progress.swift

//

import SwiftUI

struct MyProgress: View, Animatable {
    var progress: Double

    var animatableData: Double {
        get { progress }
        set { progress = newValue }
    }

    var lineWidth: CGFloat = 10
    var circleOpacity: CGFloat {
        var start = 0.95
        return max(0, progress-start)/(1-start)
    }

    var body: some View {
        let trimmed = Circle()
            .rotation(.degrees(-90))
            .trim(from: 0, to: progress)

        ZStack {
            trimmed
                .stroke(lineWidth: lineWidth)
            Circle()
                .frame(width: lineWidth, height: lineWidth)
                .opacity(circleOpacity)
        }
    }
}

struct ContentView: View {
    @State var progress: Double = 0
    var body: some View {
        VStack {
            Button("Toggle") { progress = progress == 0 ? 1 : 0}
            Slider(value: $progress, in: 0...1)
            MyProgress(progress: progress)
                .animation(.easeInOut(duration: 3), value: progress)
        }
    }
}

#Preview {
    ContentView()
}