Android - Check Network State in your app, Notifying you when an app is online/offline

MainActivity.java

/**
* This would be the activity which registers the receiver class via it's interface
*/
public class MainActivity implements NetworkStateReceiver.NetworkStateReceiverListener {

    private NetworkStateReceiver networkStateReceiver;      // Receiver that detects network state changes

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        /***/
        startNetworkBroadcastReceiver(this);
    }
    
    @Override
    protected void onPause() {
        /***/
        unregisterNetworkBroadcastReceiver(this);
        super.onPause();
    }

    @Override
    protected void onResume() {
        /***/
        registerNetworkBroadcastReceiver(this);
        super.onResume();
    }

    @Override
    public void networkAvailable() {
        Log.i(TAG, "networkAvailable()");
        //Proceed with online actions in activity (e.g. hide offline UI from user, start services, etc...)
    }

    @Override
    public void networkUnavailable() {
        Log.i(TAG, "networkUnavailable()");
        //Proceed with offline actions in activity (e.g. sInform user they are offline, stop services, etc...)
    }

    public void startNetworkBroadcastReceiver(Context currentContext) {
        networkStateReceiver = new NetworkStateReceiver();
        networkStateReceiver.addListener((NetworkStateReceiver.NetworkStateReceiverListener) currentContext);
        registerNetworkBroadcastReceiver(currentContext);
    }

    /**
     * Register the NetworkStateReceiver with your activity
     * @param currentContext
     */
    public void registerNetworkBroadcastReceiver(Context currentContext) {
        currentContext.registerReceiver(networkStateReceiver, new IntentFilter(android.net.ConnectivityManager.CONNECTIVITY_ACTION));
    }

    /**
     Unregister the NetworkStateReceiver with your activity
     * @param currentContext
     */
    public void unregisterNetworkBroadcastReceiver(Context currentContext) {
        currentContext.unregisterReceiver(networkStateReceiver);
    }
}

NetworkStateReceiver.java

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.util.Log;

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

/**
 * NetworkStateReceiver defines a BroadcastReceiver which allows us to register for system (i.e. network status) or application events.
 * All registered receivers for an event are notified by the Android runtime once this event happens.
 * Source: http://stackoverflow.com/questions/6169059/android-event-for-internet-connectivity-state-change
 */
public class NetworkStateReceiver extends BroadcastReceiver {

    protected List<NetworkStateReceiverListener> listeners;
    protected Boolean connected;
    private String TAG = "NetworkStateReceiver";

    public NetworkStateReceiver() {
        listeners = new ArrayList<>();
        connected = null;
    }

    /**
     * Called when the BroadcastReceiver is receiving an Intent broadcast (event for which the broadcast receiver has registered occurs).
     * During this time you can use the other methods on BroadcastReceiver to view/modify the current result values.
     * NOTE: When it runs on the main thread you should never perform long-running operations in it (there is a timeout of 10 seconds that the system allows before considering the receiver to be blocked and a candidate to be killed).
     * NOTE: You cannot launch a popup dialog in your implementation of onReceive().
     * @param context   Object to access additional information or to start services or activities
     * @param intent    Object with action used to register your receiver. This object contains additional information (e.g. extras)
     */
    public void onReceive(Context context, Intent intent) {
        Log.i(TAG, "Intent broadcast received");
        if(intent == null || intent.getExtras() == null)
            return;

        // Retrieve a ConnectivityManager for handling management of network connections
        ConnectivityManager manager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        // Details about the currently active default data network. When connected, this network is the default route for outgoing connections
        NetworkInfo networkInfo = manager.getActiveNetworkInfo();

        /**
         * NOTE: getActiveNetworkInfo() may return null when there is no default network e.g. Airplane Mode
         */
        if(networkInfo != null && networkInfo.getState() == NetworkInfo.State.CONNECTED) {
            connected = true;
        } else if(intent.getBooleanExtra(ConnectivityManager.EXTRA_NO_CONNECTIVITY, Boolean.FALSE)) {    //Boolean that indicates whether there is a complete lack of connectivity
            connected = false;
        }

        notifyStateToAll();
    }   //After the onReceive() of the receiver class has finished, the Android system is allowed to recycle the receiver

    /**
     * Notify the state to all needed methods
     */
    private void notifyStateToAll() {
        Log.i(TAG, "Notifying state to " + listeners.size() + " listener(s)");
        for(NetworkStateReceiverListener eachNetworkStateReceiverListener : listeners)
            notifyState(eachNetworkStateReceiverListener);
    }

    /**
     * Notify the network state, triggering interface functions based on the current state
     * @param networkStateReceiverListener  Object which implements the NetworkStateReceiverListener interface
     */
    private void notifyState(NetworkStateReceiverListener networkStateReceiverListener) {
        if(connected == null || networkStateReceiverListener == null)
            return;

        if(connected == true) {
            // Triggering function on the interface towards network availability
            networkStateReceiverListener.networkAvailable();
        } else {
            // Triggering function on the interface towards network being unavailable
            networkStateReceiverListener.networkUnavailable();
        }
    }

    /**
     * Adds a listener to the list so that it will receive connection state change updates
     * @param networkStateReceiverListener     Object which implements the NetworkStateReceiverListener interface
     */
    public void addListener(NetworkStateReceiverListener networkStateReceiverListener) {
        Log.i(TAG, "addListener() - listeners.add(networkStateReceiverListener) + notifyState(networkStateReceiverListener);");
        listeners.add(networkStateReceiverListener);
        notifyState(networkStateReceiverListener);
    }

    /**
     * Removes listener (when no longer necessary) from the list so that it will no longer receive connection state change updates
     * @param networkStateReceiverListener     Object which implements the NetworkStateReceiverListener interface
     */
    public void removeListener(NetworkStateReceiverListener networkStateReceiverListener) {
        listeners.remove(networkStateReceiverListener);
    }

    /**
     * Inner Interface (i.e. to encapsulate behavior in a generic and re-usable way) which handles connection state changes for classes which registered this receiver (Outer class NetworkStateReceiver)
     * This interface implements the 'Strategy Pattern', where an execution strategy is evaluated and applied internally at runtime
     */
    public interface NetworkStateReceiverListener {
        /**
         * When the connection state is changed and there is a connection, this method is called
         */
        void networkAvailable();

        /**
         * Connection state is changed and there is not a connection, this method is called
         */
        void networkUnavailable();
    }
}