Question

例如，当我在Fragment 5中时，当我按下“后退”按钮时，该应用程序就会出现。我想要的是当我按下“后退”按钮时，打开1号片段。我该怎么做？

我尝试了很多代码，但无法正常工作，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中的片段代码

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

添加onBackPressed方法：

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

    NextFragment nextFrag= new NextFragment();
    getActivity().getSupportFragmentManager().beginTransaction()
         .replace(R.id.Layout_container, nextFrag, "findThisFragment")
         .addToBackStack(null)
         .commit();
}

Answer 2

首先，您必须在片段中实现onBackPressed方法。

创建一个名称为IOnBackPressed的接口：单击包->新Java类->并在种类中选择接口->确定您的界面应如下所示：
```
 public interface IOnBackPressed {
 boolean onBackPressed();
 }
```

之后，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();
     }
 }

您将其实现为片段：

 public class Profil_Fragment extends Fragment implements IOnBackPressed

并将其添加到片段中：

  @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;
 }

Answer 3

添加backstack和标签（您可以使用“ HomeFragment”之类的片段名称）来替换片段。

replace(fragmentId, fragment, fragmentName).addToBackStack(fragmentName)

还有这样的弹出片段

popBackStack(fragmentName,0)

Answer 4

尝试添加transaction.addToBackStack（tag）;

例如：

FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.fragment_layout, selectedFragment);
ft.addToBackStack( "tag" ).commit();

这会将您的片段添加到后退堆栈，以便后退按钮可以正常工作。

返回时返回第一个片段

4 个答案: