DependentEnvironment.swift

//
//  ContentView.swift
//  DependentEnvironment
//
//  Created by Soroush Khanlou on 3/27/23.
//

import SwiftUI

struct Inner: View {
    @EnvironmentObject var a: OOA
    @EnvironmentObject var b: OOB

    var body: some View {
        Text("Hello, world!")
            .onAppear {
                a.sayHi()
                b.sayHiFromA()
            }
    }
}

struct ContentView: View {
    var body: some View {
            VStack {
                Image(systemName: "globe")
                    .imageScale(.large)
                    .foregroundColor(.accentColor)
                Inner()
            }
        .environmentObject(dependency: OOA.self, { OOB(ooa: $0) })
        .environmentObject(OOA())
    }
}

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

final class OOA: ObservableObject {
    func sayHi() {
        print("hi from a")
    }
}

final class OOB: ObservableObject {
    let ooa: OOA

    init(ooa: OOA) {
        self.ooa = ooa
    }

    func sayHiFromA() {
        ooa.sayHi()
    }
}

struct DependentEnvironmentModifier<Dependency: ObservableObject, Output: ObservableObject>: ViewModifier {

    let constructor: (Dependency) -> Output

    @EnvironmentObject var dependency: Dependency

    func body(content: Content) -> some View {
        content
            .environmentObject(constructor(dependency))
    }
}

extension View {
    func environmentObject<D: ObservableObject, O: ObservableObject>(dependency: D.Type, _ constructor: @escaping (D) -> O) -> some View {
        self.modifier(DependentEnvironmentModifier<D, O>(constructor: constructor))
    }
}