Develop Reference

What is Firebase Event Listener's back-off policy?

I have a project where a user can have multiple logins across multiple devices.
Now the user can subscribe to a particular topic on any device and the need is that the rest of the device logins should also do the same. Similar case is when one device unsubscribes, the rest should also follow suite.
In order to do this, I have made a Node under each user where all the subscriptions are maintained in the firebase database. I have a START_STICKY service which attaches a Firebase listener to this location and subs/unsubs from the topics when the changes occur. The code for the service is attached under the description.
In regular usage from observation, the service that i have does re-spawn due to the start sticky in case the system kills it. It will also explicitly respawn in case the user tampers with it using the developer options. The only cases which will cause it to completely cease are :
signout
data cleared
force stop
My questions are
how badly will keeping the listener attached affect the battery life. AFAIK Firebase has an exponential backoff when the web socket disconnects to prevent constant battery drain
Can the firebase listener just give up reconnecting if the connection is off for quite some time? If so, when is the backoff limit reached.
Is there a better way to ensure that a topic is subscribed and unsubscribed across multiple devices?
Is the service a good way to do this? can the following service be optimised? And yes it does need to run constantly.
Code
public class SubscriptionListenerService extends Service {
DatabaseReference userNodeSubscriptionRef;
ChildEventListener subscribedTopicsListener;
SharedPreferences sessionPref,subscribedTopicsPreference;
SharedPreferences.Editor subscribedtopicsprefeditor;
String userid;
boolean stoppedInternally = false;
SharedPreferences.OnSharedPreferenceChangeListener sessionPrefChangeListener;
#Nullable
#Override
public IBinder onBind(Intent intent) {
//do not need a binder over here
return null;
}
#Override
public void onCreate(){
super.onCreate();
Log.d("FragmentCreate","onCreate called inside service");
sessionPref = getSharedPreferences("SessionPref",0);
subscribedTopicsPreference=getSharedPreferences("subscribedTopicsPreference",0);
subscribedtopicsprefeditor=subscribedTopicsPreference.edit();
userid = sessionPref.getString("userid",null);
sessionPrefChangeListener = new SharedPreferences.OnSharedPreferenceChangeListener() {
#Override
public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
Log.d("FragmentCreate","The shared preference changed "+key);
stoppedInternally=true;
sessionPref.unregisterOnSharedPreferenceChangeListener(this);
if(userNodeSubscriptionRef!=null && subscribedTopicsListener!=null){
userNodeSubscriptionRef.removeEventListener(subscribedTopicsListener);
}
stopSelf();
}
};
sessionPref.registerOnSharedPreferenceChangeListener(sessionPrefChangeListener);
subscribedTopicsListener = new ChildEventListener() {
#Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
if(!(dataSnapshot.getValue() instanceof Boolean)){
Log.d("FragmentCreate","Please test subscriptions with a boolean value");
}else {
if ((Boolean) dataSnapshot.getValue()) {
//here we subscribe to the topic as the topic has a true value
Log.d("FragmentCreate", "Subscribing to topic " + dataSnapshot.getKey());
subscribedtopicsprefeditor.putBoolean(dataSnapshot.getKey(), true);
FirebaseMessaging.getInstance().subscribeToTopic(dataSnapshot.getKey());
} else {
//here we unsubscribed from the topic as the topic has a false value
Log.d("FragmentCreate", "Unsubscribing from topic " + dataSnapshot.getKey());
subscribedtopicsprefeditor.remove(dataSnapshot.getKey());
FirebaseMessaging.getInstance().unsubscribeFromTopic(dataSnapshot.getKey());
}
subscribedtopicsprefeditor.commit();
}
}
#Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
//either an unsubscription will trigger this, or a re-subscription after an unsubscription
