为什么 saveStateInstance 在屏幕方向上不起作用？

Question

我正在制作一个导游应用程序，它由一个主要活动和两个片段组成。顶部片段包含城市列表，当您在某个城市打卡时，底部片段会显示该城市的描述。问题是，当我改变方向时，出现错误“尝试在空对象引用上调用虚拟方法 'void android.widget.TextView.setText(java.lang.CharSequence)'”这是城市列表片段：        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]);                }            }

Answer 1

为什么不将状态存储在 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);

    }

}

Answer 2

可以使用您的xml AndroidManifest.xml 中的属性

 android:configChanges="keyboardHidden|orientation"

在这里，它将向应用程序表明您将负责处理轮换，并且不会重新启动您的活动，以便不存在空数据。