在 Fragment 切换中保留数据

Question

正在开发一个包含 3 个片段的应用。当用户决定通过 BottomNavigationView 从 FragmentB 或 FragmentC 切换回 FragmentA 时，我想在 FragmentA 中保留数据。 我的代码在更改屏幕方向时效果很好，但由于某种原因，它不会保留片段更改的数据。

为了测试，我只使用了一个应该保留字符串数据的 EditText。

MainActivity

public class MainActivity extends AppCompatActivity implements 
BottomNavigationView.OnNavigationItemSelectedListener {

Fragment Frag = new Fragment();

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

    BottomNavigationView navigation = findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(this); // mOnNavigationItemSelectedListener


    if (savedInstanceState != null)
    {
        Frag = getSupportFragmentManager().getFragment(savedInstanceState, "Frag");
    }
    else
    loadFragment(new FragmentA());
}

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
    Fragment fragment = null;
    int i = menuItem.getItemId();
    if (i == R.id.fragmentA) {
        fragment = new FragmentA();
    } else if (i == R.id.fragmentB) {
        fragment = new FragmentB();
    } else if (i == R.id.fragmentC) {
        fragment = new FragmentC();           
    }

    return loadFragment(fragment);
}

private boolean loadFragment(Fragment fragment)
{
    if (fragment != null)
    {
        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, fragment).commit();
        Frag = fragment;
        return true;
    }
    return false;
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    getSupportFragmentManager().putFragment(outState, "Frag", Frag);
}
}

片段A

public class FragmentA extends Fragment {

EditText editText;
String string;

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

    if (savedInstanceState != null)
    {
        string = savedInstanceState.getString("Text", "");
    }
}

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    // return super.onCreateView(inflater, container, savedInstanceState);
    LayoutInflater layoutInflater = getActivity().getLayoutInflater();
    View view = layoutInflater.inflate(R.layout.fragmentA, container, false);

    editText = view.findViewById(R.id.centerText);
    editText.setText(string);
    return view;
}

@Override
public void onSaveInstanceState(@NonNull Bundle outState) {
    super.onSaveInstanceState(outState);
    string = editText.getText().toString();
    outState.putString("Text", string);
}
}

当用户从 FragmentB 或 FragmentC 切换回 FragmentA 时，我希望 editText 保留数据。 我已经为这个问题苦苦挣扎了一段时间，尝试了不同的方法，但没有成功。

Answer 1

您可以保留对片段的引用，而不是像在此处那样每次都创建新片段：

if (i == R.id.fragmentA) {
    fragment = new FragmentA();
} else if (i == R.id.fragmentB) {
    fragment = new FragmentB();
} else if (i == R.id.fragmentC) {
    fragment = new FragmentC();           
}

您还可以使用ViewPager，它不仅可以让您在屏幕之间滑动，还可以设置viewPager.setOffscreenPageLimit(i);，这意味着除非片段离屏幕太远，否则不会重新创建片段。

Answer 2

这是你的错误

if (i == R.id.fragmentA) {
    fragment = new FragmentA();
} else if (i == R.id.fragmentB) {
    fragment = new FragmentB();
} else if (i == R.id.fragmentC) {
    fragment = new FragmentC();           
}

点击后创建新的片段实例

Answer 3

感谢您的回复。 这就是我改变的。它现在正在工作，虽然我不确定这是否是解决这个问题的优雅方法。导航回 FragmentA 时，数据现在保留在 EditText 中。

MainActivity

public class MainActivity extends AppCompatActivity implements 
BottomNavigationView.OnNavigationItemSelectedListener {

Fragment Frag = new Fragment();
Fragment FragmentA;
Fragment FragmentB;
Fragment FragmentC;

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

    BottomNavigationView navigation = findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(this); // mOnNavigationItemSelectedListener

    FragmentA = new FragmentA();
    FragmentB = new FragmentB();
    FragmentC = new FragmentC();

    if (savedInstanceState != null)
    {
        Frag = getSupportFragmentManager().getFragment(savedInstanceState, "Frag");
    }
    else
    loadFragment(FragmentA);
}

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
    Fragment fragment = null;
    int i = menuItem.getItemId();
    if (i == R.id.fragmentA) {
        fragment = FragmentA;
    } else if (i == R.id.fragmentB) {
        fragment = FragmentB;
    } else if (i == R.id.fragmentC) {
        fragment = FragmentC;
    }
    return loadFragment(fragment);
}

private boolean loadFragment(Fragment fragment)
{
    if (fragment != null)
    {
        getSupportFragmentManager().beginTransaction().hide(FragmentA).commit();
        getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, fragment).commit();
        getSupportFragmentManager().beginTransaction().show(fragment).commit();

        Frag = fragment;
        return true;
    }
    return false;
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    getSupportFragmentManager().putFragment(outState, "Frag", Frag);
}
}