java - 按返回时返回第一个 fragment

Question

例如，当我在 Fragment 5 中按下返回按钮时，应用程序就会出现。我想要的是当我按下后退按钮时，我打开 fragment 1。我该怎么做？

我尝试了很多代码，但都不起作用，例如

FragmentManager fm = getActivity().getSupportFragmentManager();
 for(int i = 0; i < fm.getBackStackEntryCount(); ++i) {
    fm.popBackStack();
}

和

getSupportFragmentManager().popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);

这是我在 MainActivity 中的 fragment 代码

public class MainActivity extends AppCompatActivity {

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

    // make new object and find the view "BottomNavigationView"
    BottomNavigationView navigationView = findViewById(R.id.bottom_navigation);
    // To make the first fragment shows when the app start.
    // We will ignore the "activity_main" and we will make new activity(fragment) called "fragment_home"
    // and we will add all views we want to display them on it.
    // (fragment_layout) it is a fragment have id was defined in "activity_main"
    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_layout,
            new HomeFragment()).commit();
    // make Listener to call the fragments on buttons
    // to start new fragment
    navigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            Fragment selectedFragment = null;
            switch (item.getItemId()) {
                // first fragment "home"
                case R.id.home_item:
                    selectedFragment = new HomeFragment();
                    break;
                // second fragment "videos"
                case R.id.videos_item:
                    selectedFragment = new VideosFragment();
                    break;
                // third fragment "ebadat"
                case R.id.ebadat_item:
                    selectedFragment = new EbadatFragment();
                    break;
                // fourth fragment "images"
                case R.id.images_item:
                    selectedFragment = new ImagesFragment();
                    break;
                // fifth fragment "more"
                case R.id.settings_item:
                    selectedFragment = new MoreFragment();
                    break;
            }
            assert selectedFragment != null;
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_layout, selectedFragment).commit();
            return true;
        }

    });
}

Answer 1

首先，您必须在 Fragment 中实现 onBackPressed 方法。

创建一个名为 IOnBackPressed 的接口(interface):
点击 package -> new Java Class -> 并在 Kind 中选择 interface -> OK
你的界面应该是这样的:

 public interface IOnBackPressed {
 boolean onBackPressed();
 }

之后，您打开 MainActivity 并添加以下内容:

 @Override
 public void onBackPressed() {
     //super.onBackPressed(); delete this line
     // and start your fragment:

     Fragment fragment = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
     if (!(fragment instanceof IOnBackPressed) || !((IOnBackPressed) fragment).onBackPressed()) {
         super.onBackPressed();
     }
 }

你实现这个到你的 fragment :

 public class Profil_Fragment extends Fragment implements IOnBackPressed 

并将其添加到 fragment 中:

  @Override
 public boolean onBackPressed() {
     //do what you want
     NextFragment nextFrag= new NextFragment();
 getActivity().getSupportFragmentManager().beginTransaction()
      .replace(R.id.fragment_container, nextFrag, "findThisFragment")
      .addToBackStack(null)
      .commit();

      return true;
 }