Question

我正在学习 android，我面临着一个不知道如何解决它的挑战。那是我无法回到开始活动的片段。我正在从片段打开第二个活动，但是当我在第二个活动上按下后退按钮时，它不会导航到打开该活动的片段，而是显示主页片段。我试过 this 但没有成功。

这是打开第二个片段的聊天片段

这是第二个活动，如果我按下后退按钮不会转到聊天片段

当我按下第二个活动的后退按钮时，这就是我得到的。显示首页片段内容

这是我的主要活动

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
    Fragment fragment;
    Toolbar toolbar;
    TextView title,txt_profile_contact,txt_profile_name;
    ActionBarDrawerToggle toggle;
    DrawerLayout drawer;
   
    @Override
    protected void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
       
        setContentView(R.layout.activity_main);

        notificationRepository=new NotificationRepository(this);
        toolbar=findViewById(R.id.toolbar);
        title=toolbar.findViewById(R.id.tool_bar_title);
        title.setText(R.string.home);


        setSupportActionBar(toolbar);

        getSupportActionBar().setDisplayShowTitleEnabled(false);

        drawer= findViewById(R.id.drawer_layout);
        toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView =  findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

   
    }
    
     @Override
    public void onBackPressed() {

      
        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        assert drawer != null;

        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
            return;
        }
        getSupportFragmentManager().popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);

        doExit(false);


    }
    
    
     @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        Fragment fragment=null;
        int idd=R.id.main_fragment_main;
        if (id == R.id.menu_chat)
        {
            title.setText("Chat");
            fragment=new ChatFragment();
           

        } else  (id == R.id.menu_home) {
            title.setText("Home");
            fragment=new HomeFragment();
            Utils.setIsOnChatActivity(this,false);


        }  


        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        load_fragment(fragment);

        return true;
    }

    private void load_fragment(Fragment fragment)
    {
        if(fragment==null)
        {
            return;
        }
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();

        ft.setCustomAnimations(R.anim.fadein, R.anim.fadeout);
                       toggle.getDrawerArrowDrawable().setColor(getResources().getColor(R.color.white));
                toolbar.setBackgroundResource(R.color.colorPrimary);
              title.setTextColor(getResources().getColor(R.color.white));


        try {
                ft.replace(R.id.main_fragment, fragment)
                .addToBackStack(null)
                .commit();
    } catch (IllegalStateException e) {
       //ExceptionHandler.logException(e);
    }




    }

    }

这是聊天片段

public class ChatFragment extends Fragment implements TutorListAdapter.TutorOnclickListener{
    private static final String TAG = ChatFragment.class.getSimpleName();

 


    public ChatFragment() {
        // Required empty public constructor
    }

   
    FloatingActionButton actionButton;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View view= inflater.inflate(R.layout.fragment_chat, container, false);
      
        actionButton=view.findViewById(R.id.new_chat_list_btn);


        actionButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent=new Intent(getActivity(),SecondActivity.class);
                startActivity(intent);
                mActivity.overridePendingTransition(R.anim.fadein, R.anim.fade_out);
            }
        });




        return view;
    }
    }

这是我的第二个活动。注意，我什至尝试过注释代码，但不起作用

public class SecondActivity extends AppCompatActivity {

    Toolbar toolbar;
    TextView barText;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_subject_details);
        toolbar=findViewById(R.id.custom_tool_bar);
        barText=toolbar.findViewById(R.id.toolbar_title);
        barText.setText("My Activity");
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setDisplayShowHomeEnabled(true);
        getSupportActionBar().setDisplayShowTitleEnabled(false);

    }

//    @Override
//    public boolean onOptionsItemSelected(MenuItem item) {
//        if (item.getItemId() == android.R.id.home) {
//            onBackPressed();
//            return true;
//        }
//        return super.onOptionsItemSelected(item);
//    }
//
//    @Override
//    public void onBackPressed() {
//        Toast.makeText(this, "back pressed?", Toast.LENGTH_SHORT).show();
//        if ( getFragmentManager().getBackStackEntryCount() > 0)
//        {
//            getFragmentManager().popBackStack();
//            return;
//        }
//        super.onBackPressed();
//    }
@Override
public boolean onSupportNavigateUp(){
    finish();
    return true;
}
}

Answer 1

第 1 步：创建一个自定义 ViewModelFactory，它基本上是一个 Singleton 类。这个类提供ViewModel。

CustomViewModelProvider.java

public class CommonViewModelProvider extends ViewModelProvider.NewInstanceFactory {

    private static CommonViewModelProvider commonViewModelProvider;
    private final BottomNavigationViewModel bottomNavigationViewModel;

    CommonViewModelProvider(){
        bottomNavigationViewModel = new BottomNavigationViewModel();
    }
    public static CommonViewModelProvider getInstance(){
        if (commonViewModelProvider == null)
            commonViewModelProvider = new CommonViewModelProvider();
        return commonViewModelProvider;
    }

    @NotNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass){
        return (T) bottomNavigationViewModel;
    }
}

BottomNavigationViewModel.java

public class BottomNavigationViewModel extends ViewModel {
    private final MutableLiveData<Integer> currentFragmentId;

    public Integer getCurrentFragmentId(){
        return currentFragmentId.getValue();
    }
    public void setCurrentFragmentId(int currFragmentId){
        currentFragmentId.setValue(currFragmentId);
    }
    public BottomNavigationViewModel(){
        currentFragmentId = new MutableLiveData<>();
        // R.id.navigation_snap is my first fragment. You should replace with your FirstFragment i.e HomeFragment
        currentFragmentId.setValue(R.id.navigation_snap);
    }
}

第 2 步： 您将有一个 BottomNavigationActivity，其中包含 3 个 fragments。第三个是您的 ChatFragment。在此活动中，我从 fragment 获取当前 BottomNavigationViewModel 并将其设置为 BottomNavigationView，如 navView.setSelectedItemId(currentFragment);。这样当您从 SecondActivity 返回到 BottomNavigationActivity 时，将显示 ChatFragment 而不是 HomeFragment。

BottomNavigationActivity.java

    public class BottomNavigationActivity extends AppCompatActivity {

    BottomNavigationViewModel bottomNavigationViewModel;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bottom_navigation);
        //bottomNavigationViewModel = new ViewModelProvider(this).get(BottomNavigationViewModel.class);
        CommonViewModelProvider commonViewModelProvider = CommonViewModelProvider.getInstance();
        bottomNavigationViewModel = commonViewModelProvider.create(BottomNavigationViewModel.class);
        BottomNavigationView navView = findViewById(R.id.nav_view);
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
                R.id.navigation_snap, R.id.navigation_home, R.id.navigation_chat)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
        NavigationUI.setupWithNavController(navView, navController);

        Integer currentFragment = bottomNavigationViewModel.getCurrentFragmentId();

        navView.setSelectedItemId(currentFragment);
    }

}

第 3 步： 现在您想从 SecondActivity 打开 ChatFragment。那么，在 SecondActivity 中只需更新 currentFragmentId 的 BottomNavigationViewModel 变量，这样当您按下back 按钮时，这个 currentFragment 将显示在 BottomNavigationActivity .

SecondActivity.java

    public class SecondActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);

        CommonViewModelProvider commonViewModelProvider = CommonViewModelProvider.getInstance();
        BottomNavigationViewModel bottomNavigationViewModel = commonViewModelProvider.create(BottomNavigationViewModel.class);        Log.d("tisha==>>","View Model ID inside Second Activity"+ bottomNavigationViewModel.hashCode());
        bottomNavigationViewModel.setCurrentFragmentId(R.id.navigation_chat);
    }
}

注意：每当您想打开 BottonNavigationActivity 时，请确保您已设置正确的 fragment，它将首先打开。使用以下代码将 currentFragment 设置为 ViewModel，这已在 SecondActivity 中完成。

 CommonViewModelProvider commonViewModelProvider = CommonViewModelProvider.getInstance();
 BottomNavigationViewModel bottomNavigationViewModel = commonViewModelProvider.create(BottomNavigationViewModel.class);        Log.d("tisha==>>","View Model ID inside Second Activity"+ bottomNavigationViewModel.hashCode());
bottomNavigationViewModel.setCurrentFragmentId(R.id.navigation_chat);

第二个活动后退按钮不会导航到启动活动的片段

1 个答案: