How many times did you want to use calculator without suspending the app that you are currently using? Now it's possible with Calculator application in the notification area.
The app is very minimalistic, but it includes all the essential functionality, including copying and pasting data. Design was inspired by the official Android calendar application.
On Android Nougat and later you can add settings tile for the Calculator, so you do not need to leave any app to launch the Calculator.
Source code for the application is available on GitHub.