SharedPreferences.onSharedPreferenceChangeListener not being called consistently

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?

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.

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.

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);
    }
}

As this is the most detailed page for the topic I want to add my 50ct.

I had the problem that OnSharedPreferenceChangeListener wasn't called. My SharedPreferences are retrieved at the start of the main Activity by:

prefs = PreferenceManager.getDefaultSharedPreferences(this);

My PreferenceActivity code is short and does nothing except showing the preferences:

public class Preferences extends PreferenceActivity {
	@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // load the XML preferences file
        addPreferencesFromResource(R.xml.preferences);
    }
}

Every time the menu button is pressed I create the PreferenceActivity from the main Activity:

@Override
public boolean onPrepareOptionsMenu(Menu menu) {
	super.onCreateOptionsMenu(menu);
    //start Preference activity to show preferences on screen
	startActivity(new Intent(this, Preferences.class));
    //hook into sharedPreferences. THIS NEEDS TO BE DONE AFTER CREATING THE ACTIVITY!!!
	prefs.registerOnSharedPreferenceChangeListener(this);
	return false;
}

Note that registering the OnSharedPreferenceChangeListener needs to be done AFTER creating the PreferenceActivity in this case, else the Handler in the main Activity won't be called!!! It took me some sweet time to realize that...

Kotlin Code for register SharedPreferenceChangeListener it detect when change will happening on the saved key :

  PreferenceManager.getDefaultSharedPreferences(this)
        .registerOnSharedPreferenceChangeListener { sharedPreferences, key ->
            if(key=="language") {
                //Do Something 
            }
        }

you can put this code in onStart() , or somewhere else.. *Consider that you must use

 if(key=="YourKey")

or your codes inside "//Do Something " block will be run wrongly for every change that will happening in any other key in sharedPreferences

So, I don't know if this would really help anyone though, it solved my issue. Even though I had implemented the OnSharedPreferenceChangeListener as stated by the accepted answer. Still, I had an inconsistency with the listener being called.

I came here to understand that the Android just sends it for garbage collection after some time. So, I looked over at my code. To my shame, I had not declared the listener GLOBALLY but instead inside the onCreateView. And that was because I listened to the Android Studio telling me to convert the listener to a local variable.

It make sense that the listeners are kept in WeakHashMap.Because most of the time, developers prefer to writing the code like this.

PreferenceManager.getDefaultSharedPreferences(getApplicationContext()).registerOnSharedPreferenceChangeListener(
    new OnSharedPreferenceChangeListener() {
    @Override
    public void onSharedPreferenceChanged(
	    SharedPreferences sharedPreferences, String key) {
	    Log.i(LOGTAG, "testOnSharedPreferenceChangedWrong key =" + key);
    }
});

This may seem not bad. But if the OnSharedPreferenceChangeListeners' container was not WeakHashMap, it would be very bad.If the above code was written in an Activity . Since you are using non-static (anonymous) inner class which will implicitly holds the reference of the enclosing instance. This will cause memory leak.

What's more, If you keep the listener as a field, you could use registerOnSharedPreferenceChangeListener at the start and call unregisterOnSharedPreferenceChangeListener in the end. But you can not access a local variable in a method out of it's scope. So you just have the opportunity to register but no chance to unregister the listener. Thus using WeakHashMap will resolve the problem. This is the way I recommend.

If you make the listener instance as a static field, It will avoid the memory leak caused by non-static inner class. But as the listeners could be multiple, It should be instance-related. This will reduce the cost of handling the onSharedPreferenceChanged callback.

While reading Word readable data shared by first app,we should

Replace

getSharedPreferences("PREF_NAME", Context.MODE_PRIVATE);

with

getSharedPreferences("PREF_NAME", Context.MODE_MULTI_PROCESS);

in second app to get updated value in second app.

But still it is not working...