如何不使用 Android 中的底部导航栏导航回某些片段

Question

I have a bottom navigation bar with 4 buttons. One of them is a back-button that just navigates back to the previously displayed fragment (I use a one-activity multiple-fragments approach). Basically everything works fine. Now I wanted to know whether it is possible not to navigate back to a certain fragment A? So basically whenever this Fragement A was the last fragment that was being displayed and the user clicks on the back-button, it should not navigate back but rather stay on the current fragment (and maybe display a small toast message).

So here you can see my main activity where the navigation is implemented:

package com.example.td.bapp;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.databinding.DataBindingUtil;
import androidx.fragment.app.FragmentManager;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.NavigationUI;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.util.Log;
import android.view.MenuItem;
import android.view.View;

import com.example.td.bapp.databinding.ActivityMainBinding;
import com.google.android.material.bottomnavigation.BottomNavigationView;

import java.io.IOException;
import java.io.InputStream;
import java.sql.SQLException;


public class MainActivity extends AppCompatActivity  {

   public static DataBaseHelper myDB;
   public static boolean veryFirstCreation = true;


    private ActivityMainBinding binding;


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

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        View view = binding.getRoot();

        setContentView(view);

        final  NavController navController = Navigation.findNavController(this,
                R.id.navHostfragment);
       NavigationUI.setupWithNavController(binding.bottomNavigation, navController);

        myDB=new DataBaseHelper(this);

        if(veryFirstCreation) {
            navController.navigate(R.id.FR_LanguageSelection);
            veryFirstCreation = false;
        }





          // Define the bahaviour when clicking on items of the bottomNavigationBar; Overwrites the JetpackNavigation behaviour (automatic navigation using the id specified in the BottomNavigation XML menu file)
        binding.bottomNavigation.setOnNavigationItemSelectedListener(
                new BottomNavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                        if (item.getItemId()== R.id.BottomNavigation_Back) {
                            //Here I navigate back to the previous fragment
                            navController.navigateUp();

                        }

                        if (item.getItemId()== R.id.BottomNavigation_Language) {
                            navController.navigate(R.id.FR_LanguageSelection);
                        }

                        if (item.getItemId()== R.id.BottomNavigation_MyItems) {
                            navController.navigate(R.id.FR_MyItems);
                        }

                        if (item.getItemId()== R.id.BottomNavigation_Menu) {
                            navController.navigate(R.id.FR_Menu);
                        }



                        return true;
                    }
                });


        };




    }

I'd appreciate every comment and would be quite thankful for your help.

Answer 1

You could use the SupportFragmentManager, with this you could know if your back stack contains at least one fragment. Use this in your back function (or in your onBackPressed if necessary)

FragmentManager fm = getSupportFragmentManager();
if (fm.getBackStackEntryCount() ==1) {
    Log.d("ppl","This is the last fragment. Show your message, poppup or whatever you want here")

} else if (fm.getBackStackEntryCount() >1) {
    Log.d("ppl","the backstack still have more to show")
    fm.popBackStackImmediate();
}