CircleProgressViewStyleSizing.swift

import SwiftUI

// this demonstrate preference/onPreferenceChange propagate size value
// to set View size do not work. The value is propagated out as can be
// verified by seeing the print() output from closure of .onPreferenceChange()
// but size value is not reflected inside makeBody()
struct CircleProgressViewStyle: ProgressViewStyle, PreferenceKey {
    static var defaultValue: CGFloat = 0
    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value = nextValue()
    }

    // this is value is set with onPreferenceChange()
    @State var size: CGFloat?

    func makeBody(configuration: Configuration) -> some View {
        GeometryReader { proxy in
            ZStack {
                Circle()
                    .stroke(Color.orange, lineWidth: 5)
                Text("size: \(size ?? CGFloat.nan)")
            }
            .preference(key: Self.self, value: min(proxy.size.width, proxy.size.height))
        }
        // the closure here is called, but size is not reflected inside makeBody
        .onPreferenceChange(Self.self) { print("size is \($0)"); size = $0 }
        // setting frame size here has no effect, why?
        .frame(width: size, height: size)
        .border(Color.red)
    }
}

struct CircleProgressViewStyleDemo: View {
    var body: some View {
        ProgressView(value: 0.5)
            .progressViewStyle(CircleProgressViewStyle())
            .border(Color.yellow)
            .padding()
    }
}

struct CircleProgressViewStyleDemo_Previews: PreviewProvider {
    static var previews: some View {
        CircleProgressViewStyleDemo()
    }
}