PreferenceFragment NullPointerException

Question

public class MyPrefs extends PreferenceActivity {

    public static class MyPreferenceFragment extends PreferenceFragment
    {
        @Override
        public void onCreate(final Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            addPreferencesFromResource(R.xml.preferences);
        }
    }

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

        MyPreferenceFragment frag =  new MyPreferenceFragment();

        getFragmentManager().beginTransaction().replace(android.R.id.content, frag).commit();

        Preference pref;
        pref = frag.findPreference("pref_gps_updates");

        pref.setOnPreferenceChangeListener(new OnPreferenceChangeListener() {
            @Override
            public boolean onPreferenceChange(Preference preference, Object newValue) {
                return true;
            }
        });

    }
}

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.preference.Preference.setOnPreferenceChangeListener(android.preference.Preference$OnPreferenceChangeListener)' on a null object reference

我不明白为什么我会撞车，或者我做错了什么？

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android" >

    <EditTextPreference
        android:key="pref_gps_updates"
        android:inputType="number"
        android:title="@string/pref_gps_updates"
        android:summary="@string/pref_gps_updates_summary"
        android:defaultValue="60" />

</PreferenceScreen>    

Answer 1

Preference pref;
pref = frag.findPreference("pref_updates");

pref.setOnPreferenceChangeListener(new OnPreferenceChangeListener() {
    @Override
    public boolean onPreferenceChange(Preference preference, Object newValue) {
        return true;
    }
});

将此部分放入您的片段‘s onCreate() (活动onCreate中的NOT)中，并再次检查在xml中是否使用了密钥pref_updates。

更新：

public class MyPrefs extends PreferenceActivity {

    public static class MyPreferenceFragment extends PreferenceFragment
    {
        @Override
        public void onCreate(final Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            addPreferencesFromResource(R.xml.preferences);


            Preference pref;
            pref = findPreference("pref_gps_updates");

            pref.setOnPreferenceChangeListener(new OnPreferenceChangeListener() {
                @Override
                public boolean onPreferenceChange(Preference preference, Object newValue) {
                    return true;
                }
            });
        }
    }

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

        MyPreferenceFragment frag =  new MyPreferenceFragment();

        getFragmentManager().beginTransaction().replace(android.R.id.content, frag).commit();

    }
}

Answer 2

与其为每个首选项创建一个PreferenceChangedListener，不如让PreferenceFragment实现OnSharedPreferenceChangeListener接口。

这样，无论您添加了多少首选项，您都不需要为每个首选项添加一个新的侦听器，并且逻辑是自包含在PreferenceFragment中的。

会是这样的：

public class MyPrefs extends PreferenceActivity {

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

        MyPreferenceFragment frag = new MyPreferenceFragment();

        getFragmentManager().beginTransaction().replace(android.R.id.content, frag).commit();

        /* remove all of this
        Preference pref;
        pref = frag.findPreference("pref_gps_updates");
        pref.setOnPreferenceChangeListener(new OnPreferenceChangeListener() {
            @Override
            public boolean onPreferenceChange(Preference preference, Object newValue) {
                return true;
            }
        });
        */

    }


    public static class MyPreferenceFragment extends PreferenceFragment
            implements OnSharedPreferenceChangeListener {
        @Override
        public void onCreate(final Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            addPreferencesFromResource(R.xml.preferences);
        }

        @Override
        public void onResume() {
            super.onResume();
            getPreferenceScreen().getSharedPreferences().registerOnSharedPreferenceChangeListener(this);
        }

        @Override
        public void onPause() {
            super.onPause();
            getPreferenceScreen().getSharedPreferences().unregisterOnSharedPreferenceChangeListener(this);
        }

        @Override
        public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {

            if (key.equals("pref_gps_updates")) {
                //do something
            }
        }
    }
}

Answer 3

在您使用的代码中

pref = frag.findPreference("pref_updates");

但是在xml中，它说

android:key="pref_gps_updates"

更改键需要匹配才能找到。