java - PreferenceFragment 空指针异常

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;
            }
        });

    }
}

由以下原因引起:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“void android.preference.Preference.setOnPreferenceChangeListener(android.preference.Preference$OnPreferenceChangeListener)”

我不明白为什么会发生这个崩溃或者我做错了什么？

<?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 Fragment 的 onCreate() 中(NOT 位于 Activity onCreate 中)，并仔细检查键 pref_updates 在您的 xml 中使用。

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();

    }
}