Canvas で Apple ロゴっぽいレインボーなレンダリングをするやつ。

ContentView.swift

// 🌱 Inspired by and special thanks!
// https://gist.github.com/Koshimizu-Takehito/737381f5e55678e691205fe11fe16e93

import SwiftUI

struct ContentView: View {
    var body: some View {
        HStack(spacing: 12) {
            ForEach(["applelogo", "swift"], id: \.self) { name in
                VStack(spacing: 12) {
                    Group {
                        RainbowLogo(systemName: name)
                            .frame(width: 160, height: 160)
                        RainbowLogo(systemName: name)
                            .frame(width: 160, height: 120)
                        RainbowLogo(systemName: name)
                            .frame(width: 120, height: 160)
                    }
                    .border(.red)
                }
            }
        }
    }
}

private let colors: [Color] = [
    .green,
    .green,
    .green,
    .yellow,
    .orange,
    .red,
    .purple,
    .blue
]

struct RainbowLogo: View {
    let systemName: String

    @State private var ratio: CGFloat = 1.0

    var body: some View {
        ZStack {
            Canvas { context, size in
                let canvasRatio = size.width / size.height
                
                //
                // Compute frame of image.
                //
                let w: CGFloat
                let h: CGFloat
                
                if ratio > canvasRatio {
                    w = size.width
                    h = size.width * (1 / ratio)
                } else {
                    w = size.height * ratio
                    h = size.height
                }
                
                let x: CGFloat = (size.width - w) / 2
                let y: CGFloat = (size.height - h) / 2
                
                //
                // Clip to image.
                //
                context.clipToLayer { context in
                    let rect = CGRect(x: x, y: y, width: w, height: h)
                    context.draw(Image(systemName: systemName), in: rect)
                }
                
                //
                // Render borders.
                //
                var rect = CGRect(x: x, y: y, width: w, height: h / CGFloat(colors.count))
                for color in colors {
                    context.fill(Path(rect), with: .color(color))
                    rect.origin.y += rect.height
                }
            } symbols: {
                Color.clear.frame(width: ratio) // ☑️ Invalidates canvas by `ratio` was changed.
            }
            
            //
            // Get ratio of image. (not rendered)
            //
            Image(systemName: systemName)
                .hidden()
                .readSize {
                    ratio = $0.width / $0.height
                }
        }
    }
}

extension View {
    func readSize(perform: @escaping (CGSize) -> ()) -> some View {
        self.background(
            GeometryReader { geometry in
                Color.clear
                    .preference(key: SizeKey.self, value: geometry.size)
            }
        )
        .onPreferenceChange(SizeKey.self, perform: perform)
    }
}

struct SizeKey: PreferenceKey {
    static var defaultValue: CGSize = .zero
    
    static func reduce(value: inout CGSize, nextValue: () -> CGSize) {
        value = nextValue()
    }
}