SharedPreferences.onSharedPreferenceChangeListener not being called consistently

Question

I'm registering a preference change listener like this (in the onCreate() of my main activity):

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);

prefs.registerOnSharedPreferenceChangeListener(
   new SharedPreferences.OnSharedPreferenceChangeListener() {
       public void onSharedPreferenceChanged(
         SharedPreferences prefs, String key) {

         System.out.println(key);
       }
});

The trouble is, the listener is not always called. It works for the first few times a preference is changed, and then it is no longer called until I uninstall and reinstall the app. No amount of restarting the application seems to fix it.

I found a mailing list thread reporting the same problem, but no one really answered him. What am I doing wrong?

Answer 1

This is a sneaky one. SharedPreferences keeps listeners in a WeakHashMap. This means that you cannot use an anonymous inner class as a listener, as it will become the target of garbage collection as soon as you leave the current scope. It will work at first, but eventually, will get garbage collected, removed from the WeakHashMap and stop working.

Keep a reference to the listener in a field of your class and you will be OK, provided your class instance is not destroyed.

i.e. instead of:

prefs.registerOnSharedPreferenceChangeListener(
  new SharedPreferences.OnSharedPreferenceChangeListener() {
  public void onSharedPreferenceChanged(SharedPreferences prefs, String key) {
    // Implementation
  }
});

do this:

// Use instance field for listener
// It will not be gc'd as long as this instance is kept referenced
listener = new SharedPreferences.OnSharedPreferenceChangeListener() {
  public void onSharedPreferenceChanged(SharedPreferences prefs, String key) {
    // Implementation
  }
};

prefs.registerOnSharedPreferenceChangeListener(listener);

The reason unregistering in the onDestroy method fixes the problem is because to do that you had to save the listener in a field, therefore preventing the issue. It's the saving the listener in a field that fixes the problem, not the unregistering in onDestroy.

UPDATE: The Android docs have been updated with warnings about this behavior. So, oddball behavior remains. But now it's documented.

Answer 2

this accepted answer is ok, as for me it is creating new instance each time the activity resumes

so how about keeping the reference to the listener within the activity

OnSharedPreferenceChangeListener listener = new OnSharedPreferenceChangeListener(){
      public void onSharedPreferenceChanged(SharedPreferences prefs, String key) {
         // your stuff
      }
};

and in your onResume and onPause

@Override     
public void onResume() {
    super.onResume();          
    getPreferenceScreen().getSharedPreferences().registerOnSharedPreferenceChangeListener(listener);     
}

@Override     
public void onPause() {         
    super.onPause();          
    getPreferenceScreen().getSharedPreferences().unregisterOnSharedPreferenceChangeListener(listener);

}

this will very similar to what you are doing except we are maintaining a hard reference.

Answer 3

The accepted answer creates a SharedPreferenceChangeListener every time onResume is called. @Samuel solves it by making SharedPreferenceListener a member of the Activity class. But there's a third and a more straightforward solution that Google also uses in this codelab. Make your activity class implement the OnSharedPreferenceChangeListener interface and override onSharedPreferenceChanged in the Activity, effectively making the Activity itself a SharedPreferenceListener.

public class MainActivity extends Activity implements SharedPreferences.OnSharedPreferenceChangeListener {

    @Override
    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String s) {

    }

    @Override
    protected void onStart() {
        super.onStart();
        PreferenceManager.getDefaultSharedPreferences(this)
                .registerOnSharedPreferenceChangeListener(this);
    }

    @Override
    protected void onStop() {
        super.onStop();
        PreferenceManager.getDefaultSharedPreferences(this)
                .unregisterOnSharedPreferenceChangeListener(this);
    }
}