Develop Reference

Is there a unique id for applications like IOS vendor id?

Is there a unique id for every Android application that changes when the application is re-installed? Some people say Application Id = Package Name for Android applications but I need a unique identifier to track applications.
EDIT: I need something other than package name because Although package name is unique for applications it's not unique for devices/users. For example, my app has a package name like com.example.myapp, it's same as every device that has the app. And yet it doesn't change when I re-install the app.

I had a similar requirement for one of my apps. What I did, is whenever the app is running for a first time after install, it connects to my server and requests a new unique ID. It's similar to authentication process, yet it runs automatically on first app launch.
Also, there is one more thing you might find useful. There is such thing as Linux user ID, it's unique for each app installation.
At install time, Android gives each package a distinct Linux user ID.
The identity remains constant for the duration of the package's life
on that device. On a different device, the same package may have a
different UID; what matters is that each package has a distinct UID on
a given device.

If you want it to change upon reinstallation of app, it is better to create a UUID in your app and save it in SharedPreferences. Then you will need to check SharedPreferences each time you need this identifier. If it exists, use it and if not regenerate it and save it.
The only draw back of this approach is that if user do a clear data on the app, this id will be lost. If it is important for you to not lost it in this case, you can save it in a file so as it only deletes when your app is deleted.
Other than this, you can use androidId which is mostly stays the same even upon reinstallation of app and is a better approach to track each installation uniquely, although its has some details to note specially on android oreo.

It depends on what is the purpose of this identification. For privacy, you shouldn't not be able to identify certain user with your application.
You can follow this article here
And depending on your requirement of the scope, resettability, uniqueness and integrity of that ID, you can choose the one that works for you best.

How to "flag" users in firebase?

I have developed an android "Group Chat" app. Me and 20 of my friends are using this app to group chat with each other. When I look at the firebase database, I obviously see 21 users (including myself). However, I can't know whether all of them actually have the app installed on their phones, or some of them have uninstalled it. Is there any way I can differentiate among my uses? Generally speaking, can I "flag" my users as, say, active, inactive, and the like?

What is if you add a timestamp every time a use log in. With this timestamp you can see how long the user not log in and so if he is in active for a long time or not. It's not say if the user have delete the app. It's only a prognosis.

If i understand correctly your question, it can be split in two parts.
First part is whether you want to know if your users are online or offline this could be achieved by an online presence system as described here
The second part is whether or not users have installed or uninstalled your app.
In my opinion easiest way to do this is by using the Firebase Analytics SDK. This way you can check the app_remove stats and whenever a user uninstalls an app, it gets updated in the console under events section.
This will give you how many devices uninstalled the app and some info on country, gender and age of the user but you could couple this with a "last login" timestamp to pinpoint the exact user.
More details along with how to include analytics SDK to your app can be found here
Or you can use the .info/connected to have this functionality in your client code. You can read more info about this here and the sample presence app at the bottom of the page will help you get a grip on how to do it.

One possible solution is to add a flag in the database as the child of the user. This flag will have an initial value of active.
And instead of uninstalling the app directly, you can have a delete account functionality in the app, and whenever someone wants to delete his/her account, it simply updates the flag to inactive in firebase database.
Now you will know actually who is active / inactive.

Can I permanently change a variable in an Android app even if they reinstall?

I have an Android application I am working on in which the client wants a promotional page in which the first 100 people who download the app get a 10% discount on their order. From what I researched I understand that Google Play doesn't have a very user-friendly way to do this, but for my work-around I wanted to have a "not used coupon" and a "used coupon" image that the waitress could see and type in a short verification code to permanently change the coupon on the app to "used."
From what I read I can use the SharedPreferences to make this happen, but what if the person uninstalls the application and then reinstalls it to get a fresh coupon? Is there anyway in Android to prevent this from happening?

There's actually a very simple API for backing up and a specific helper for SharedPreferences. You don't need to have your own server for this.
http://developer.android.com/training/cloudsync/backupapi.html
footnote:
Never use device ID. Use the account ID to identify the user and the ANDROID_ID to identify the device. If you use IMEI, MAC, serial number or anything that stays the same when device ownership changes, you're gonna have a bad time.