if(!(dataSnapshot.getValue() instanceof Boolean)){
Log.d("FragmentCreate","Please test subscriptions with a boolean value");
}else{
if((Boolean)dataSnapshot.getValue()){
Log.d("FragmentCreate","Subscribing to topic "+dataSnapshot.getKey());
subscribedtopicsprefeditor.putBoolean(dataSnapshot.getKey(),true);
FirebaseMessaging.getInstance().subscribeToTopic(dataSnapshot.getKey());
}else{
Log.d("FragmentCreate","Unsubscribing from topic "+dataSnapshot.getKey());
subscribedtopicsprefeditor.remove(dataSnapshot.getKey());
FirebaseMessaging.getInstance().unsubscribeFromTopic(dataSnapshot.getKey());
}
subscribedtopicsprefeditor.commit();
}
}
#Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
//Log.d("FragmentCreate","Unubscribing from topic "+dataSnapshot.getKey());
//FirebaseMessaging.getInstance().unsubscribeFromTopic(dataSnapshot.getKey());
}
#Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
//do nothing, this won't happen --- rather this isnt important
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.d("FragmentCreate","Failed to listen to subscriptions node");
}
};
if(userid!=null){
Log.d("FragmentCreate","Found user id in service "+userid);
userNodeSubscriptionRef = FirebaseDatabase.getInstance().getReference().child("Users").child(userid).child("subscriptions");
userNodeSubscriptionRef.addChildEventListener(subscribedTopicsListener);
userNodeSubscriptionRef.keepSynced(true);
}else{
Log.d("FragmentCreate","Couldn't find user id");
stoppedInternally=true;
stopSelf();
}
}
#Override
public int onStartCommand(Intent intent,int flags,int startId){
//don't need anything done over here
//The intent can have the following extras
//If the intent was started by the alarm manager ..... it will contain android.intent.extra.ALARM_COUNT
//If the intent was sent by the broadcast receiver listening for boot/update ... it will contain wakelockid
//If it was started from within the app .... it will contain no extras in the intent
//The following will not throw an exception if the intent does not have an wakelockid in extra
//As per android doc... the following method releases the wakelock if any specified inside the extra and returns true
//If no wakelockid is specified, it will return false;
if(intent!=null){
if(BootEventReceiver.completeWakefulIntent(intent)){
Log.d("FragmentCreate","Wakelock released");
}else{
Log.d("FragmentCreate","Wakelock not acquired in the first place");
}
}else{
Log.d("FragmentCreate","Intent started by regular app usage");
}
return START_STICKY;
}
#Override
public void onDestroy(){
if(userNodeSubscriptionRef!=null){
userNodeSubscriptionRef.keepSynced(false);
}
userNodeSubscriptionRef = null;
subscribedTopicsListener = null;
sessionPref = null;
subscribedTopicsPreference = null;
subscribedtopicsprefeditor = null;
userid = null;
sessionPrefChangeListener = null;
if(stoppedInternally){
Log.d("FragmentCreate","Service getting stopped due to no userid or due to logout or data clearance...do not restart auto.. it will launch when user logs in or signs up");
}else{
Log.d("FragmentCreate","Service getting killed by user explicitly from running services or by force stop ... attempt restart");
//well basically restart the service using an alarm manager ... restart after one minute
AlarmManager alarmManager = (AlarmManager) this.getSystemService(ALARM_SERVICE);
Intent restartServiceIntent = new Intent(this,SubscriptionListenerService.class);
restartServiceIntent.setPackage(this.getPackageName());
//context , uniqueid to identify the intent , actual intent , type of pending intent
PendingIntent pendingIntentToBeFired = PendingIntent.getService(this,1,restartServiceIntent,PendingIntent.FLAG_ONE_SHOT);
if(Build.VERSION.SDK_INT>=23){
alarmManager.setExactAndAllowWhileIdle(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime()+600000,pendingIntentToBeFired);
}else{
alarmManager.setExact(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime()+600000,pendingIntentToBeFired);
}
}
super.onDestroy();
}
}

