StateObjectvsObservedObject.swift

import SwiftUI
import PlaygroundSupport

class Presenter: ObservableObject {
    @Published var viewModel: ViewModel
    
    private var timer: Timer!
    private var memoryAddress: String!
    
    init(prefix: String) {
        self.viewModel = ViewModel(number: 0, prefix: prefix)
        memoryAddress = String(format: "%p", unsafeBitCast(self, to: Int.self))
        print("Presenter init: \(memoryAddress!) - '\(prefix)'")
    }
    
    func onAppear() {
        print("Presenter onAppear: \(memoryAddress!)")
        timer = Timer.scheduledTimer(withTimeInterval: 2, repeats: true) { [weak self] timer in
            guard let self = self else { return }
            print("Presenter onAppear completion: \(self.memoryAddress!)")
            self.viewModel = ViewModel(number: self.viewModel.number + 1, prefix: self.viewModel.prefix)
        }
    }
    
    deinit {
        print("Presenter deinit: \(memoryAddress!)")
        timer?.invalidate()
    }
}

extension Presenter: Equatable {
    static func == (lhs: Presenter, rhs: Presenter) -> Bool {
        lhs.memoryAddress == rhs.memoryAddress
    }
}

struct ViewModel: Equatable {
    let number: Int
    let prefix: String
    var name: String {
        "\(prefix) \(number)"
    }
}

struct ParentView: View {
    @State var isPresented = false
    
    var body: some View {
        VStack {
            NavigationLink("Push view", destination: getNextView())
        }
    }
    
    private func getNextView() -> some View {
        ContentView(presenterObserved: Presenter(prefix: "ObservedObject"))
            .environmentObject(Presenter(prefix: "EnvironmentObject"))
    }
}

struct ContentView: View {
    @StateObject var presenter = Presenter(prefix: "StateObject")
    @ObservedObject var presenterObserved: Presenter
    @EnvironmentObject var presenterEnvironment: Presenter
    
    @State var text1 = "Hello 1!"
    @State var text2 = "Hello 2!"
    @State var text3 = "Hello 3!"
    
    @State var isPresented = false
    
    @StateObject private var runOnce = RunOnce()
    
    var body: some View {
        VStack {
            Text(text1)
                .padding()
                .onChange(of: presenter.viewModel) { value in
                    text1 = presenter.viewModel.name
                }
            
            Text(text2)
                .padding()
                .onChange(of: presenterObserved.viewModel) { value in
                    text2 = presenterObserved.viewModel.name
                }
            
            Text(text3)
                .padding()
                .onChange(of: presenterEnvironment.viewModel) { value in
                    text3 = presenterEnvironment.viewModel.name
                }
            
            NavigationLink("Push view", destination: getNextView(from: "Navigation"))
            
            Button("Modal view") {
                isPresented.toggle()
            }.sheet(isPresented: $isPresented) {
                getNextView(from: "Sheet")
            }
        }
        .onAppear {
            onAppear()
        }
    }
    
    private func onAppear() {
        runOnce.execute {
            presenter.onAppear()
            presenterObserved.onAppear()
            presenterEnvironment.onAppear()
        }
    }
    
    private func getNextView(from: String) -> some View {
        ContentView(presenterObserved: Presenter(prefix: "\(presenterObserved.viewModel.prefix): \(from)"))
            .environmentObject(Presenter(prefix: "\(presenterEnvironment.viewModel.prefix): \(from)"))
    }
}

let view = NavigationView { ParentView() }
PlaygroundPage.current.liveView = UIHostingController(rootView: view)

// Helpers

class RunOnce: ObservableObject {
    var didRun: Bool = false
    
    func execute(_ block: () -> Void) {
        guard !didRun else { return }
        
        block()
        didRun = true
    }
}