A Bitty Software application - https://bittysoftware.blogspot.com/

bitty controller lets you remote control Bluetooth connected devices like robots, cars or lights which are connected to micro-controllers and computers such as a BBC micro:bit, an Arduino or a Raspberry Pi. It provides a choice of user interfaces including a series of on/off switches, a classic dual d-pad controller and an analog touchpad for fine control. Variants of the touchpad UI allow sensor data to be displayed and ancillary features like LEDs and servos to be controlled. It's basically awesome!

The Bitty Software web site contains code examples and further information. The Bitty Software github repository contains code examples and templates, too: https://github.com/bittysoftware/bitty_controller_device_code