শর্তসাপেক্ষ নেভিগেশন

class ProfileFragment : Fragment() {
    private val userViewModel: UserViewModel by activityViewModels()
    ...
}

public class ProfileFragment extends Fragment {
    private UserViewModel userViewModel;
    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        userViewModel = new ViewModelProvider(requireActivity()).get(UserViewModel.class);
        ...
    }
    ...
}

class ProfileFragment : Fragment() {
    private val userViewModel: UserViewModel by activityViewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val navController = findNavController()
        userViewModel.user.observe(viewLifecycleOwner, Observer { user ->
            if (user != null) {
                showWelcomeMessage()
            } else {
                navController.navigate(R.id.login_fragment)
            }
        })
    }

    private fun showWelcomeMessage() {
        ...
    }
}

public class ProfileFragment extends Fragment {
    private UserViewModel userViewModel;

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        userViewModel = new ViewModelProvider(requireActivity()).get(UserViewModel.class);
        final NavController navController = Navigation.findNavController(view);
        userViewModel.user.observe(getViewLifecycleOwner(), (Observer<User>) user -> {
            if (user != null) {
                showWelcomeMessage();
            } else {
                navController.navigate(R.id.login_fragment);
            }
        });
    }

    private void showWelcomeMessage() {
        ...
    }
}

class LoginFragment : Fragment() {
    companion object {
        const val LOGIN_SUCCESSFUL: String = "LOGIN_SUCCESSFUL"
    }

    private val userViewModel: UserViewModel by activityViewModels()
    private lateinit var savedStateHandle: SavedStateHandle

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        savedStateHandle = findNavController().previousBackStackEntry!!.savedStateHandle
        savedStateHandle.set(LOGIN_SUCCESSFUL, false)
    }
}

public class LoginFragment extends Fragment {
    public static String LOGIN_SUCCESSFUL = "LOGIN_SUCCESSFUL"

    private UserViewModel userViewModel;
    private SavedStateHandle savedStateHandle;

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        userViewModel = new ViewModelProvider(requireActivity()).get(UserViewModel.class);

        savedStateHandle = Navigation.findNavController(view)
                .getPreviousBackStackEntry()
                .getSavedStateHandle();
        savedStateHandle.set(LOGIN_SUCCESSFUL, false);
    }
}

class LoginFragment : Fragment() {
    companion object {
        const val LOGIN_SUCCESSFUL: String = "LOGIN_SUCCESSFUL"
    }

    private val userViewModel: UserViewModel by activityViewModels()
    private lateinit var savedStateHandle: SavedStateHandle

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        savedStateHandle = findNavController().previousBackStackEntry!!.savedStateHandle
        savedStateHandle.set(LOGIN_SUCCESSFUL, false)

        val usernameEditText = view.findViewById(R.id.username_edit_text)
        val passwordEditText = view.findViewById(R.id.password_edit_text)
        val loginButton = view.findViewById(R.id.login_button)

        loginButton.setOnClickListener {
            val username = usernameEditText.text.toString()
            val password = passwordEditText.text.toString()
            login(username, password)
        }
    }

    fun login(username: String, password: String) {
        userViewModel.login(username, password).observe(viewLifecycleOwner, Observer { result ->
            if (result.success) {
                savedStateHandle.set(LOGIN_SUCCESSFUL, true)
                findNavController().popBackStack()
            } else {
                showErrorMessage()
            }
        })
    }

    fun showErrorMessage() {
        // Display a snackbar error message
    }
}

public class LoginFragment extends Fragment {
    public static String LOGIN_SUCCESSFUL = "LOGIN_SUCCESSFUL"

    private UserViewModel userViewModel;
    private SavedStateHandle savedStateHandle;

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        userViewModel = new ViewModelProvider(requireActivity()).get(UserViewModel.class);

        savedStateHandle = Navigation.findNavController(view)
                .getPreviousBackStackEntry()
                .getSavedStateHandle();
        savedStateHandle.set(LOGIN_SUCCESSFUL, false);

        EditText usernameEditText = view.findViewById(R.id.username_edit_text);
        EditText passwordEditText = view.findViewById(R.id.password_edit_text);
        Button loginButton = view.findViewById(R.id.login_button);

        loginButton.setOnClickListener(v -> {
            String username = usernameEditText.getText().toString();
            String password = passwordEditText.getText().toString();
            login(username, password);
        });
    }

    private void login(String username, String password) {
        userViewModel.login(username, password).observe(viewLifecycleOwner, (Observer<LoginResult>) result -> {
            if (result.success) {
                savedStateHandle.set(LOGIN_SUCCESSFUL, true);
                NavHostFragment.findNavController(this).popBackStack();
            } else {
                showErrorMessage();
            }
        });
    }

    private void showErrorMessage() {
        // Display a snackbar error message
    }
}

class ProfileFragment : Fragment() {
    ...

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val navController = findNavController()

        val currentBackStackEntry = navController.currentBackStackEntry!!
        val savedStateHandle = currentBackStackEntry.savedStateHandle
        savedStateHandle.getLiveData<Boolean>(LoginFragment.LOGIN_SUCCESSFUL)
                .observe(currentBackStackEntry, Observer { success ->
                    if (!success) {
                        val startDestination = navController.graph.startDestination
                        val navOptions = NavOptions.Builder()
                                .setPopUpTo(startDestination, true)
                                .build()
                        navController.navigate(startDestination, null, navOptions)
                    }
                })
    }

    ...
}

public class ProfileFragment extends Fragment {
    ...

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        NavController navController = NavHostFragment.findNavController(this);

        NavBackStackEntry navBackStackEntry = navController.getCurrentBackStackEntry();
        SavedStateHandle savedStateHandle = navBackStackEntry.getSavedStateHandle();
        savedStateHandle.getLiveData(LoginFragment.LOGIN_SUCCESSFUL)
                .observe(navBackStackEntry, (Observer<Boolean>) success -> {
                    if (!success) {
                        int startDestination = navController.getGraph().getStartDestination();
                        NavOptions navOptions = new NavOptions.Builder()
                                .setPopUpTo(startDestination, true)
                                .build();
                        navController.navigate(startDestination, null, navOptions);
                    }
                });
    }

    ...
}