Drawing rounded rectangle progress bar, start/end at 315°

Question

I have a rounded rectangular progress bar via a UIBezierPath and CAShapeLayer. The progress stroke animated currently draws 360 degrees clockwise beginning from top center.

My current setup has the stroke starting at 315 degrees, but ends at top center, and am admittedly lost. My goal is to start/end the stroke at 315 degrees. Any guidance would be appreciated!

class ProgressBarView: UIView {
    
    let progressLayer = CAShapeLayer()
    let cornerRadius: CGFloat = 20
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        setupProgressLayer()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupProgressLayer()
    }
    
    private func setupProgressLayer() {
        
        progressLayer.lineWidth = 6
        progressLayer.fillColor = nil
        progressLayer.strokeColor = Constants.style.offWhite.cgColor
        progressLayer.strokeStart = 135 / 360
        progressLayer.lineCap = .round
        
        
        let lineWidth: CGFloat = 6
        let radius = bounds.height / 2 - lineWidth / 2
        let progressPath = UIBezierPath()
        progressPath.move(to: CGPoint(x: lineWidth / 2 + cornerRadius, y: lineWidth / 2))
        progressPath.addLine(to: CGPoint(x: bounds.width - lineWidth / 2 - cornerRadius, y: lineWidth / 2))
        progressPath.addArc(withCenter: CGPoint(x: bounds.width - lineWidth / 2 - cornerRadius, y: lineWidth / 2 + cornerRadius), radius: cornerRadius, startAngle: -CGFloat.pi / 2, endAngle: 0, clockwise: true)
        progressPath.addLine(to: CGPoint(x: bounds.width - lineWidth / 2, y: bounds.height - lineWidth / 2 - cornerRadius))
        progressPath.addArc(withCenter: CGPoint(x: bounds.width - lineWidth / 2 - cornerRadius, y: bounds.height - lineWidth / 2 - cornerRadius), radius: cornerRadius, startAngle: 0, endAngle: CGFloat.pi / 2, clockwise: true)
        progressPath.addLine(to: CGPoint(x: lineWidth / 2 + cornerRadius, y: bounds.height - lineWidth / 2))
        progressPath.addArc(withCenter: CGPoint(x: lineWidth / 2 + cornerRadius, y: bounds.height - lineWidth / 2 - cornerRadius), radius: cornerRadius, startAngle: CGFloat.pi / 2, endAngle: CGFloat.pi, clockwise: true)
        progressPath.addLine(to: CGPoint(x: lineWidth / 2, y: lineWidth / 2 + cornerRadius))
        progressPath.addArc(withCenter: CGPoint(x: lineWidth / 2 + cornerRadius, y: lineWidth / 2 + cornerRadius), radius: cornerRadius, startAngle: CGFloat.pi, endAngle: -CGFloat.pi / 2, clockwise: true)
        progressPath.close()
        
        progressLayer.path = progressPath.cgPath
        
        
        layer.addSublayer(progressLayer)
    }
    
    func setProgress(_ progress: CGFloat) {
        let animation = CABasicAnimation(keyPath: "strokeEnd")
        animation.fromValue = progressLayer.strokeStart
        animation.toValue = progress
        animation.duration = 1
        progressLayer.add(animation, forKey: "progressAnimation")
        progressLayer.strokeEnd = progress
    }
}

Answer 1

If you really want rounded line caps, or you want to be able to easily animate the progress using UIView or CALayer animation, you need to draw the path so that it starts and ends at 315°.

Since 315° is in the middle of one of the rounded corners, you have to start the drawing with a circular arc spanning 45°, and end it with another arc of 45°. However, you can draw the other three corners using a different method of CGMutablePath designed specifically for drawing rounded corners, which simplifies the code.

class ProgressBarView: UIView {
    var progress: CGFloat {
        get { (layer as! CAShapeLayer).strokeEnd }
        set { (layer as! CAShapeLayer).strokeEnd = newValue }
    }

    var cornerRadius: CGFloat { 20 }
    var lineWidth: CGFloat { 6 }

    override class var layerClass: AnyClass { CAShapeLayer.self }

    override func layoutSubviews() {
        super.layoutSubviews()

        let layer = layer as! CAShapeLayer
        layer.lineCap = .round
        layer.lineWidth = lineWidth
        layer.fillColor = nil
        layer.strokeColor = UIColor.gray.cgColor

        let rect = layer.bounds.insetBy(dx: 0.5 * lineWidth, dy: 0.5 * lineWidth)
        let (x0, y0, x1, y1) = (rect.minX, rect.minY, rect.maxX, rect.maxY)

        let r = cornerRadius

        let path = CGMutablePath()
        // No prior move, so path automatically moves to the start of this arc.
        path.addRelativeArc(
            center: CGPoint(x: x0 + r, y: y0 + r),
            radius: r,
            startAngle: 0.625 * 2 * .pi, // 45° above -x axis
            delta: 0.125 * 2 * .pi // 45°
        )
        var current = CGPoint(x: x1, y: y0)
        for next in [
            CGPoint(x: x1, y: y1),
            CGPoint(x: x0, y: y1),
            CGPoint(x: x0, y: y0)
        ] {
            path.addArc(tangent1End: current, tangent2End: next, radius: r)
            current = next
        }

        path.addRelativeArc(
            center: CGPoint(x: x0 + r, y: y0 + r),
            radius: r,
            startAngle: 0.5 * 2 * .pi, // -x axis
            delta: 0.125 * 2 * .pi // 45°
        )
        path.closeSubpath()

        layer.path = path
    }
}

Instead of setting up the CAShapeLayer as a sublayer of the view's layer, I tell the view to use a CAShapeLayer directly. This has the advantage that UIKit will set up animations for changes to the layer's properties, if we make those changes inside a UIView.animate block. Here's the demo code. Notice how when I update the view's progress property, I do it inside an animation block. I don't have to create a CABasicAnimation directly.

struct ContentView: View {
    @State var isComplete = true

    var body: some View {
        VStack {
            ProgressBarViewRep(progress: isComplete ? 1 : 0)
                .aspectRatio(1, contentMode: .fit)

            Toggle("Complete", isOn: $isComplete)
                .toggleStyle(.button)
                .fixedSize()
        }
        .padding()
    }
}

struct ProgressBarViewRep: UIViewRepresentable {
    var progress: CGFloat

    func makeUIView(context: Context) -> ProgressBarView {
        let view = ProgressBarView()
        view.progress = progress
        return view
    }

    func updateUIView(_ uiView: ProgressBarView, context: Context) {
        UIView.animate(withDuration: 1, delay: 0, options: .curveLinear) {
            uiView.progress = progress
        }
    }
}


struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .padding()
    }
}

Answer 2

One way to write this, if you're willing to forgo the rounded line caps, is to use a mask layer. The idea is that your main layer's path is always the full rounded rect, and the mask layer is a circle with a wedge taken out to hide part of the rounded rect. It looks like this:

Here's my revised ProgressBarView code. Note that the code for drawing the rounded rect is considerably simpler because I'm always drawing the whole thing, and because I used insetBy(dx:dy:) instead of repeatedly adding/subtracting lineWidth / 2.

class ProgressBarView: UIView {
    var progress: CGFloat = 0.5 {
        didSet {
            setMaskShape()
        }
    }

    let maskLayer = CAShapeLayer()

    var cornerRadius: CGFloat { 20 }
    var lineWidth: CGFloat { 6 }

    override class var layerClass: AnyClass { CAShapeLayer.self }

    override func layoutSubviews() {
        super.layoutSubviews()

        let layer = self.layer as! CAShapeLayer

        if layer.mask != maskLayer {
            layer.lineWidth = lineWidth
            layer.fillColor = nil
            layer.strokeColor = UIColor.gray.cgColor
            // layer.strokeStart = 135 / 360

            layer.mask = maskLayer
        }

        layer.path = CGPath(
            roundedRect: bounds.insetBy(dx: 0.5 * lineWidth, dy: 0.5 * lineWidth),
            cornerWidth: cornerRadius,
            cornerHeight: cornerRadius,
            transform: nil
        )
        maskLayer.frame = bounds

        setMaskShape()
    }

    private func setMaskShape() {
        let bounds = maskLayer.bounds
        let center = CGPoint(x: bounds.midX, y: bounds.midY)

        let path = CGMutablePath()
        path.move(to: center)
        path.addRelativeArc(
            center: center,
            radius: bounds.size.width + bounds.size.height,
            startAngle: 0.625 * 2 * .pi,
            delta: progress * 2 * .pi
        )
        path.closeSubpath()
        maskLayer.path = path
    }
}

And here is the SwiftUI code I used to test it out:

struct ContentView: View {
    @State var progress: CGFloat = 0.5

    var body: some View {
        VStack {
            ProgressBarViewRep(progress: progress)
                .aspectRatio(1, contentMode: .fit)

            Slider(value: $progress, in: 0 ... 1)
        }
        .padding()
    }
}

struct ProgressBarViewRep: UIViewRepresentable {
    var progress: CGFloat

    func makeUIView(context: Context) -> ProgressBarView {
        return ProgressBarView()
    }

    func updateUIView(_ uiView: ProgressBarView, context: Context) {
        uiView.progress = progress
    }
}


struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .padding()
    }
}