Check for "unlocker" in my free app?

Question

my music theory app is free and has no ads. I made this for a school project, the goal is to make some money. My idea is that some functions are only available if you have purchased the "unlocker" version of my app.

Can I tell my app to check for the "unlocker" at first launch? I don't want the "unlocker" app to act as the real app, just a "file" that unlocks the free version.

If the user has purchased the "unlocker" I want it to activate some locked buttons.

I would prefer it to work like for example Solid Explorer.

Solide Explorer (Trial) - Google Play

Solide Explorer Unlocker - Google Play

Answer 1

You can use PackageManager.getPackageInfo() and request PakageInfo for your unlocker app. The app must be there. Second step would be to validate your app's signature. To make it even more difficult to hack, your main app can send some intents to unlock app, and then receive and check responses for correctness.

Although this will technically work, it is not as safe as In App Billing, which is advised way to go. Moreover in your case users will have to install two apps, which is not as convenient.