AdaptedTextColor.swift

import SwiftUI

struct ContentView: View {
    @State private var sliderValue: Double = 0.0
    @State private var backgroundColor: Color = .red

    let colors: [Color] = [.red, .orange, .yellow, .green, .blue, .purple, .pink, .mint, .indigo, .teal,  .cyan, .brown, .black]
    
    var body: some View {
        ZStack {
            Rectangle()
                .ignoresSafeArea()
                .foregroundColor(backgroundColor)
            
            VStack {
                Text("This is a Demo")
                    .font(.largeTitle)
                    .foregroundStyle(backgroundColor.adaptedTextColor())
                    .padding(40)
                Slider(value: $sliderValue, in: 0...1)
                    .tint(backgroundColor.adaptedTextColor())
                    .padding()
            }
            
        }
        .onAppear {
            updateColor()
        }
        .onChange(of: sliderValue) {
            updateColor()
        }
    }
    
    private func updateColor() {
        let colorIndex = Int(sliderValue * Double(colors.count - 1))
        backgroundColor = colors[colorIndex]
    }
}

#Preview {
    ContentView()
}

extension Color {
    func luminance() -> Double {
        // Convert SwiftUI Color to UIColor
        let uiColor = UIColor(self)
        
        // Extract RGB values
        var red: CGFloat = 0
        var green: CGFloat = 0
        var blue: CGFloat = 0
        uiColor.getRed(&red, green: &green, blue: &blue, alpha: nil)
        
        // Compute luminance.
        return 0.2126 * Double(red) + 0.7152 * Double(green) + 0.0722 * Double(blue)
    }
    
    func isLight() -> Bool {
        return luminance() > 0.5
    }
    
    func adaptedTextColor() -> Color {
        return isLight() ? Color.black : Color.white
    }
}