Android SharedPreferences / PreferenceFragment无效

Question

I recently studied the section about handling application preferences with shared preferences and the PreferenceFragment in the Android Developer Documentation and made the following simple example:

SettingsActivity.java

public class SettingsActivity extends PreferenceActivity {

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

        if(savedInstanceState == null)
            getFragmentManager().beginTransaction()
                .replace(android.R.id.content, new SettingsFragment())
                .commit()

    }

}

SettingsFragment.java

public class SettingsFragment extends PreferenceFragment {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        addPreferencesFromResource(R.xml.settings);
    }

}

res/xml/settings.xml

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

    <PreferenceCategory android:title="@string/pref_general_category">    

        <EditTextPreference
            android:key="@id/pref_key_apiBaseUri"
            android:title="@string/pref_apiBaseUri_title"
            android:defaultValue="@string/pref_apiBaseUri_default"
            android:persistent="true"
            android:inputType="text"
            android:singleLine="true" />

    </PreferenceCategory>

</PreferenceScreen>

Other stuff:

id        pref_key_apiBaseUri
string    pref_apiBaseUri_title       "Api Base Uri"
string    pref_api_baseUri_default    "http://acme.com/api/

Problem

Straightforward stuff, eh? I started the app, went to the preference menu / activity, the preference fragment got inflated and I clicked on " Api Base Uri ". An EditText dialog popped up, I changed " http://acme.com/api " to " http://acme.com/api2 " and pressed on OK. Now, as far as I understood the documentation, preferences should get saved automatically into the shared preferences . When I reopen the EditText dialog, I see that my changes are stored.

HOWEVER.. when I close the activity (or the app) and go back to the settings, again, the default value is back, no changes have been made! What can I do to permanently save my settings?

Answer 1

In your settings.xml, you have:

<EditTextPreference
    android:key="@id/pref_key_apiBaseUri"
    android:title="@string/pref_apiBaseUri_title"
    android:defaultValue="@string/pref_apiBaseUri_default"
    android:persistent="true"
    android:inputType="text"
    android:singleLine="true" />

You must specify a string for key, as the docs for android:key say:

This attribute is required for preferences that persist a data value. It specifies the unique key (a string) the system uses when saving this setting's value in the SharedPreferences.

It seems that your pref_key_apiBaseUri is an empty String. Change it to a valid String and move to strings.xml:

<string name="pref_key_apiBaseUri">KEY</string>

then use as:

<EditTextPreference
    android:key="@string/pref_key_apiBaseUri"
    ...

OR keep it as it is, and just add + in front of id tag, which will create id which in turn will be used as key:

<EditTextPreference
    android:key="@+id/pref_key_apiBaseUri"
    ...