Change the summary of a ListPreference with the new value (Android)

Nauman Zubair is right.

The %s implementation is buggy. The view shows the correct value on first load (if a default list value is set), but the view does not update when selecting a list item. You have to toggle a checkbox or some other preference to update the view.

As a workaround, implement OnSharedPreferenceChangeListener to override the summary for the list preference.

/src/apps/app_settings/SettingsActivity.java

package apps.app_settings;

import android.os.Bundle;
import android.preference.PreferenceActivity;

public class SettingsActivity extends PreferenceActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        /* set fragment */
        getFragmentManager().beginTransaction().replace(android.R.id.content, new SettingsFragment()).commit();
    }
}

/src/apps/app_settings/SettingsFragment.java

package apps.app_settings;

import android.content.SharedPreferences;
import android.content.SharedPreferences.OnSharedPreferenceChangeListener;
import android.os.Bundle;
import android.preference.ListPreference;
import android.preference.Preference;
import android.preference.PreferenceFragment;

public class SettingsFragment extends PreferenceFragment implements OnSharedPreferenceChangeListener {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        /* set preferences */
        addPreferencesFromResource(R.xml.preferences);
    }

    @Override
    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
        /* get preference */
        Preference preference = findPreference(key);

        /* update summary */
        if (key.equals("list_0")) {
            preference.setSummary(((ListPreference) preference).getEntry());
        }
    }

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

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

/res/xml/preferences.xml

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen
  xmlns:android="http://schemas.android.com/apk/res/android">
  <ListPreference
    android:key="list_0"
    android:title="@string/settings_list_0_title"
    android:summary="%s"
    android:entries="@array/settings_list_0_entries"
    android:entryValues="@array/settings_list_0_entry_values"
    android:defaultValue="@string/settings_list_0_default_value"/>
</PreferenceScreen>

/res/values/strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="settings_list_0_title">list 0</string>
  <string-array name="settings_list_0_entries">
    <item>item 0</item>
    <item>item 1</item>
    <item>item 2</item>
  </string-array>
  <string-array name="settings_list_0_entry_values">
    <item>0</item>
    <item>1</item>
    <item>2</item>
  </string-array>
  <string name="settings_list_0_default_value">0</string>
</resources>

Just set the summary value to %s in the xml description.

EDIT: I've tested it on several devices and it doesn't work really. That's strange because according to docs it must work: ListPreference.getSummary().

But it's possible to implement this functionality by oneself. The implementation requires to inherit from the ListPreference class:

public class MyListPreference extends ListPreference {
    public MyListPreference(final Context context) {
        this(context, null);
    }

    public MyListPreference(final Context context, final AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public CharSequence getSummary() {
        final CharSequence entry = getEntry();
        final CharSequence summary = super.getSummary();
        if (summary == null || entry == null) {
             return null;
        } else {
            return String.format(summary.toString(), entry);
        }
    }

    @Override
    public void setValue(final String value) {
        super.setValue(value);
        notifyChanged();
    }
}

As you can see MyListPreference has its own summary field which can contain formatting markers. And when a preference's value changes, Preference.notifyChanged() method is called and it causes MyListPreference.getSummary() method to be called from Preference.onBindView().

P.S.: This approach hasn't been tested sufficiently so it may contain errors.