Preference Summaries

It's not a great experience for our user that they have to click on the minimum magnitude preference to find out what its current setting is.

It'd be much nicer if, when we opened our Setting Activity, we could see the value of our preference right below the preference name, and when we change it, we see the summary update immediately.

It's often useful for the app to know immediately when a preference is changed, especially when that preference change should have some visible impact on the UI.

To do this, our PreferenceFragment can implement the OnPreferenceChangeListener interface to get notified when a preference changes. Then when a single Preference has been changed by the user and is about to be saved, the onPreferenceChange() method will be invoked with the key of the preference that was changed. Note that this method returns a boolean, which allows us to prevent a proposed preference change by returning false.

First declare that the EarthquakePreferenceFragment class should implement the OnPreferenceChangeListener interface, and override the onPreferenceChange() method. The code in this method takes care of updating the displayed preference summary after it has been changed.

In SettingsActivity, in EarthquakePreferenceFragment class:

  public static class EarthquakePreferenceFragment extends PreferenceFragment implements Preference.OnPreferenceChangeListener {

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

         @Override
         public boolean onPreferenceChange(Preference preference, Object value) {
             String stringValue = value.toString();
             preference.setSummary(stringValue);
             return true;
         }
     }

However, we still need to update the preference summary when the settings activity is launched. Given the key of a preference, we can use PreferenceFragment's findPreference() method to get the Preference object, and setup the preference using a helper method called bindPreferenceSummaryToValue().

In EarthquakePreferenceFragment:

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

     Preference minMagnitude = findPreference(getString(R.string.settings_min_magnitude_key));
     bindPreferenceSummaryToValue(minMagnitude);
 }

Now we need to define the bindPreferenceSummaryToValue() helper method to set the current EarthquakePreferenceFragment instance as the listener on each preference. We also read the current value of the preference stored in the SharedPreferences on the device, and display that in the preference summary (so that the user can see the current value of the preference).

In EarthquakePreferenceFragment:

 private void bindPreferenceSummaryToValue(Preference preference) {
     preference.setOnPreferenceChangeListener(this);
     SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(preference.getContext());
     String preferenceString = preferences.getString(preference.getKey(), "");
     onPreferenceChange(preference, preferenceString);
 }

The full SettingsActivity file will look like this after all the changes.

In SettingsActivity.java:

 package com.example.android.quakereport;

 import android.content.SharedPreferences;
 import android.os.Bundle;
 import android.preference.Preference;
 import android.preference.PreferenceFragment;
 import android.preference.PreferenceManager;
 import android.support.v7.app.AppCompatActivity;

 public class SettingsActivity extends AppCompatActivity {

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.settings_activity);
     }

     public static class EarthquakePreferenceFragment extends PreferenceFragment implements Preference.OnPreferenceChangeListener {

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

             Preference minMagnitude = findPreference(getString(R.string.settings_min_magnitude_key));
             bindPreferenceSummaryToValue(minMagnitude);
         }

         @Override
         public boolean onPreferenceChange(Preference preference, Object value) {
             String stringValue = value.toString();
             preference.setSummary(stringValue);
             return true;
         }

         private void bindPreferenceSummaryToValue(Preference preference) {
             preference.setOnPreferenceChangeListener(this);
             SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(preference.getContext());
             String preferenceString = preferences.getString(preference.getKey(), "");
             onPreferenceChange(preference, preferenceString);
         }
     }
 }

All code changes by the end of this lesson can be found here: https://github.com/udacity/ud843-QuakeReport/commit/8b84bc757c1ace9addce75396caa75ab076bf4ac

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