Equivalent of iOS NSNotificationCenter in Android?

Question

Is there an equivalent of the iOS class NSNotificationCenter in Android ? Are there any libraries or useful code available to me ?

Answer 1

If you don't want to use Observer - it can be problematic in cases you want a Fragment to be your Observer cause you can't extend more then one class- You can use google's Guava Library (https://code.google.com/p/guava-libraries/) for "Function" and "Multimap" - although you can use as well HashMap> for the subscibersCollection

and implement something like this:

    import java.util.Collection;
import com.google.common.collect.ArrayListMultimap;
import com.google.common.base.Function;

public class EventService {

    ArrayListMultimap<String, Function<Object, Void>> subscibersCollection;

    private static EventService instance = null;

    private static final Object locker = new Object();

    private EventService() {
        subscibersCollection = ArrayListMultimap.create();
    }

    public static EventService getInstance() {
        if (instance == null) {
            synchronized (locker) {
                if (instance == null) {
                    instance = new EventService();
                }
            }
        }
        return instance;
    }

    /**
     * Subscribe to the notification, and provide the callback functions in case
     * notification is raised.
     * 
     * @param notification
     *            - notification name
     * @param func
     *            - function to apply when notification is raised
     */
    public void addSubscription(String notification, Function<Object, Void> func) {
        synchronized (subscibersCollection) {
            if (!subscibersCollection.containsEntry(notification, func)) {
                subscibersCollection.put(notification, func);
            }
        }
    }

    /**
     * Remove subscription from notification
     */
    public void removeSubscription(String notification,
            Function<Object, Void> func) {
        synchronized (subscibersCollection) {
            subscibersCollection.remove(notification, func);
        }
    }

    /**
     * raise notification for all its subscribers
     * 
     * @param notification
     *            - notification name
     * @param data
     *            - update data
     */
    public void publish(String notification, Object data) {
        Collection<Function<Object, Void>> observableList = subscibersCollection
                .get(notification);
        for (Function<Object, Void> func : observableList) {
            func.apply(data);
        }
    }
}

Answer 2

On the basis of Behlül answer, I change the code to make it closer to iOS NSNotificationCenter.

Another thing: the notifications will fire on the main thread

package com.oxygen.utils;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import android.os.Handler;

public class NotificationCenter {

//---------------- event type list ---------------------

    public static enum NotificationID{
        IMAGES_CACHE_READY
    } 


//---------------- singelton ---------------------------    

        private static NotificationCenter instance = null;

        private NotificationCenter() { observables = new HashMap<NotificationID, MyObservable>();   }

        public static synchronized NotificationCenter singelton() {
            if (instance == null) {
                        instance = new NotificationCenter ();
            }
            return instance;
        }

//-------------------------------------------

                public class Notification {

                    private Object poster;  // the object that post the event
                    private Object info;    // event specific data 
                    private NotificationID id;      // event name

                    public Notification(Object poster, NotificationID id, Object info) {
                        super();
                        this.poster = poster;
                        this.info = info;
                        this.id = id;
                    }

                    public Object getPoster() {
                        return poster;
                    }

                    public Object getInfo() {
                        return info;
                    }

                    public NotificationID getId() {
                        return id;
                    }
                }
        //-------------------------------------------

                public interface Notifiable {
                    public void onNotification(Notification notify);
                }

//-------------------------------------------

        protected class MyObservable {
            List<Notifiable> observers = new ArrayList<Notifiable>();

            public MyObservable() {
            }

            public void addObserver(Notifiable observer) {
                if (observer == null) {
                    throw new NullPointerException("observer == null");
                }
                synchronized (this) {
                    if (!observers.contains(observer))
                        observers.add(observer);
                }
            }

            public int countObservers() {
                return observers.size();
            }

            public synchronized void deleteObserver(Notifiable observer) {
                observers.remove(observer);
            }

            public synchronized void deleteObservers() {
                observers.clear();
            }

            public void notifyObservers(Notification notify) {
                int size = 0;
                Notifiable[] arrays = null;
                synchronized (this) {
                        size = observers.size();
                        arrays = new Notifiable[size];
                        observers.toArray(arrays);
                }
                if (arrays != null) {
                    for (Notifiable observer : arrays) {
                        observer.onNotification(notify);
                    }
                }
            }
        }

