[xxgreg]

"A widget that provides a canvas on which to draw during the paint phase."

https://docs.flutter.io/flutter/widgets/CustomPaint-class.ht...

Edit: All of the built-in framework widgets are written in Dart and rendered via canvas-like draw calls at the lowest layer (See dart:ui). Since the widgets are available under a liberal open source license you can fork them and customise them however you like.