UIScrollView缩放

参考这几篇文章：

https://developer.apple.com/library/ios/documentation/windowsviews/conceptual/UIScrollView_pg/ZoomZoom/ZoomZoom.html

http://www.raywenderlich.com/10518/how-to-use-uiscrollview-to-scroll-and-zoom-content

为了使scrollview支持缩放，必须使用scroll view 的代理。

- (UIView*)viewForZoomingInScrollView:(UIScrollView *)scrollView {
    //返回需要缩放的view
    return self.imageView;
}

minimumZoomScale和maximumZoomScale，设置最小和最大的缩放的比例，默认值都为1.0。

- (void)viewDidLoad {
    [super viewDidLoad];
    self.scrollView.minimumZoomScale=0.5;
    self.scrollView.maximumZoomScale=6.0;
    self.scrollView.contentSize=CGSizeMake(1280, 960);
    self.scrollView.delegate=self;
}

UIScrollView提供了两个方法来缩放：setZoomScale:animated: 和 zoomToRect:animated:
setZoomScale:animated: 设置当前的scale zoom scale 到特定的值，值必须在minimumZoomScale 和 maximumZoomScale 之间。也可以直接设置zoomScale属性，等同于调用
setZoomScale:animated:，animated参数为NO。

- (CGRect)zoomRectForScrollView:(UIScrollView *)scrollView withScale:(float)scale withCenter:(CGPoint)center {
 
    CGRect zoomRect;
 
    // The zoom rect is in the content view's coordinates.
    // At a zoom scale of 1.0, it would be the size of the
    // imageScrollView's bounds.
    // As the zoom scale decreases, so more content is visible,
    // the size of the rect grows.
    zoomRect.size.height = scrollView.frame.size.height / scale;
    zoomRect.size.width  = scrollView.frame.size.width  / scale;
 
    // choose an origin so as to get the right center.
    zoomRect.origin.x = center.x - (zoomRect.size.width  / 2.0);
    zoomRect.origin.y = center.y - (zoomRect.size.height / 2.0);
 
    return zoomRect;
}
}

当UIScrollView的zoom结束的时候，会调用

scrollViewDidEndZooming:withView:atScale: 

代理方法。

一个例子：

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // Set a nice title
    self.title = @"Image";
    
    // Set up the image we want to scroll & zoom and add it to the scroll view
    UIImage *image = [UIImage imageNamed:@"photo1.png"];
    self.imageView = [[UIImageView alloc] initWithImage:image];
    self.imageView.frame = (CGRect){.origin=CGPointMake(0.0f, 0.0f), .size=image.size};
    [self.scrollView addSubview:self.imageView];
    
    // Tell the scroll view the size of the contents
    self.scrollView.contentSize = image.size;
    
    //放大
    UITapGestureRecognizer *doubleTapRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(scrollViewDoubleTapped:)];
    doubleTapRecognizer.numberOfTapsRequired = 2;
    doubleTapRecognizer.numberOfTouchesRequired = 1;
    [self.scrollView addGestureRecognizer:doubleTapRecognizer];
    //缩小
    UITapGestureRecognizer *twoFingerTapRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(scrollViewTwoFingerTapped:)];
    twoFingerTapRecognizer.numberOfTapsRequired = 1;
    twoFingerTapRecognizer.numberOfTouchesRequired = 2;
    [self.scrollView addGestureRecognizer:twoFingerTapRecognizer];
}

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    
    // Set up the minimum & maximum zoom scales
    CGRect scrollViewFrame = self.scrollView.frame;
    CGFloat scaleWidth = scrollViewFrame.size.width / self.scrollView.contentSize.width;
    CGFloat scaleHeight = scrollViewFrame.size.height / self.scrollView.contentSize.height;
    CGFloat minScale = MIN(scaleWidth, scaleHeight);
    
    self.scrollView.minimumZoomScale = minScale;
    self.scrollView.maximumZoomScale = 1.0f;
    self.scrollView.zoomScale = minScale;
    
    [self centerScrollViewContents];
}

//使图片居中
- (void)centerScrollViewContents {
    CGSize boundsSize = self.scrollView.bounds.size;
    CGRect contentsFrame = self.imageView.frame;
    
    if (contentsFrame.size.width < boundsSize.width) {
        contentsFrame.origin.x = (boundsSize.width - contentsFrame.size.width) / 2.0f;
    } else {
        contentsFrame.origin.x = 0.0f;
    }
    
    if (contentsFrame.size.height < boundsSize.height) {
        contentsFrame.origin.y = (boundsSize.height - contentsFrame.size.height) / 2.0f;
    } else {
        contentsFrame.origin.y = 0.0f;
    }
    self.imageView.frame = contentsFrame;
}

- (void)scrollViewDoubleTapped:(UITapGestureRecognizer*)recognizer {
    // 1
    CGPoint pointInView = [recognizer locationInView:self.imageView];
 
    // 2
    CGFloat newZoomScale = self.scrollView.zoomScale * 1.5f;
    newZoomScale = MIN(newZoomScale, self.scrollView.maximumZoomScale);
 
    // 3
    CGSize scrollViewSize = self.scrollView.bounds.size;
 
    CGFloat w = scrollViewSize.width / newZoomScale;
    CGFloat h = scrollViewSize.height / newZoomScale;
    CGFloat x = pointInView.x - (w / 2.0f);
    CGFloat y = pointInView.y - (h / 2.0f);
 
    CGRect rectToZoomTo = CGRectMake(x, y, w, h);
 
    // 4
    [self.scrollView zoomToRect:rectToZoomTo animated:YES];
}

- (void)scrollViewTwoFingerTapped:(UITapGestureRecognizer*)recognizer {
    // Zoom out slightly, capping at the minimum zoom scale specified by the scroll view
    CGFloat newZoomScale = self.scrollView.zoomScale / 1.5f;
    newZoomScale = MAX(newZoomScale, self.scrollView.minimumZoomScale);
    [self.scrollView setZoomScale:newZoomScale animated:YES];
}

UIScrollView的代理方法

- (UIView*)viewForZoomingInScrollView:(UIScrollView *)scrollView {
    // Return the view that you want to zoom
    return self.imageView;
}

- (void)scrollViewDidZoom:(UIScrollView *)scrollView {
    // The scroll view has zoomed, so you need to re-center the contents
    [self centerScrollViewContents];
}