOfTapsRequired = 2;
    [imageView addGestureRecognizer:tap];
    
}

- (void)tapAction {

    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:.5];
    
//    _scrollView.zoomScale = 2;

//    if (_scrollView.zoomScale == 2) {
//        _scrollView.zoomScale = 1;
//    }else {
//        _scrollView.zoomScale = 2;
//    }
    
    _scrollView.zoomScale = _scrollView.zoomScale == 2 ? 1:2;
    
    [UIView commitAnimations];
}

#pragma mark - UIScrollView delegate

//用于设置需要放大缩小的视图
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView {

    UIImageView *imageView = (UIImageView *)[self.view viewWithTag:101];

    return imageView;
}

//如果在对视图进行所放的时候会实时调用
- (void)scrollViewDidZoom:(UIScrollView *)scrollView {

    //缩放比例
    CGFloat scale = scrollView.zoomScale;
    NSLog(@"正在缩放,缩放倍数：%f",scale);
}

//将要开始缩放
//- (void)scrollViewWillBeginZooming:(UIScrollView *)scrollView withView:(UIView *)view NS_AVAILABLE_IOS(3_2); // called before the scroll view begins zooming its content
//结束缩放
//- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(CGFloat)scale; // scale between minimum and maximum. called after any 'bounce' animations

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

使用ScrollView缩放图片