使用片段保存和恢复状态

Question

我试图理解使用片段保存和恢复状态的过程。我用它创建了滑动导航菜单。

其中一个片段中有以下代码：

public class FifthFragment extends Fragment {

    CheckBox cb;
    View view;

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

        view = inflater.inflate(R.layout.fifth_layout, container, false);
        cb = (CheckBox) view.findViewById(R.id.checkBox);
        return view;

    }

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

        if (savedInstanceState != null) {
            // Restore save state
        }

    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        // save state
    }

}

例如，我想在用户退出片段之前保存复选框的状态，并在再次创建片段时恢复它。如何实现这一点？

编辑：

根据raxellson的回答，我将片段更改为：

public class FifthFragment extends Fragment {

    private static final String CHECK_BOX_STATE = "string";
    CheckBox cb;
    View view;

    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fifth_layout, container, false);

        cb = (CheckBox) view.findViewById(R.id.checkBox);
        if (savedInstanceState == null) {
            Log.i("statenull", "null");
        }
        if (savedInstanceState != null) {
            // Restore last state for checked position.
            boolean checked = savedInstanceState.getBoolean(CHECK_BOX_STATE, false);
            cb.setChecked(checked);
        }

        return view;
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putBoolean(CHECK_BOX_STATE, cb.isChecked());
    }

}

我已被记录，因此未保存savedInstanceState。我做错了什么？

Answer 1

查看所有示例后。这是保存片段状态的解决方案：

为此，有两个步骤：

1.

String saveValue;
 
 
 @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState == null) {
            saveValue = "";
        } else {

            saveValue = savedInstanceState.getString("saveInstance");
          

        }
    }

@Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        //save the values of fragment if destroy on second to back
        if (!saveValue.isEmpty())
            savedInstanceState.putString("saveInstance", saveValue);
       
    }

在onSaveInstanceState中，您可以保存您的值。销毁片段后，您可以通过onCreate接收您的值。

Answer 2

您想将当前选中状态的值保存在onSaveInstanceState中。

像这样的东西：

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putBoolean(CHECK_BOX_STATE, cb.getChecked());
}

然后，当你的视图被创建时，如果它存在的话，你想要得到它的值。并用它设置您的复选框状态。

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fifth_layout, container, false);

    cb = (CheckBox) view.findViewById(R.id.checkBox);
    if (savedInstanceState != null) {
        // Restore last state for checked position.
        boolean checked = savedInstanceState.getBoolean(CHECK_BOX_STATE, false);
        cb.setChecked(checked);              
    }

    return view;
}

编辑：

添加片段时，请确保使用标签或id添加它，以便您可以检索相同的实例。

您可以使用助手方法来检索片段并设置片段。

private void setFragment(String tag, Fragment newFragment) {
   FragmentManager fm = getSupportFragmentManager();
   Fragment savedFragment = fm.getFragmentByTag(tag);
   fm.replace(R.id.container, savedFragment != null ? savedFragment : newFragment, tag);
   fm.commit();
}

因此，您可以调用helper方法。

switch (position) {
   case 0:
   setFragment("A", new FragmentA());
   break;
   ....
}

注意：这只是一个示例，不是最佳实践，因为您现在每次都在切换案例中创建新片段。但它可能会为你指明正确的方向。