Example of how GradientMap can be used to control the color of a view dependent on its size

ColorBasedOnSizeDemo.swift

//
//  ColorBasedOnSizeDemo.swift
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
//  of the Software, and to permit persons to whom the Software is furnished to do so,
//  subject to the following conditions:

//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
//  INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
//  PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
//  COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
//  AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
//  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//
//  Created by Adam Fordyce on 24/01/2020.
//  Copyright © 2020 Adam Fordyce. All rights reserved.
//

import SwiftUI
import PureSwiftUITools

private let gradientMap = GradientMap([.pureYellow, .pureRed], withResolution: 50)

struct ColorBasedOnSizeDemo: View {
    @State private var toggle = false
    var body: some View {
        VStack(spacing: 5) {
            ForEach(0..<30) { index in
                ColorBand(trigger: self.toggle)
            }
        }
        .onAppear {
            every (1) { timer in
                withAnimation(Animation.easeInOut(duration: 2)) {
                    self.toggle.toggle()
                }
            }
        }
    }
}

private struct ColorBand: View {
    let trigger: Bool
    var body: some View {
        let randomValue = 1.0.random()
        return Rectangle()
            .fillColor(gradientMap.colorAt(randomValue))
            .height(4 + randomValue * 150)
            .greedyWidth()
    }
}

struct ColorBasedOnSizeDemo_Previews: PreviewProvider {
    struct ColorBasedOnSizeDemo_Harness: View {

        var body: some View {
            ColorBasedOnSizeDemo()
        }
    }

    static var previews: some View {
        ColorBasedOnSizeDemo_Harness()
    }
}