為什么 saveStateInstance 在屏幕方向上不起作用？

Question

我正在制作一個由一個主要活動和兩個片段組成的導游應用程序。 頂部片段包含一個城市列表，當您在一個城市打卡時，底部片段會顯示該城市的描述。 The problem is that when I change orientation I get an error saying "Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference"

這是城市列表片段：

        package com.example.tourguide;


        import android.os.Bundle;
        import android.support.annotation.NonNull;
        import android.support.v4.app.Fragment;
        import android.view.LayoutInflater;
        import android.view.View;
        import android.view.ViewGroup;
        import android.widget.AdapterView;
        import android.widget.ArrayAdapter;
        import android.widget.ListView;


        /**
         * A simple {@link Fragment} subclass.
         */
        public class CityFragment extends Fragment {

            View view;
            String[] cities;
            String[] descriptions;
            ListView listView;
            DescriptionFragment text;
            int mPosition;

            public CityFragment() {
                // Required empty public constructor
            }


            @Override
            public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                     Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                // Check whether we're recreating a previously destroyed instance
                if (savedInstanceState != null) {
                    // Restore value of member from saved state
                    text = (DescriptionFragment) getFragmentManager().getFragment(savedInstanceState, getString(R.string.DESCRIPTION_FRAG));
                    mPosition = savedInstanceState.getInt(getString(R.string.POSITION));
                    cities = savedInstanceState.getStringArray(getString(R.string.CITY_ARRAY));
                    descriptions = savedInstanceState.getStringArray(getString(R.string.DESCRIPTION_ARRAY));
                    text.change(descriptions[mPosition], cities[mPosition]);
                }

                    // Get views
                    this.view = inflater.inflate(R.layout.fragment_city, container, false);
                    cities = getResources().getStringArray(R.array.cities);
                    descriptions = getResources().getStringArray(R.array.cities_description);
                    listView = this.view.findViewById(R.id.city_list);

                    // Create an ArrayAdapter
                    ArrayAdapter<String> listViewAdapter = new ArrayAdapter<String>(getActivity(),
                            android.R.layout.simple_list_item_1, cities);

                    // Set adapter on the listView
                    listView.setAdapter(listViewAdapter);

                // Set an item click listener for ListView
                listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

                    @Override
                    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                        // Get the selected item text from ListView
                        text = (DescriptionFragment) getFragmentManager().findFragmentById(R.id.fragmentBottom);
                        text.change(descriptions[Integer.parseInt(String.valueOf(position))], cities[Integer.parseInt(String.valueOf(position))]);
                        mPosition = position;
                    }
                });

                // Inflate the layout for this fragment
                return this.view;

            }

            @Override
            public void onSaveInstanceState(@NonNull Bundle outState) {
                super.onSaveInstanceState(outState);

                // Save the fragments Instance
                getFragmentManager().putFragment(outState, getString(R.string.DESCRIPTION_FRAG), text);
                outState.putInt(getString(R.string.POSITION), mPosition);
                outState.putStringArray(getString(R.string.CITY_ARRAY), cities);
                outState.putStringArray(getString(R.string.DESCRIPTION_ARRAY), descriptions);
            }
        }

這是描述片段：

        package com.example.tourguide;


        import android.os.Bundle;
        import android.support.annotation.NonNull;
        import android.support.v4.app.Fragment;
        import android.view.LayoutInflater;
        import android.view.View;
        import android.view.ViewGroup;
        import android.widget.TextView;


        /**
         * A simple {@link Fragment} subclass.
         */
        public class DescriptionFragment extends Fragment {

            TextView cityName;
            TextView text;

            public DescriptionFragment() {
                // Required empty public constructor
            }


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

                if (savedInstanceState != null) {

                }

                // Inflate the layout for this fragment
                View view = inflater.inflate(R.layout.fragment_description, container, false);
                text = view.findViewById(R.id.city_description);
                cityName = view.findViewById(R.id.city_name);
                return view;
            }

            public void change(String description, String city) {
                text.setText(description);
                cityName.setText(city);
            }
        }

text.change 方法似乎是問題所在，但我還能如何實現它來保存描述 state？

            if (savedInstanceState != null) {
                // Restore value of member from saved state
                text = (DescriptionFragment) getFragmentManager().getFragment(savedInstanceState, getString(R.string.DESCRIPTION_FRAG));
                mPosition = savedInstanceState.getInt(getString(R.string.POSITION));
                cities = savedInstanceState.getStringArray(getString(R.string.CITY_ARRAY));
                descriptions = savedInstanceState.getStringArray(getString(R.string.DESCRIPTION_ARRAY));
                text.change(descriptions[mPosition], cities[mPosition]);
            }

Answer 1

可以在您的 xml AndroidManifest.xml 中使用該屬性

 android:configChanges="keyboardHidden|orientation"

在這里它將向應用程序指示您將負責處理輪換並且不會重新啟動您的活動，因此沒有 null 數據。

Answer 2

為什么不將 state 存儲在 DescriptionFragment 中。

/**
 * A simple {@link Fragment} subclass.
 */
public class DescriptionFragment extends Fragment {

    private static final String DESCRIPTION = "DESCRIPTION";
    private static final String CITY = "CITY";

    TextView cityName;
    TextView text;

    public DescriptionFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_description, container, false);
    }

    @Override
    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        text = view.findViewById(R.id.city_description);
        cityName = view.findViewById(R.id.city_name);

        if (savedInstanceState != null) {
            String description = savedInstanceState.getString(DESCRIPTION);
            String city = savedInstanceState.getString(CITY);
            change(description, city);
        }
    }

    @Override
    public void onSaveInstanceState(@NonNull Bundle outState) {
        super.onSaveInstanceState(outState);

        // Save the fragments Instance
        outState.putString(DESCRIPTION, text.getText().toString());
        outState.putString(CITY, cityName.getText().toString());
    }

    public void change(String description, String city) {
        text.setText(description);
        cityName.setText(city);
    }
}