how to detect whether user had installed this app before

Question

I am trying to check whether user has installed this app on his iOS device before or not.

For example I install app X , then Uninstall it, and again Install it.

I want that app X be able to detect that the device had installed this app before and this is a re-installation.

Update

By thanks to all friends that shared solutions like keychain, vendor Id and etc. The main subject that I should notice (as mentioned in comments) is that; Is there a solution that works even after system restore or system factory reset? According to @Andrea's answer using receipts may work but what about the cases that we want to publish the app in enterprises account? Or what if user uses a jailbreak device and install the app with and IPA? Some times because of security problems we want to detect that whether this app is installed before or not? So how we can detect it?

Answer 1

Maybe trying to check the app receipt you can infer the original purchase date (even if the app is free it comes with a receipt), but this could be difficult.
If you want to do that you can can add a flag to the keychain. Keychain values persist after an uninstall, but not after a system restore.
Of course this can work only in future releases.

Answer 2

iOS is built to reserve a portion of disk to each application installed, when you uninstall application the space is erased so there isn't a "cookie" that describes when app has been installed. You can reach your goal in two ways: through the keychain, for instance you save a value with specific key and you check every installation that key or through a simple backend call that send you the date o whatever you want. I think the first solution is better.

Answer 3

I would define a method to uniquely identify a device. Whenever an install is issued on a device, I would send a request to a server, where device keys are stored. If the key is not already present in the database, then I would store it, otherwise the install has been already issued.

Answer 4

You can save something to keychain and then try to read it.

Answer 5

As the OP mentions in one comment Apple does not allow access to the device UDID so we need a different approach to uniquely identify the user.

In order to get an identifier that uniquely identifies the user you could use:

[[UIDevice currentDevice] identifierForVendor]

But be careful, according to Apple documentation:

The value in this property remains the same while the app (or another app from the same vendor) is installed on the iOS device. The value changes when the user deletes all of that vendor’s apps from the device and subsequently reinstalls one or more of them. The value can also change when installing test builds using Xcode or when installing an app on a device using ad-hoc distribution. Therefore, if your app stores the value of this property anywhere, you should gracefully handle situations where the identifier changes.

A possible solution to this is to store the identifier in NSUserDefaults and also in the server and have a logic that at startup checks if there is a change in the identifier. In that case you could update the identifier locally and in the server to provide continuity for your analytics, for example.

For example to check for a change in the identifier:

NSString *previousUUID = [[NSUserDefaults standardUserDefaults] stringForKey:DEVICE_UUID];
NSString *currentUUID = [[[UIDevice currentDevice] identifierForVendor] UUIDString];
BOOL UUIDChanged = ![previousUUID isEqualToString:currentUUID];
// Handle this situation in your backend to offer continuity in your analytics

I have three apps in the App Store that use this and were approved by Apple without problems.

Answer 6

You can use APNS. Apple push notification service.

Device token never will change whatever device reset.

So, you can use Push notification.

Push notification(device token) is unique for each device and always will be same.

When app will install save device token on your server database so, whenever app will again install on same device you can check device token from server database.