I've got a concept of adding feature to my app which will register user location in interval of few seconds. Then sending this coordinates through cellular or Wi-Fi all when staying active in background. I need to send this in almost real time, so the app can't be killed in backgorund.
I know this concept is very power consuming but it is only a conception.
The conception of getting constant location in backgorund is in this theard https://forums.developer.apple.com/thread/69152 so I think it is possible.
But refreshing only the process in my app of sending coordinates to server it's a little bit difficult. I could not get straight answer that it is possible to set the time interval in which the app will refresh in backgrund.
Is there a method for telling the app how often it should refresh in background?
Apple won't let your app idea onto the app store. They're quite strict about which types of apps are allowed to run in the background, and are allowed to use continuous GPS. (Both of those things really drain the user's battery.) Sending a continuous stream of location updates to a server will also keep the cellular/WiFi transmitter powered up, making things even worse.
For your own experimentation, though, you can probably set up your app to be a navigation app. Those are allowed to keep the GPS "lit" constantly and to run in the background. You could then set up the location manager with the highest accuracy setting and start updating your location.
I don't think you have the ability to control how often you get location updates though. You could create a timer that fires on a regular interval and fetches the current location from the location manager. However I don't think there's much value in that, since you WILL get called when the user's location changes, and fetching the current location more often will just give you the same answers repeatedly.