Develop Reference

How to detect a user after reinstalling the app

I am creating an iOS and Android App and I want to create a screen where the player can start with a guest account or can connect his account with our own accountsystem.
But my question is: Can I detect a user after the app was uninstalled and installed again?
I know that there is something like the vendor. But this will change.
I know that other apps also can do this.

With the user's permission- have them log into an account. Or provide you with their google of facebook account info. So far as hardware ids, those are discouraged and actively being removed to prevent people from the API to prevent this.
Also remember- that unless the user logs in with an account, you don't really know whether it is the same person. You could know its the same phone, but you don't know if he gave it to his kid sister to play on. Or sold it when he got a new one, and now you've given the new owner access to someone else's account. Also, if you rely on hardware ids you won't know its me when I buy a new phone and download it on that.
So yeah- either have him log in with a username and password, or use a 3rd party signon mechanism like Google or Facebook.

For iOS, there is a recommended approach to do that: by using the DeviceCheck framework. The idea here is that it allows you to persist 2 bits of data across app installations on each device. You can set the first bit to 1 if the user has already installed the app or 0 otherwise. And use the second bit, for example, to check if the user has signed in or not.
The official documentation is pretty good, please check it out.
The downside of this approach is that you will also have to do some work on the backend side.
UPDATE:
If you specifically want to detect the account, there is no reliable approach. One of the options is to use identifierForVendor or generate some kind of device fingerprint (for example, by combining the device model, timezone, locale, etc.), but of course, this will not work every time.
Uniquely identifying a device is a security leak, and all platforms are putting serious restrictions on unique persistent identifiers because of privacy concerns.

Google Developer Account Terminated: What to do?

So we just launched a beta closed test, and our account was completely terminated. I believe the reason was a deceptive name.
We filed an appeal, but we're not sure what to do next. We had no idea our name was violating anything, and if this appeal doesn't go through, we have to publish this app. We some big financial backers, and getting the account terminated completely was kind of a shock to all of us.
So, what to do next, if the appeal doesn't go through? Should we make a new account, and republish the app? Should I contact someone and tell them I just need to change the name? We're kind of freaking out here.
Thank you.

Please write an email to googleplay-developer-support#google.com
Ask if they can reactivate your account with changing the name to resolve the issue. Otherwise sure you can pay 25€ to register a new developer account if that is an option for you.
Make sure your developer name (and your apps) are not deceptive, check if your app description matches to their actual features, check if your app and developer names are already used (simply google the names) and if you feel like it it is always a good idea to check copyright and name brands. (Trademark checks)
If you want to do a trademark check it is a good idea to consult a lawyer, however you can do a simple trademark check yourself which is almost always enough for a small android publisher. Go to a trademark research site for your region, for example UK and EU https://trademarks.ipo.gov.uk/ipo-tmtext . I can recommend to search for "text" and "contains any" to get the best results. For example text="adidas" contains-any in clothing will return that it is already registered, therefore you shouldn't call a fashion app "adidas" or a deceptive version like "adi-das" or alike.

Same here in US region. I used a name that's not a trademark or anything. It took them 2 months (which's supposed to be "Up to 7 days") to get back to me saying they won't reactivate my account. I've tried to appeal and reach their customer service (it shows "server error" on online chat and the online help page is a dead end too). You may seek a legal assistance to get their attention.

How to limit my paid app for only one device on same account in Android and iOS

I will publish paid app for android and ios. I want that one account can only install and run on one device. Otherwise I want to block the app on first device. Is there any way to do this?
Note: I know there is a lvl for Google Play. But I am not sure, it does this exactly. Sorry for my english.

What you seem to need to do, is to get the device_ID and pair it with the account.That shouldn't be hard.
You get the device ID when your customer registers ,than at login you check the login credentials AND the device ID...
Here's pretty much all you need regarding the device ID (not the login,I trust you can handle it yourself)
How to get unique device hardware id in Android?

Can a android device start giving null value for device id?

I have an app on the market. When you start playing your device id is linked to your account so you don't have to log in manually. For some time now a few people had a problem. They started logging in into different accounts. What seems to be the problem is that some devices give a null value when asked for device id. With a few accounts with a null device id, people were logging into different accounts.
I read that some devices will do that. But it seems that there was no problem during account creation and then playing for a while.
I know this is not the best method and google discourages using device ids since they can change on fabric reset.
Can a device for some reason stop giving its id(besides the fabric reset)?
Anyone know of a different way to identify the user with out him needing to put in a login or password?

Yes some phones have bugs in them where they will return NULL for Android ID. I think if you want to continue to use them you'll always need a way to tie that handset to an account, email, username. That way if the Android ID is unrecognized they could login using their email or username, and password to associate them again. You'll always have issues where someone buys a new phone and needs a way recover their account if you don't.
A better option is every Android phone is attached to a Google account. And using the AccountManager class you can use those credentials to authenticate to your software automatically and the user doesn't have to enter username/password. And you'e centralized their identity. Also with this method you don't require the extra permissions to read data that has potential security issues too.

You could create a random UUID after the user has installed your app.
Do not use hardware ids, because not every type of hardware is found in every type of Android device. Another reason not to use them is, if someone sells his phone, the user changes but the device id not! This has serious privacy and usability implications you do not want to have.
See the talk Android Protips (from google io 2011). Tune in at 15:00 to see more reasons and a detailed descriptions on how to use random UUIDs for your usecase.

I am facing the same problem In my case when first install the app it is giving null id . but after that it is giving me all three IMEI, ANDROID_ID and deviceID.
BTW, you can find the ways to identify the devices here
http://innovator.samsungmobile.com/cms/cnts/knowledge.detail.view.do?platformId=1&cntsId=9640

Also some research prototypes try to prevent phones from collecting this information.

How to know if an app is downloaded for first time or many times from market?

I am placing an app for free in the Market. I want to restrict the downloads (i.e only 1 download per account). If the person uninstalls my app and tries to download again and re-install it with same gmail account, I want to restrict it.
Is that possible? Please help. I am stuck here. Thanks in advance!

I strongly recommend to skip what ever you try in that direction for the given reason:
you will upset your user!
you will lose user when they had to reset their device or if they bought a new one
and finally: its not supported!
Guillaume Brunerie mentioned the Application Licensing, but that will only work after they have downloaded the app the second time and you will just piss off your user when they can't start the app after downloading it the second time.
Seriously: drop the idea!

You have to create a server.
When you app starts for first time after installation, it should make a request to your server and send some unique information about the user like gmail address, but it is highly recomended that you hash this information to protect user's privacy. The server should check the hash if exists in it`s database and return an appropriative response to the application.

The standard way to do this kind of things is to use Application Licensing, but this is only available for paid apps.
So I don’t think this is possible if you want your app to be free.