SwiftUI onAppear bug

OnAppearCheck.swift

//
//  ContentView.swift
//  OnAppearCheck
//
//  Created by VAndrJ on 21.12.2023.
//

import SwiftUI

struct ContentView: View {
    @State var isLoggedIn: Bool = false

    var body: some View {
        if isLoggedIn {
            TabView {
                TabViewView(title: "Store", tabIcon: "trash", isLoggedIn: $isLoggedIn)
                TabViewView(title: "New", tabIcon: "newspaper", isLoggedIn: $isLoggedIn)
                TabViewView(title: "Gallery", tabIcon: "photo", isLoggedIn: $isLoggedIn)
                TabViewView(title: "Discover", tabIcon: "magnifyingglass", isLoggedIn: $isLoggedIn)
                TabViewView(title: "Profile", tabIcon: "person", isLoggedIn: $isLoggedIn)
            }
        } else {
            LoginView {
                isLoggedIn = true
            }
        }
    }
}

struct TabViewView: View {
    let title: String
    let tabIcon: String
    @Binding var isLoggedIn: Bool

    var body: some View {
        NavigationStack {
            Text(title)
                .navigationBarTitle(title)
                .task {
                    print("🏋️ \(title) some task")
                }
                .onAppear {
                    print("👋 \(title) onAppear")
                }
                .onDisappear {
                    print("😶‍🌫️ \(title) onDisappear")
                }
                .toolbar {
                    ToolbarItem(placement: .navigationBarTrailing) {
                        Button("Logout") {
                            isLoggedIn = false
                        }
                    }
                }
        }
        .task {
            print("🏋️ \(title) NavigationStack some task")
        }
        .onAppear {
            print("👋 \(title) NavigationStack onAppear")
        }
        .onDisappear {
            print("😶‍🌫️ \(title) NavigationStack onDisappear")
        }
        .tabItem {
            Image(systemName: tabIcon)
            Text(title)
        }
    }
}

struct LoginView: View {
    let onLogin: () -> Void

    var body: some View {
        Button("Login", action: onLogin)
    }
}

OnAppearCheck1.swift

//
//  ContentView.swift
//  OnAppearCheck
//
//  Created by VAndrJ on 21.12.2023.
//

import SwiftUI

struct ContentView: View {
    @State var isLoggedIn: Bool = false

    var body: some View {
        if isLoggedIn {
            TabView {
                TabViewView(title: "Store", tabIcon: "trash")
                TabViewView(title: "New", tabIcon: "newspaper")
                TabViewView(title: "Gallery", tabIcon: "photo")
                TabViewView(title: "Discover", tabIcon: "magnifyingglass")
                TabViewView(title: "Profile", tabIcon: "person")
            }
            .environment(\.logout, LogoutAction(action: logout))
        } else {
            LoginView {
                isLoggedIn = true
            }
        }
    }

    func logout() {
        isLoggedIn = false
    }
}

struct LogoutAction {
    let action: () -> Void

    func callAsFunction() {
        action()
    }
}

struct LogoutActionKey: EnvironmentKey {
    static var defaultValue: LogoutAction? = nil
}

extension EnvironmentValues {
    var logout: LogoutAction? {
        get { self[LogoutActionKey.self] }
        set { self[LogoutActionKey.self] = newValue }
    }
}

struct TabViewView: View {
    let title: String
    let tabIcon: String
    @Environment(\.logout) var logout

    var body: some View {
        NavigationStack {
            Text(title)
                .navigationBarTitle(title)
                .task {
                    print("🏋️ \(title) some task")
                }
                .onAppear {
                    print("👋 \(title) onAppear")
                }
                .onDisappear {
                    print("😶‍🌫️ \(title) onDisappear")
                }
                .toolbar {
                    ToolbarItem(placement: .navigationBarTrailing) {
                        Button("Logout") {
                            logout?()
                        }
                    }
                }
        }
        .task {
            print("🏋️ \(title) NavigationStack some task")
        }
        .onAppear {
            print("👋 \(title) NavigationStack onAppear")
        }
        .onDisappear {
            print("😶‍🌫️ \(title) NavigationStack onDisappear")
        }
        .tabItem {
            Image(systemName: tabIcon)
            Text(title)
        }
    }
}

struct LoginView: View {
    let onLogin: () -> Void

    var body: some View {
        Button("Login", action: onLogin)
    }
}

Looks like most of your problem is failing to provide the TabView with its selection state and also not properly providing the appropriate tag for each tab item. The following code should work as expected.

import SwiftUI

struct MyTabView: View {
    @State var tab: Int = 1
    @State var isLoggedIn: Bool = false
    var body: some View {
        if isLoggedIn {
            TabView(selection: $tab) {
                TabViewView(title: "Store", tabIcon: "trash", isLoggedIn: $isLoggedIn)
                    .tag(1)
                TabViewView(title: "New", tabIcon: "newspaper", isLoggedIn: $isLoggedIn)
                    .tag(2)
                TabViewView(title: "Gallery", tabIcon: "photo", isLoggedIn: $isLoggedIn)
                    .tag(3)
                TabViewView(title: "Discover", tabIcon: "magnifyingglass", isLoggedIn: $isLoggedIn)
                    .tag(4)
                TabViewView(title: "Profile", tabIcon: "person", isLoggedIn: $isLoggedIn)
                    .tag(5)
            }
        } else {
            LoginView(isLoggedIn: $isLoggedIn)
        }
    }
}

struct TabViewView: View {
    let title: String
    let tabIcon: String
    @Binding var isLoggedIn: Bool
    var body: some View {
        NavigationStack {
            Text(title)
                .navigationBarTitle(title)
                .task {
                    print("🏋️ \(title) some task")
                }
                .onAppear {
                    print("👋 \(title) onAppear")
                }
                .onDisappear {
                    print("😶‍🌫️ \(title) onDisappear")
                }
                .toolbar {
                    ToolbarItem(placement: .navigationBarTrailing) {
                        Button("Logout") {
                            isLoggedIn = false
                        }
                    }
                }
        }
        .tabItem {
            Image(systemName: tabIcon)
            Text(title)
        }
        .task {
            print("🏋️ \(title) NavigationStack some task")
        }
        .onAppear {
            print("👋 \(title) NavigationStack onAppear")
        }
        .onDisappear {
            print("😶‍🌫️ \(title) NavigationStack onDisappear")
        }
    }
}

struct LoginView: View {
    @Binding var isLoggedIn: Bool
    var body: some View {
        Button("Login") {
            isLoggedIn = true
        }
    }
}

@hmlongco , only this workaround helps:

public extension View {

    func onDidAppear(perform action: @escaping () -> Void) -> some View {
        background(DidAppearView(action: action))
    }
}

struct DidAppearView: UIViewControllerRepresentable {
    class Coordinator: ActionControllerRepresentableDelegate {
        var action: () -> Void

        init(action: @escaping () -> Void) {
            self.action = action
        }

        func performAction() {
            action()
        }
    }

    let action: () -> Void

    func makeUIViewController(context: Context) -> DidAppearViewController {
        let controller = DidAppearViewController()
        controller.delegate = context.coordinator

        return controller
    }

    func updateUIViewController(_ controller: DidAppearViewController, context: Context) {
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(action: action)
    }
}

protocol ActionControllerRepresentableDelegate: AnyObject {
    func performAction()
}

class DidAppearViewController: UIViewController {
    weak var delegate: ActionControllerRepresentableDelegate?

    override func viewDidAppear(_ animated: Bool) {
        delegate?.performAction()
    }
}