A service is not really necessary for what you're trying to do. There's no advantage to having a service, except that it may keep your app's process alive longer than it would without the service started. If you don't need to actually take advantage of the special properties of a Service, there's no point in using one (or you haven't really made a compelling case why it does need to be started all the time). Just register the listener when the app process starts, and let it go until the app process is killed for whatever reason. I highly doubt that your users will be upset about not having subscription updates if the app just isn't running (they certainly aren't using it!).
The power drain on an open socket that does no I/O is minimal. Also, an open socket will not necessarily keep the device's cell radio on at full power, either. So if the listen location isn't generating new values, then your listener is never invoked, and there is no network I/O. If the value being listened to is changing a lot, you might want reconsider just how necessary it is to keep the user's device busy with those updates.
The listener itself isn't "polling" or "retrying". The entire Firebase socket connection is doing this. The listener has no clue what's going on behind the scenes. It's either receiving updates, or not. It doesn't know or care about the state of the underlying websocket. The fact that a location is of interest to the client is actually managed on the server - that is what's ultimately responsible for noticing a change and propagating that to listening clients.

Message specific user in a Phonegap/Cordova app using SignalR 2

I am attempting to create a real-time communication capability for a Phonegap/Cordova app. I am using SignalR 2 to handle the communication.
The thing I am struggling with is getting a message to a particular user. Every single example out there shows saving Context.User.Identity.Name, which is useless to me because the remote site's User.Identity context is not shared by my phonegap app.
In essence, I am not authenticating a user in the traditional sense, so I need another way of linking the SignalR connectionID with the username I pass along.
Taken from the official ASP.NET signalr Examples, I have the following code which overrides the OnConnected event. Unfortunately it takes no parameters and expects User.Identity to be not null:
public override Task OnConnected()
{
using (var db = new UserContext())
{
// Retrieve user.
var user = db.Users
.Include(u => u.Rooms)
.SingleOrDefault(u => u.UserName == Context.User.Identity.Name);
// If user does not exist in database, must add.
if (user == null)
{
user = new User()
{
UserName = Context.User.Identity.Name
};
db.Users.Add(user);
db.SaveChanges();
}
else
{
// Add to each assigned group.
foreach (var item in user.Rooms)
{
Groups.Add(Context.ConnectionId, item.RoomName);
}
}
}
return base.OnConnected();
}
Now, maybe what I'd need is to have a version of this method that takes a string as a parameter and then I'd use that as my user identifier.
But how to go about that?

You need to create a new IUserIdProvider for the user and use dependency injection to register your provider and use it.
public interface IUserIdProvider
{
string GetUserId(IRequest request);
}
Register your provider with Global Host
GlobalHost.DependencyResolver.Register(typeof(IUserIdProvider), () => new MyIdProvider());
Usage:
public class MyHub : Hub
{
public void Send(string userId, string message)
{
Clients.User(userId).send(message);
}
}
Taken from: http://www.asp.net/signalr/overview/guide-to-the-api/mapping-users-to-connections#IUserIdProvider

Why does Android AppOps (privacy manager) send duplicate events when user change his/her privacy settings?

When a user changes his/her privacy settings through AppOps (e.g. denying an application access to phone contacts), AppOpsManager sends to anyone who listens what the users have changed (i.e. the package name and the operation (e.g. Read contacts)).
So I wrote a listener to do so. However, we the user make only one change, I receive too many duplicate events (e.g. 10 events that the user decided to deny Angry Bird access to his/her location) and then the app crashes.
Here is my code to register listners for each pair of package & operation:
public void startWatchingOperations(AppOpsManager appOps, List<AppOpsManager.PackageOps> opsforapps) {
SharedPreferences myAppListnerPreferences = getSharedPreferences(APP_OPS_PREFERENCES, Activity.MODE_PRIVATE);
for (AppOpsManager.PackageOps o:opsforapps) {
List<OpEntry> opEntry = o.getOps();
//if I already assigned a listener to this pari of package & operation, then skip
if (myAppListnerPreferences.getBoolean(o.getPackageName(), false)==false) {
for (OpEntry entry:opEntry) {
//for each pair of package & operation, assign a new listener
ChangePrivacySettingsListener opsListner = new ChangePrivacySettingsListener(getApplicationContext());
appOps.startWatchingMode(entry.getOp(),o.getPackageName(),opsListner);
}
myAppListnerPreferences.edit().putBoolean(o.getPackageName(), true).apply();
}
}
}
Here is a snippet of the listener
public class ChangePrivacySettingsListener implements AppOpsManager.Callback {
public void opChanged(int op, String packageName) {
AppOpsManager appOps= (AppOpsManager)context.getSystemService(Context.APP_OPS_SERVICE);
PackageManager pkg = context.getPackageManager();
try {
//this is an object to store the event: package name,
// the operation that has been changed, & time stamp
PrivacySetting privacySetting = new PrivacySetting();
privacySetting.setPackageName(packageName);
privacySetting.setOperation(OPERATIONS_STRINGS[op]);
privacySetting.setDecisionTime(Calendar.getInstance(TimeZone.getDefault()).getTimeInMillis());
privacySetting.setUserId(userId);
} catch (NameNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
Her is the part of AppOpsManager.java that allows me to listen to user's changes.
public class AppOpsManager {
final HashMap<Callback, IAppOpsCallback> mModeWatchers
= new HashMap<Callback, IAppOpsCallback>();
public void startWatchingMode(int op, String packageName, final Callback callback) {
synchronized (mModeWatchers) {
IAppOpsCallback cb = mModeWatchers.get(callback);
if (cb == null) {
cb = new IAppOpsCallback.Stub() {
public void opChanged(int op, String packageName) {
callback.opChanged(op, packageName);
}
};
mModeWatchers.put(callback, cb);
}
try {
mService.startWatchingMode(op, packageName, cb);
} catch (RemoteException e) {
}
}
}
I double checked to ensure that I've never assigned more than one listener to each pair of package & operation.
I would appreciate hints about potential causes.
Here is a link to AppOpsManager.java