List of Installed App From Google Play Against Google Account

Following are my problems:-
Is it possible to get the list of applications that a user has installed, against their google account through Google Play, programmatically through the use of any api?. Please note that I am not asking about the list of apps currently installed in the device, but ones that have at some point been installed.
I need a solution to the above as I am thinking of a scenario in my app, which is:
I want to give my app to the user for free during the first three months, after three months if user uninstalls the app and then installs it again I want to detect through any api (from Google Play) that the user has installed the app a second time (and should not get any free usage). Please note that I don't want to use any web service to store the account id & device id of the user at my side.

For option 2, you can create a file on the SD card. This will remain there when the user uninstalls/installs. But the user can always delete your special file. Unless you do something at your side, you are never sure if the user already used your app before.
To be completely sure, store it online:
You will need to have a very simple database which holds a list of device_id that installed your app.
Further more a webpage which fills this database.
In your app you download/open this webpage which the webpage will fill the DB.
App > WebClient (or other) which opens http://www.example.com/registerDevice.php?device_id=. The php site fills the db.
You app will need to check the database if the current device already has installed this app inorder to work/not work. You can do this via the same php and check the response. You could for example return 'ok' or 'not ok' or something else.

The simplest method is to get the account of the user the first time he/she runs the app, and send that detail over to you.
How this is can be done is well-documented over here: How to get the Android device's primary e-mail address
You will have to add another line of code to check back to the database at the first start of the app.
EDIT: For a non-web solution, simply add a timer to the start of your app:
schedule(TimerTask task, Date when)
the task should be adding a token AFTER the period of time you wish to give, to the phone's memory with the user's account details for authentication (see first solution on getting the account details)
Finally, as above, add a check-back to the phone's memory for that particular file to see if the user has used the app before.
The problem with any type of authentication that is based on the phone's memory is that people can easily remove the token, if they can find it, and reuse the app again.

Try this device specific implementation:
PackageManager packageManager = getContext().getPackageManager();
List<ApplicationInfo> applications = packageManager.getInstalledApplications(
PackageManger.GET_UNINSTALLED_PACKAGES);
// retrieves some information about all applications (even uninstalled ones)
// which have data directories
Of course this method won't work if user replaces the device. But you don't need to use any web service.

You can use this by keeping a database of hashed device id and users google id on a 3rd party server.
Also see http://android-developers.blogspot.de/2011/03/identifying-app-installations.html

How to protect my app properly?

I have an app that creates an account for a web app, which is basically sending and receiving SMS messages from the web. This is how it works (not released yet, nearing the end of the first-release features I had planned):
The user purchases the app.
The user enters their name, email, and password.
The account is created on the server-end, and the final view is shown telling the user where to access the web app.
The background processes are opened (C2DM and ContentObserver for SMS).
All goes well. The android part of this app all works flawlessly, but I'm scared of people making multiple accounts from one purchase. How could I stop this from happening? I am clueless when it comes to this subject. First of all, when the final view is shown, a user could just hit back and then recreate another account. How can I prevent them from going to that form ever again? I am thinking I can just set a SharedPreference, but then all the user has to do to make another account is uninstall the app and then reinstall it, and bam another account is made.
I need a way, so once the user registers for the first time, there is in no possible way they can register again, on that specific phone (or specific Google account). Is there any real way to accomplish this? Any help is appreciated, I am stuck when it comes to this topic.

Since SIM identification functions (getSimSerialNumber) return null on CDMA devices and *ANDROID_ID* is said to be the same value on CDMA devices; with addition of tablets which do not have either of them, I highly suggest implementing your own unique identifier in your database and matching it with user's Google Account.
However, since a poweruser can always reset their app data storage and clear their identification from device (thus making your app session on device brand new on app launch) this approach has it's caveats.
You may want to check this blogpost for ideas about generating your unique id
http://android-developers.blogspot.com/2011/03/identifying-app-installations.html

I would make it a check on the server side.
Every device has a unique identifier. If you save this on the webserver side during the account creation, you check whether or not an account has already been made on this device. Also add the same check for the Google ID, just in case.
String android_id = Secure.getString(getBaseContext().getContentResolver(),
Secure.ANDROID_ID);
Might be worth a try. Your safest bet will always be something on the server side, since the device side is easily tempered with if people really wanted to.