read the name (Activity.title) of the currently active and visible app

I want to ask a question in the round, because I think in spite of the forum and Google has no answer. I want to program a service that reads the name (Activity.title) of the currently active and visible app stores as a string. For example, the name "settings" in settings (see screenshot)
I've also experimented with the OS.Library, but I do not get a satisfactory result. Can anyone help me? A thank you in advance

I think that the OS library is your only option.

I want to program a service that waits in the background on a very specific window of another app and then becomes active. With getactivity, getClass or GetWindow I'm not for profit. Can you help me.