Tag: onCreatePreferences

PreferenceChangeListener

The differences between PreferenceChangeListener and SharedPreferenceChangeListener are:

  • SharedPreferenceChangeListener is triggered after any value is saved to the SharedPreferences file.
  • PreferenceChangeListener is triggered before a value is saved to the SharedPreferences file. Because of this, it can prevent an invalid update to a preference.
  • PreferenceChangeListeners are also attached to a single preference.

So it can be used as follows:

  • User updates a preference.
  • PreferenceChangeListener triggered for that preference e.g. to validate input.
  • The new value is saved to the SharedPreference file.
  • onSharedPreferenceChanged listeners are triggered.
Implement Preference.OnPreferenceChangeListener:
public class SettingsFragment extends PreferenceFragmentCompat implements OnSharedPreferenceChangeListener, Preference.OnPreferenceChangeListener



In the override for onCreatePreferences attach the listener to the chosen Preference:


@Override
public void onCreatePreferences(Bundle bundle, String s) {
         /* Other preference setup code code */
        //...
        Preference preference = findPreference(getString(R.string.pref_xxxx_key)); // Key for the Preference to watch. Customise
        preference.setOnPreferenceChangeListener(this); // Attach the listener to the Preference
}



In this example we will use the onPreferenceChange override to validate the input and restrict it to a number between 0.1 and 3:


@Override
public boolean onPreferenceChange(Preference preference, Object newValue) {
   Toast error = Toast.makeText(getContext(), "Please select a number between 0.1 and 3", Toast.LENGTH_SHORT); // Error Toast if input invalid

   String xxxxKey = getString(R.string.pref_xxxx_key);
   if (preference.getKey().equals(xxxxKey)) {
       String stringInput = (String) newValue; // Cast the Object o a String
       try {
           float input = Float.parseFloat(stringInput); // Try converting to a float
           if (input > 3 || input <= 0) { // If outside the accepted range...
               error.show(); // ...show the error Toast...
               return false; // ...and fail to register the new change
           }
       } catch (NumberFormatException nfe) { // If cannot convert to float then throw an exception
           error.show();
           return false;
       }
   }
   return true; // Otherwise accept the value
}



ud851-Exercises-student\Lesson06-Visualizer-Preferences\T06.10

Setting Preference Summary/Label for ListPreference

ListPreference items in Preference screens do not automatically provide a label showing their current value. You can create labels for all list Preference items as so:

In the Settings Fragment onCreatePreferences override, get the Shared Preferences and the Preference screen. Then iterate through all of the Preferences in the screen, checking whether they are tickboxes. Any that aren't will have their value stored and passed to a setPreferenceSummary function along with the Preference itself:
    @Override
    public void onCreatePreferences(Bundle bundle, String s) {

        addPreferencesFromResource(R.xml.pref_xxxxx); // Reference to xml resource
        SharedPreferences sharedPreferences = getPreferenceScreen().getSharedPreferences(); // Retrieve the Shared Preferences from the Preference screen
        PreferenceScreen preferenceScreen = getPreferenceScreen(); // Retrieve the Preference screen
        int count = preferenceScreen.getPreferenceCount(); // The number of Preferences within the screen

        for (int i = 0; i < count; i++ ) { // For each Preference...
            Preference p = preferenceScreen.getPreference(i); // ...retrieve the Preference...
            if (!(p instanceof CheckBoxPreference)) { // ...make sure it isn't a checkbox...
                String value = sharedPreferences.getString(p.getKey(), ""); // ...then retrieve its value (can't use getString on a Boolean, so another reason to exclude tickboxes)...
                setPreferenceSummary(p, value); // ...and send the Preference and its value to the setPreferenceSummary function
            }
        }
    }



The setPreferenceSummary function is as follows:


    private void setPreferenceSummary(Preference preference, String value) {
        if (preference instanceof ListPreference) { // If it is a ListPreference...
            ListPreference listPreference = (ListPreference) preference; // ...cast it to an object of type ListPreference...
            int prefIndex = listPreference.findIndexOfValue(value); // ...retrieve the index of that value...
            if (prefIndex >= 0) {
                listPreference.setSummary(listPreference.getEntries()[prefIndex]); // ...and get the corresponding label for it, setting it as the Preference Summary
            }
        } else if (preference instanceof EditTextPreference) {
            // For EditTextPreferences, set the summary to the value's simple string representation.
            preference.setSummary(value);
        }
    }



Implement an OnSharedPreferenceChangeListener to make sure changes are implemented without having to force an onCreate (e.g. device rotation). Within the required onSharedPreferenceChanged override, again check to see if the preference is not a checkbox, and if so send the preference to the setPreferenceSummary function along with its value:


public class SettingsFragment extends PreferenceFragmentCompat implements OnSharedPreferenceChangeListener {

    @Override
    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
        Preference preference = findPreference(key);
        if (preference != null) {
            if (!(preference instanceof CheckBoxPreference)) {
                String value = sharedPreferences.getString(preference.getKey(), "");
                setPreferenceSummary(preference, value);
            }
        }
    }
}



Register and unregister the OnSharedPreferenceChangeListener with the following overrides:


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getPreferenceScreen().getSharedPreferences().registerOnSharedPreferenceChangeListener(this);
    }

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



ud851-Exercises-student\Lesson06-Visualizer-Preferences\T06.08