UIProgressView reveal progressImage image

I have a UIProgressView and I'm giving it a progressImage that I'd like to reveal as the progress bar progresses. How can I do this?

What currently happens is the whole images always fits inside the progress bar, it just gets compressed. I'd like to, instead, just show a partial (e.g. the left side) of the image.

You'll need to create your own "custom progress view."

One approach is to use a CALayer as a mask on the image view, adjusting the size of the layer to be a percentage of the width of the custom view.

Here's a quick example...

Custom Progress View

class MyProgressView: UIView {
    // image that will be "revealed"
    public var image: UIImage? {
        didSet {
            bkgView.image = image
        }
    }
    public var progress: Float {
        set {
            // keep the value between 0.0 and 1.0
            _progress = max(min(newValue, 1.0), 0.0)
            setNeedsLayout()
        }
        get {
            return _progress
        }
    }
    private var _progress: Float = 0.0
    
    private let bkgView = UIImageView()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() -> Void {
        bkgView.translatesAutoresizingMaskIntoConstraints = false
        addSubview(bkgView)
        let g = self
        NSLayoutConstraint.activate([
            bkgView.topAnchor.constraint(equalTo: g.topAnchor, constant: 0.0),
            bkgView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 0.0),
            bkgView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: 0.0),
            bkgView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: 0.0),
        ])
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        // width to "reveal" will be the percentage of self's width
        let w: CGFloat = bounds.width * CGFloat(_progress)
        var r = bounds
        r.size.width = w
        
        // create a mask layer
        let msk = CALayer()
        // can be any color other than clear
        msk.backgroundColor = UIColor.black.cgColor
        msk.frame = r
        bkgView.layer.mask = msk
    }

}

Sample view controller

class MyProgessVC: UIViewController {

    let myProgressView = MyProgressView()
    let standardProgressView = UIProgressView()
    let infoLabel = UILabel()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        let g = view.safeAreaLayoutGuide

        // make sure we can load the image we want to use for our custom progress view
        guard let img = UIImage(named: "pvBKG") else { return }
        
        // add the standard progress view
        standardProgressView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(standardProgressView)

        // add our custom progress view
        myProgressView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(myProgressView)

        // add a slider to set the progress view percentage
        let slider = UISlider()
        slider.addTarget(self, action: #selector(sliderChanged(_:)), for: .valueChanged)
        
        slider.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(slider)
        
        // add a label to show the current progress
        infoLabel.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(infoLabel)
        infoLabel.textAlignment = .center
        
        NSLayoutConstraint.activate([

            // let's put the standard progress view near the top
            standardProgressView.topAnchor.constraint(equalTo: g.topAnchor, constant: 40.0),
            // with 20-points on each side
            standardProgressView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            standardProgressView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            
            // put our custom "progress view" below the standard one
            myProgressView.topAnchor.constraint(equalTo: standardProgressView.bottomAnchor, constant: 40.0),
            // with 20-points on each side
            myProgressView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            myProgressView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            // we'll make the height equal to the image height
            myProgressView.heightAnchor.constraint(equalToConstant: img.size.height),

            // put the slider below the progress views
            slider.topAnchor.constraint(equalTo: myProgressView.bottomAnchor, constant: 40.0),
            slider.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            slider.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            
            // put the info label below the slider
            infoLabel.topAnchor.constraint(equalTo: slider.bottomAnchor, constant: 40.0),
            infoLabel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            infoLabel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            
        ])
        
        // color for progress view "right-side"
        myProgressView.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
        
        // set the "reveal" image
        myProgressView.image = img
        
        updateInfoLabel()
    }
    @objc func sliderChanged(_ sender: UISlider) {
        // set .progress on each to the slider value
        myProgressView.progress = sender.value
        standardProgressView.progress = sender.value
        updateInfoLabel()
    }
    func updateInfoLabel() {
        infoLabel.text = "\(myProgressView.progress)"
    }

}

We add a "standard" UIProgressView, an instance of our custom MyProgressView, a UISlider to interactively set the progress value, and a label to show the value.

Using this image for the progress view "reveal" image:

It looks like this when running:

If you want to emulate the animation capability of the default UIProgressView (as in calling .setProgress(0.75, animated: true)) you'll have a little more work to do :)