Environment.swift

import SwiftUI

extension View {
  func printType() -> Self {
    print(type(of: self))
    return self
  }
}

// Need to make a key; all it needs is a default value.

struct SecretMessageKey: EnvironmentKey {
  static let defaultValue = "some secret"
}

// We need to extend Environment with our key 
extension EnvironmentValues {
  var secretMessage: String {
    get {
      self[SecretMessageKey.self]
    }
    set {
      self[SecretMessageKey.self] = newValue
    }
  }
}

// Need to add a modifier for views so they can inject a value for the key into the environment
extension View {
  func secretMessage(_ message: String) -> some View {
    environment(\.secretMessage, message)
  }
}

// We can now use the environment

struct MyView: View {
  @Environment(\.secretMessage) var message
  var body: some View {
    Text(message)
  }
}

// Experiment with EnvironmentObject... You must remember to
// inject in or your app crashes.  You can only inject one instance of 
// a type.

final class ViewModel: ObservableObject {
  @Published var total: Int = 0
}

struct ContentView: View {
  
  @Environment(\.colorScheme) var colorScheme
  @EnvironmentObject var scoreboard: ViewModel
  @EnvironmentObject var scoreboard2: ViewModel
  
  var body: some View {
    VStack {
      MyView().secretMessage("sdfs")
      MyView()
      Text("The score is \(scoreboard.total)")
      Text("The score is \(scoreboard2.total)")
        .foregroundColor(colorScheme == .dark ? Color.purple : Color.black)
    }.font(.largeTitle)
      .secretMessage("Yay")
      .printType()
      .onAppear {
        self.scoreboard.total += 1 // This will modify both scoreboard and scoreboard2 !
    }
  }
}