A Bitty Software application - http://www.bittysoftware.com
bitty controller lets you remote control a Bluetooth connected device which uses a micro-controller or computer like a BBC micro:bit, Arduino or Raspberry Pi. It provides a choice of user interfaces including several digital switches for switching things and and off, a dual d-pad controller, an analog touchpad for fine control and an enhanced UI which includes support for up to 5 sensors and has an extra button.
The Bitty Software web site contains code examples and further information.
More information can be found at http://www.bittysoftware.com.