iOS UISLider with gradient layer

Question

I'm building an iOS application in which I have to implement a custom UISlider. The problem is that the built-in UISlider doesn't support gradient track. Another issue is my UI style guide shows that the current tracking value rectangle should be a gradient of two colors as shown in the image

How can I build a customized version of the UISlider? I have thought of either subclassing the existing one or by building a UIControl subclass.

I'm using xcode 9.4 and swift 4.2

Thanks in advance!

Answer 1

I ended up solving the problem by setting the gradient layer as an image for the slider minimum track image as following:

@IBDesignable
class GradientSlider: UISlider {

    @IBInspectable var thickness: CGFloat = 20 {
        didSet {
            setup()
        }
    }

    @IBInspectable var sliderThumbImage: UIImage? {
        didSet {
            setup()
        }
    }

    func setup() {
        let minTrackStartColor = Palette.SelectiveYellow
        let minTrackEndColor = Palette.BitterLemon
        let maxTrackColor = Palette.Firefly
        do {
            self.setMinimumTrackImage(try self.gradientImage(
            size: self.trackRect(forBounds: self.bounds).size,
            colorSet: [minTrackStartColor.cgColor, minTrackEndColor.cgColor]),
                                  for: .normal)
            self.setMaximumTrackImage(try self.gradientImage(
            size: self.trackRect(forBounds: self.bounds).size,
            colorSet: [maxTrackColor.cgColor, maxTrackColor.cgColor]),
                                  for: .normal)
            self.setThumbImage(sliderThumbImage, for: .normal)
        } catch {
            self.minimumTrackTintColor = minTrackStartColor
            self.maximumTrackTintColor = maxTrackColor
        }
    }

    func gradientImage(size: CGSize, colorSet: [CGColor]) throws -> UIImage? {
        let tgl = CAGradientLayer()
        tgl.frame = CGRect.init(x:0, y:0, width:size.width, height: size.height)
        tgl.cornerRadius = tgl.frame.height / 2
        tgl.masksToBounds = false
        tgl.colors = colorSet
        tgl.startPoint = CGPoint.init(x:0.0, y:0.5)
        tgl.endPoint = CGPoint.init(x:1.0, y:0.5)

        UIGraphicsBeginImageContextWithOptions(size, tgl.isOpaque, 0.0);
        guard let context = UIGraphicsGetCurrentContext() else { return nil }
        tgl.render(in: context)
        let image = 

    UIGraphicsGetImageFromCurrentImageContext()?.resizableImage(withCapInsets: 
        UIEdgeInsets.init(top: 0, left: size.height, bottom: 0, right: size.height))
        UIGraphicsEndImageContext()
        return image!
    }

    override func trackRect(forBounds bounds: CGRect) -> CGRect {
        return CGRect(
            x: bounds.origin.x,
            y: bounds.origin.y,
            width: bounds.width,
            height: thickness
        )
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }


}