What mobile cross-platform tool can run in the background?

I’m an expert at Unity and I make games.

Sometimes I get potential contracts, where I just have to tell the client, that what they are asking for is not a game, Unity is not a good fit for their project and they should look for someone else.

I’ve been thinking about having a go at one of these tools with which you can make apps for Android and iOS, so I can take on these sorts of contracts. (something like react native for example)

Heres a simple use case…

An app to highlight areas on interest on a walking trail..

It runs in the background. (or if their phone is locked)

It uses GPS to detect if u are close to a particular point of interest.

When u get close to a new point of interest it sends a notification.

When u maximize the app it shows info on that particular thing.

​

This would be impossible with Unity because Unity cannot run in the background.. But I’ve been looking into some of the other tools (like react native) and whether its possible to do background stuff like this with them, is not clear.

Could anyone recommend a tool with which u can build to iOS & Android, and in which this sort of background stuff is possible?

Thanks.