Custom ViewModifier in SwiftUI - https://programmingwithswift.com/viewmodifier-in-swiftui/

CustomerViewModifier.swift

import SwiftUI

struct GreenButtonStyle: ViewModifier {
    func body(content: Content) -> some View {
        return content
        .foregroundColor(.white)
        .background(Color.green)
        .border(Color(red: 7/255,
                      green: 171/255,
                      blue: 67/255),
                width: 5)
    }
}

struct BlueButtonStyle: ViewModifier {
    func body(content: Content) -> some View {
        return content
        .foregroundColor(.white)
        .background(Color.blue)
        .border(Color(red: 7/255,
                      green: 42/255,
                      blue: 171/255),
                width: 5)
    }
}

struct StyledButton: ViewModifier {
    enum ButtonColor {
        case green
        case blue
        
        func backgroundColor() -> Color {
            switch self {
            case .green:
                return Color.green
            case .blue:
                return Color.blue
            }
        }
        
        func borderColor() -> Color {
            switch self {
            case .green:
                return Color(red: 7/255,
                             green: 171/255,
                             blue: 67/255)
            case .blue:
                return Color(red: 7/255,
                             green: 42/255,
                             blue: 171/255)
            }
        }
    }
    
    let buttonColor: ButtonColor
    
    func body(content: Content) -> some View {
        return content
            .foregroundColor(.white)
            .background(buttonColor.backgroundColor())
            .border(buttonColor.borderColor(),
                    width: 5)
    }
}

struct Track: ViewModifier {
    let eventName: String

    func body(content: Content) -> some View {
        return content.simultaneousGesture(TapGesture().onEnded({
            print(self.eventName)
        }))
    }
}

struct BorderChange: ViewModifier {
    enum BorderColor: CaseIterable {
        case black
        case blue
        case red
        
        func color() -> Color {
            switch self {
            case .black:
                return .black
            case .blue:
                return .blue
            case .red:
                return .red
            }
        }
        
        func next() -> BorderColor {
            // 1
            let allColors = BorderColor.allCases

            // 2
            if let lastIndex = allColors.lastIndex(of: self) {
                
                // 3
                if lastIndex + 1 == allColors.count {
                    return allColors[0]
                }
                
                // 4
                return allColors[lastIndex + 1]
            }

            // 5
            return self
        }
    }
    
    @State var currentState = BorderColor.black
    
    func body(content: Content) -> some View {
        return content.border(self.currentState.color(),
                              width: 5).simultaneousGesture(TapGesture().onEnded({
            self.currentState = self.currentState.next()
        }))
    }
}

struct ContentView: View {
    var body: some View {
        Button(action: {
            print("Button Pressed")
        }, label: {
            Text("Button").padding()
        })
        .modifier(BorderChange())
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}