This is a custom control for Flutter which implements a touchpad-like surface (like a joystick but without the control point springing back to the center).
The control takes a change handler and reports back the x and y position only when it is being moved.
I had a lot of help in putting this together from the Flutter developers on stackoverflow here: https://stackoverflow.com/questions/44515611/flutter-creating-a-custom-control-with-flutter
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