UIScrollView with Embedded UIImageView; how to get the image to fill the screen

UIKit/Programmatic UI

I have an UIScrollView with an UIImageView inside. The image is set by user selection and can have all kinds of sizes. What I want is that the image initially fills the screen (view) and then can be zoomed and scrolled all the way to the edges of the image.

If I add the ImageView directly to the view (no scrollView), I get it to fill the screen with the following code:

        mapImageView.image = ProjectImages.projectDefaultImage
        mapImageView.translatesAutoresizingMaskIntoConstraints = false
        mapImageView.contentMode = .scaleAspectFill
        view.addSubview(mapImageView)

Now the same with the scrollView and the embedded imageView:

        view.insertSubview(mapImageScrollView, at: 0)
        mapImageScrollView.delegate = self
        mapImageScrollView.translatesAutoresizingMaskIntoConstraints = false
        mapImageScrollView.contentMode = .scaleAspectFill
        mapImageScrollView.maximumZoomScale = 4.0
        mapImageScrollView.pinToEdges(of: view, safeArea: true)
        
        mapImageView.image = ProjectImages.projectDefaultImage
        mapImageView.translatesAutoresizingMaskIntoConstraints = false
        mapImageView.contentMode = .scaleAspectFill
        mapImageScrollView.addSubview(mapImageView)

And now, if the image's height is smaller than the view's height, the image does not fill the screen and I'm left with a blank view area below the image. I can zoom and scroll ok, and then the image does fill the view.

Adding contsraints will fill the view as I want, but interferes with the zooming and scrolling and prevents me getting to the edges of the image when zoomed in.

How to set this up correctly ?

You might find this useful...

It allows you to zoom an image in a scrollView, starting with it centered and maintaining aspect ratio.

Here's a complete implementation. It has two important variables at the top:

// can be .scaleAspectFill or .scaleAspectFit
var fitMode: UIView.ContentMode = .scaleAspectFill

// if fitMode is .scaleAspectFit, allowFullImage is ignored
// if fitMode is .scaleAspectFill, image will start zoomed to .scaleAspectFill
//  if allowFullImage is false, image will zoom back to .scaleAspectFill if "pinched in"
//  if allowFullImage is true, image can be "pinched in" to see the full image
var allowFullImage: Bool = true

Everything is done via code - no @IBOutlet or other connections - so just create add a new view controller and assign its custom class to ZoomAspectViewController (and edit the name of the image you want to use):

class ZoomAspectViewController: UIViewController, UIScrollViewDelegate {
    
    var scrollView: UIScrollView!
    var imageView: UIImageView!
    var imageViewBottomConstraint: NSLayoutConstraint!
    var imageViewLeadingConstraint: NSLayoutConstraint!
    var imageViewTopConstraint: NSLayoutConstraint!
    var imageViewTrailingConstraint: NSLayoutConstraint!
    
    // can be .scaleAspectFill or .scaleAspectFit
    var fitMode: UIView.ContentMode = .scaleAspectFit
    
    // if fitMode is .scaleAspectFit, allowFullImage is ignored
    // if fitMode is .scaleAspectFill, image will start zoomed to .scaleAspectFill
    //  if allowFullImage is false, image will zoom back to .scaleAspectFill if "pinched in"
    //  if allowFullImage is true, image can be "pinched in" to see the full image
    var allowFullImage: Bool = true
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        guard let img = UIImage(named: "myImage") else {
            fatalError("Could not load the image!!!")
        }
        
        scrollView = UIScrollView()
        imageView = UIImageView()
        
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        imageView.translatesAutoresizingMaskIntoConstraints = false
        
        imageView.contentMode = .scaleToFill
        
        scrollView.addSubview(imageView)
        view.addSubview(scrollView)
        
        // respect safe area
        let g = view.safeAreaLayoutGuide
        
        imageViewTopConstraint = imageView.topAnchor.constraint(equalTo: scrollView.topAnchor)
        imageViewBottomConstraint = imageView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor)
        imageViewLeadingConstraint = imageView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor)
        imageViewTrailingConstraint = imageView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor)

        NSLayoutConstraint.activate([
            
            scrollView.topAnchor.constraint(equalTo: g.topAnchor),
            scrollView.bottomAnchor.constraint(equalTo: g.bottomAnchor),
            scrollView.leadingAnchor.constraint(equalTo: g.leadingAnchor),
            scrollView.trailingAnchor.constraint(equalTo: g.trailingAnchor),

            imageViewTopConstraint,
            imageViewBottomConstraint,
            imageViewLeadingConstraint,
            imageViewTrailingConstraint,
            
        ])
        
        scrollView.delegate = self
        scrollView.minimumZoomScale = 0.1
        scrollView.maximumZoomScale = 5.0
        
        imageView.image = img
        imageView.frame.size = img.size
        
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)
        coordinator.animate(alongsideTransition: { _ in
            self.updateMinZoomScaleForSize(size, shouldSize: (self.scrollView.zoomScale == self.scrollView.minimumZoomScale))
            self.updateConstraintsForSize(size)
        }, completion: {
            _ in
        })
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        updateMinZoomScaleForSize(scrollView.bounds.size)
        updateConstraintsForSize(scrollView.bounds.size)
        
        if fitMode == .scaleAspectFill {
            centerImageView()
        }
        
    }
    
    func updateMinZoomScaleForSize(_ size: CGSize, shouldSize: Bool = true) {
        guard let img = imageView.image else {
            return
        }
        
        var bShouldSize = shouldSize
        
        let widthScale = size.width / img.size.width
        let heightScale = size.height / img.size.height
        
        var minScale = min(widthScale, heightScale)
        let startScale = max(widthScale, heightScale)
        
        if fitMode == .scaleAspectFill && !allowFullImage {
            minScale = startScale
        }
        if scrollView.zoomScale < minScale {
            bShouldSize = true
        }
        scrollView.minimumZoomScale = minScale
        if bShouldSize {
            scrollView.zoomScale = fitMode == .scaleAspectFill ? startScale : minScale
        }
    }
    
    func scrollViewDidZoom(_ scrollView: UIScrollView) {
        updateConstraintsForSize(scrollView.bounds.size)
    }
    
    func centerImageView() -> Void {
        let yOffset = (scrollView.frame.size.height - imageView.frame.size.height) / 2
        let xOffset = (scrollView.frame.size.width - imageView.frame.size.width) / 2
        scrollView.contentOffset = CGPoint(x: -xOffset, y: -yOffset)
    }
    
    func updateConstraintsForSize(_ size: CGSize) {
        let yOffset = max(0, (size.height - imageView.frame.height) / 2)
        imageViewTopConstraint.constant = yOffset
        imageViewBottomConstraint.constant = yOffset
        
        let xOffset = max(0, (size.width - imageView.frame.width) / 2)
        imageViewLeadingConstraint.constant = xOffset
        imageViewTrailingConstraint.constant = xOffset
        
        view.layoutIfNeeded()
    }
    
    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }
    
}

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Edit

As an example, I used this image (2560 x 1440):

and I get this result on launch:

and maximum zoom in (5.0) scrolled to top-center:

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Edit 2

Same image, at launch, with:

var fitMode: UIView.ContentMode = .scaleAspectFill

instead of .scaleAspectFit: