Cocoa-touch – How to zoom in/out an UIImage object when user pinches screen

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I would like to zoom in/out an UIImage object when the user performs the standard pinch action on my application. I'm currently using a UIImageView to display my image, if that detail helps in any way.

I'm trying to figure out how to do this, but no such luck so far.

Any clues?

Best Answer

As others described, the easiest solution is to put your UIImageView into a UIScrollView. I did this in the Interface Builder .xib file.

In viewDidLoad, set the following variables. Set your controller to be a UIScrollViewDelegate.

- (void)viewDidLoad {
    [super viewDidLoad];
    self.scrollView.minimumZoomScale = 0.5;
    self.scrollView.maximumZoomScale = 6.0;
    self.scrollView.contentSize = self.imageView.frame.size;
    self.scrollView.delegate = self;
}

You are required to implement the following method to return the imageView you want to zoom.

- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView
{
    return self.imageView;
}

In versions prior to iOS9, you may also need to add this empty delegate method:

- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(CGFloat)scale
{
}

The Apple Documentation does a good job of describing how to do this:

Related Solutions

R – Memory management when using UIImage

[UIImage imageNamed:]

caches the image you're loading into memory. This is great if you want to reuse the same set of images over and over again, but if you are constantly showing different (or large) images, then you should use:

NSString *fileLocation = [[NSBundle mainBundle] pathForResource:fileName ofType:extension];
NSData *imageData = [NSData dataWithContentsOfFile:fileLocation];

[UIImage imageWithData:imageData];

instead.

Ios – How to crop a UIImageView to a new UIImage in ‘aspect fit’ mode

Based on my other answer to a similar question I have written this method for a UIImageView category:

-(CGRect) cropRectForFrame:(CGRect)frame
{
    NSAssert(self.contentMode == UIViewContentModeScaleAspectFit, @"content mode must be aspect fit");

    CGFloat widthScale = self.bounds.size.width / self.image.size.width;
    CGFloat heightScale = self.bounds.size.height / self.image.size.height;

    float x, y, w, h, offset;
    if (widthScale<heightScale) {
        offset = (self.bounds.size.height - (self.image.size.height*widthScale))/2;
        x = frame.origin.x / widthScale;
        y = (frame.origin.y-offset) / widthScale;
        w = frame.size.width / widthScale;
        h = frame.size.height / widthScale;
    } else {
        offset = (self.bounds.size.width - (self.image.size.width*heightScale))/2;
        x = (frame.origin.x-offset) / heightScale;
        y = frame.origin.y / heightScale;
        w = frame.size.width / heightScale;
        h = frame.size.height / heightScale;
    }
    return CGRectMake(x, y, w, h);
}

You can pass in the frame of your green rect (assuming it is a subview of the image view) and get the crop rect back for cropping the UIImage. Note that it only works for 'aspect fit' mode! I haven't tested it though. So, tell me if it works!

Best Answer

Related Solutions

R – Memory management when using UIImage

Ios – How to crop a UIImageView to a new UIImage in ‘aspect fit’ mode

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