Try moving the deceleration of ChangePrivacySettingsListener opsListner to be out side of the for block:
public void startWatchingOperations(AppOpsManager appOps, List<AppOpsManager.PackageOps> opsforapps) {
ChangePrivacySettingsListener opsListner;
SharedPreferences myAppListnerPreferences = getSharedPreferences(APP_OPS_PREFERENCES, Activity.MODE_PRIVATE);
for (AppOpsManager.PackageOps o:opsforapps) {
List<OpEntry> opEntry = o.getOps();
//if I already assigned a listener to this pari of package & operation, then skip
if (myAppListnerPreferences.getBoolean(o.getPackageName(), false)==false) {
for (OpEntry entry:opEntry) {
//for each pair of package & operation, assign a new listener
opsListner = new ChangePrivacySettingsListener(getApplicationContext());
appOps.startWatchingMode(entry.getOp(),o.getPackageName(),opsListner);
}
myAppListnerPreferences.edit().putBoolean(o.getPackageName(), true).apply();
}
}
}
And please let me know what happened?

Just in case this is helpful to someone, up to at least Android Oreo, calling AppOpsManager.startWatchingMode(op, packageName, callback) will cause callback to be invoked when the setting is changed (1) for the op with any package, AND (2) for any AppOps setting changes with packageName. This can be seen from the AppOpsService.java source, particularly AppOpsService.startWatchingMode() which registers the callback, AppOpsService.setMode() which calls the callback when the AppOps setting is changed.
For example, if you register a callback with startWatchingMode(appOps1, package1, callback) and startWatchingMode(appOps2, package1, callback),
when there is a change in the setting for appOps3 for package1, the callback will be called twice since you have registered for package1 two times. If there is a change in appOps1 for package1, the callback will be invoked 3 times, because you have registered once for appOps1, and twice for package1.
The solution is to register either for the set of AppOps you are interested in (without duplications), with the packageName parameter set to null, or register for the set of packages you are interested in, with op parameter set to AppOpsManager.OP_NONE.
Also you need to ensure that all listeners are unregistered (e.g. in onDestroy of your activity) using stopWatchingMode. Otherwise, the callback entries will accumulate across Activity lifecycles (until the app is terminated) and you will start getting duplicates. This also means that you should keep references to all the listeners created.

