go 按返回时返回第一个片段

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，我打开第 1 号片段。我该怎么做？

我尝试了很多代码，但它不起作用，比如

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 Fragment 中实现 onBackPressed 方法。

1. 创建一个名为 IOnBackPressed 的界面：单击 package -> new Java Class -> 并选择界面类型 -> 确定您的界面应如下所示：

    public interface IOnBackPressed { boolean onBackPressed(); }

2. 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(); } }

3. 你实现这个到你的片段：

    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（您可以使用像“HomeFragment”这样的片段名称）来替换片段。

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

和这样的流行片段

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台堆栈，以便后退按钮可以正常工作。