How to Animate Path in SwiftUI

Question

Being unfamiliar with SwiftUI and the fact that there is not much documentation on this new framework yet. I was wondering if anyone was familiar with how it would be possible to animate a Path in SwiftUI.

For example given a view, lets say this simple RingView:

struct RingView : View {   
    var body: some View {
        GeometryReader { geometry in
            Group {
                // create outer ring path
                Path { path in
                    path.addArc(center: center,
                                radius: outerRadius,
                                startAngle: Angle(degrees: 0),
                                endAngle: Angle(degrees: 360),
                                clockwise: true)
                }
                .stroke(Color.blue)

                // create inner ring
                Path { path in
                    path.addArc(center: center,
                                radius: outerRadius,
                                startAngle: Angle(degrees: 0),
                                endAngle: Angle(degrees: 180),
                                clockwise: true)
                }
                .stroke(Color.red)
                .animation(.basic(duration: 2, curve: .linear))
            }
        }
        .aspectRatio(1, contentMode: .fit)
    }
}

What is displayed is:

Now I was wondering how we could go about animating the inner ring, that is the red line inside the blue line. The animation I'm looking to do would be a simple animation where the path appears from the start and traverses to the end.

This is rather simple using CoreGraphics and the old UIKit framework but it doesn't seem like adding a simple .animation(.basic(duration: 2, curve: .linear)) to the inner path and displaying the view with a withAnimation block does anything.

I've looked at the provided Apple tutorials on SwiftUI but they really only cover move/scale animations on more in-depth views such as Image.

Any guidance on how to animate a Path or Shape in SwiftUI?

Answer 1

Animation of paths is showcased in the WWDC session 237 (Building Custom Views with SwiftUI). The key is using AnimatableData. You can jump ahead to 31:23, but I recommend you start at least at minute 27:47.

You will also need to download the sample code, because conveniently, the interesting bits are not shown (nor explained) in the presentation: https://developer.apple.com/documentation/swiftui/drawing_and_animation/building_custom_views_in_swiftui

---

More documentation: Since I originally posted the answer, I continued to investigate how to animate Paths and posted an article with an extensive explanation of the Animatable protocol and how to use it with Paths: https://swiftui-lab.com/swiftui-animations-part1/

---

Update:

I have been working with shape path animations. Here's a GIF.

And here's the code:

IMPORTANT: The code does not animate on Xcode Live Previews. It needs to run either on the simulator or on a real device.

import SwiftUI

struct ContentView : View {
    var body: some View {
        RingSpinner().padding(20)
    }
}

struct RingSpinner : View {
    @State var pct: Double = 0.0

    var animation: Animation {
        Animation.basic(duration: 1.5).repeatForever(autoreverses: false)
    }

    var body: some View {

        GeometryReader { geometry in
            ZStack {
                Path { path in

                    path.addArc(center: CGPoint(x: geometry.size.width/2, y: geometry.size.width/2),
                                radius: geometry.size.width/2,
                                startAngle: Angle(degrees: 0),
                                endAngle: Angle(degrees: 360),
                                clockwise: true)
                }
                .stroke(Color.green, lineWidth: 40)

                InnerRing(pct: self.pct).stroke(Color.yellow, lineWidth: 20)
            }
        }
        .aspectRatio(1, contentMode: .fit)
            .padding(20)
            .onAppear() {
                withAnimation(self.animation) {
                    self.pct = 1.0
                }
        }
    }

}

struct InnerRing : Shape {
    var lagAmmount = 0.35
    var pct: Double

    func path(in rect: CGRect) -> Path {

        let end = pct * 360
        var start: Double

        if pct > (1 - lagAmmount) {
            start = 360 * (2 * pct - 1.0)
        } else if pct > lagAmmount {
            start = 360 * (pct - lagAmmount)
        } else {
            start = 0
        }

        var p = Path()

        p.addArc(center: CGPoint(x: rect.size.width/2, y: rect.size.width/2),
                 radius: rect.size.width/2,
                 startAngle: Angle(degrees: start),
                 endAngle: Angle(degrees: end),
                 clockwise: false)

        return p
    }

    var animatableData: Double {
        get { return pct }
        set { pct = newValue }
    }
}