Android In app billing - remove Security class dependency

I'm using the In App Billing sample app to add this feature to my application.
After I finished adding it to my app, and tested all working, I noticed the comment in this Security class:
Security-related methods. For a secure implementation, all of
this code should be implemented on a server that communicates with
the application on the device. For the sake of simplicity and
clarity of this example, this code is included here and is executed
on the device. If you must verify the purchases on the phone, you
should obfuscate this code to make it harder for an attacker to
replace the code with stubs that treat all purchases as verified.
As Google suggests, I do the purchase verification on the server side so I really don't need the Security class in my project.
The problem is, I can't figure out how to remove the BillingService class dependency in the Security class.
I started by deleting the Security class and following the errors in the BillingService and most places it's being used I can remove easily, except in one place:
private void purchaseStateChanged(int startId, String signedData, String signature) {
ArrayList<Security.VerifiedPurchase> purchases;
purchases = Security.verifyPurchase(signedData, signature);
if (purchases == null) {
return;
}
ArrayList<String> notifyList = new ArrayList<String>();
for (VerifiedPurchase vp : purchases) {
if (vp.notificationId != null) {
notifyList.add(vp.notificationId);
}
ResponseHandler.purchaseResponse(this, vp.purchaseState, vp.productId,
vp.orderId, vp.purchaseTime, vp.developerPayload);
}
if (!notifyList.isEmpty()) {
String[] notifyIds = notifyList.toArray(new String[notifyList.size()]);
confirmNotifications(startId, notifyIds);
}
}
Would love if someone can share his/hers purchaseStateChanged method (based on the in app billing sample app) without the use of the Security class.

So here's what I did. First the calls to BillingService occur on the applications main thread, so you need to issue your server calls in a background thread. I chose to finish up processing on the main thread, since I wasn't sure what impact calling methods like 'confirmNotifications' on a background thread might have.
I created a callback interface VerifyTransactionCompletion which could be dispatched back to the main thread after the remote call completed.
I keep around the Security class and have it manage the call to the server now, instead of what it originally performed in the sample. So when you see the call to Security, that's where I call out to my server and perform signature validation.
/**
* Callback interface to <em>finish</em> processing a transaction once the remote
* servers have processed it.
*/
public interface VerifyTransactionCompletion {
public void transactionVerified(List<Security.VerifiedPurchase> purchases);
}
private void purchaseStateChanged(final int startId, String signedData, String signature) {
// verifyPurchase issues remote call to server (in a background thread), then
// calls transactionVerified on the main thread to continue processing.
Security.verifyPurchase(signedData, signature, new VerifyTransactionCompletion() {
#Override
public void transactionVerified(List<VerifiedPurchase> purchases) {
if (purchases == null) {
return;
}
ArrayList<String> notifyList = new ArrayList<String>();
for (VerifiedPurchase vp : purchases) {
if (vp.notificationId != null) {
notifyList.add(vp.notificationId);
}
ResponseHandler.purchaseResponse(BillingService.this, vp.purchaseState, vp.productId,
vp.orderId, vp.purchaseTime, vp.developerPayload);
}
if (!notifyList.isEmpty()) {
String[] notifyIds = notifyList.toArray(new String[notifyList.size()]);
confirmNotifications(startId, notifyIds);
}
}
});
}

Android - security through inheritence

I want to extend a common security check to nearly every view of my application. To do this, I have made this class
public class ProtectedActivity extends ActivityBase {
boolean isAuthenticated = false;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Thread validationThread = new Thread()
{
#Override
public void run()
{
try
{
isAuthenticated = UserService.validateToken();
}
catch (FTNIServiceException e)
{
//eat it
}
finally
{
if (!isAuthenticated)
{
startActivity(new Intent(ProtectedActivity.this, SignInActivity.class));
finish();
}
}
}
};
validationThread.start();
}
}
The logic is simple. Validate the user against my restful api to make sure they are signed in. If they aren't, show them to the signin page.
This works great, because to add the security check, all I need to do is inherit from my ProtectedActivity.
public class MainMenuActivity extends ProtectedActivity{
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
}
The problem is, however, that I periodically receive View not attached to window manager errors. I understand why this is happening. I am starting a new intent in the parent class, and the child lives on. to attempt to alter it's view even though a new intent has started. What is a better way to handle this so that if a user is not authenticated (such as their session expires serverside), it won't error when sending the user to the sign in screen?

Don't you Thread. Use AsyncTask instead which should handle your references to windows correctly.
On a different note, I would change this to a different implementation. Why don't use the Preferences storage on the phone to store some kind token. If the token is not valid then request a new token and all the stuff you are doing currently. This way is better because you don't want to request a REST call every time.
I imagine something like this (pseudo code)
Check if credentials exist in Preference
if(valid) then do nothing
else use AsyncTask and pop up a loader screen "Waiting..."