How can I find out which application is currently open?

Question

Android

Subject.
For example, a service works that, when an application is opened, executes a method.
Here, in fact, I want to know how to find out which application is currently open, and how to find out that the user has switched to another application / minimized the current one

Answer 1

[]-> www.itcuties.com/android/how-to-get-running-proces...
[] -> stackoverflow.com/questions/4212992/how-can-i-chec...
[] -> stackoverflow.com/questions/8489993/check-android-...
[] -> stackoverflow.com/questions/2166961/determining-th...
[] -> stackoverflow.com/questions/27330747/android-how-t ...