//-------------------------------------------

        HashMap<NotificationID, MyObservable > observables;

        public void addObserver(NotificationID id, Notifiable observer) {
            MyObservable observable = observables.get(id);
            if (observable==null) {
                observable = new MyObservable ();
                observables.put(id, observable);
            }
            observable.addObserver(observer);
        }

        public void removeObserver(NotificationID id, Notifiable observer) {
            MyObservable observable = observables.get(id);
            if (observable!=null) {         
                observable.deleteObserver(observer);
            }
        }

        public void removeObserver(Notifiable observer) {
             for (MyObservable observable : observables.values()) {
                 if (observable!=null) {         
                    observable.deleteObserver(observer);
                }    
            }
        }

        public void postNotification(final Object notificationPoster, final NotificationID id, final Object notificationInfo) {

            final MyObservable  observable = observables.get(id);
            if (observable!=null) {

                // notification post to the maim (UI) thread    
                // Get a handler that can be used to post to the main thread
                Handler mainHandler = new Handler(AppContext.get().getMainLooper());

                Runnable myRunnable = new Runnable() {

                    @Override
                    public void run() {
                        observable.notifyObservers(new Notification(notificationPoster, id, notificationInfo) );
                    }
                };

                mainHandler.post(myRunnable);
            }
        }
}

Listener sample:

public class CustomGridViewAdapter extends ArrayAdapter<Category>  implements Notifiable {
    int layoutResourceId;

    public CustomGridViewAdapter(Context context, int layoutResourceId) {
        super(context, layoutResourceId);
        this.layoutResourceId = layoutResourceId;

        loadCategories(false);
        NotificationCenter.singelton().addObserver(NotificationID.IMAGES_CACHE_READY, this);
    }

    public void onDestroy() {
        NotificationCenter.singelton().removeObserver(this);            
    }

    @Override
    public void onNotification(Notification notify) {
        switch (notify.getId()) {
        case IMAGES_CACHE_READY:
            loadCategories(true);
            break;
        }
    }
...
}

Answer 3

i had the same wondrings.. so i wrote this:

public class NotificationCenter {

    //static reference for singleton
    private static NotificationCenter _instance;

    private HashMap<String, ArrayList<Runnable>> registredObjects;

    //default c'tor for singleton
    private NotificationCenter(){
        registredObjects = new HashMap<String, ArrayList<Runnable>>();
    }

    //returning the reference
    public static synchronized NotificationCenter defaultCenter(){
        if(_instance == null)
            _instance = new NotificationCenter();
        return _instance;
    }

    public synchronized void addFucntionForNotification(String notificationName, Runnable r){
        ArrayList<Runnable> list = registredObjects.get(notificationName);
        if(list == null) {
            list = new ArrayList<Runnable>();
            registredObjects.put(notificationName, list);
        }
        list.add(r);
    }

    public synchronized void removeFucntionForNotification(String notificationName, Runnable r){
        ArrayList<Runnable> list = registredObjects.get(notificationName);
        if(list != null) {
            list.remove(r);
        }
    }

    public synchronized void postNotification(String notificationName){
        ArrayList<Runnable> list = registredObjects.get(notificationName);
        if(list != null) {
            for(Runnable r: list)
                r.run();
        }
    }

}

and a usage for this will be:

  NotificationCenter.defaultCenter().addFucntionForNotification("buttonClick", new Runnable() {
        @Override
        public void run() {
            Toast.makeText(MainActivity.this, "Hello There", Toast.LENGTH_LONG).show();
        }
    });

tried to make the interface as similar to IOS as possible, but simpler (no object registration needed).

hope that helps:)

Answer 4

In Android there is not a central notification center as in ios. But you can basically use Observable and Observer objects to achieve your task.

You can define a class like something below, just modify it for singleton use and add synchronized for concurrent use but the idea is the same:

public class ObservingService {
    HashMap<String, Observable> observables;

    public ObservingService() {
        observables = new HashMap<String, Observable>();
    }

    public void addObserver(String notification, Observer observer) {
        Observable observable = observables.get(notification);
        if (observable==null) {
            observable = new Observable();
            observables.put(notification, observable);
        }
        observable.addObserver(observer);
    }

    public void removeObserver(String notification, Observer observer) {
        Observable observable = observables.get(notification);
        if (observable!=null) {         
            observable.deleteObserver(observer);
        }
    }       

    public void postNotification(String notification, Object object) {
        Observable observable = observables.get(notification);
        if (observable!=null) {
            observable.setChanged();
            observable.notifyObservers(object);
        }
    }
}

Answer 5

Take a look at the Otto event bus from Square:

http://square.github.com/otto/

It has essentially the same features as NSNotificationCenter but thanks to annotations and static typing it is easier to follow the dependencies of components and paths that events follow. It's much simpler to use than the stock Android broadcast API, IMO.