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.
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Does flutter have a canvas yet? Whats the story for making arbitrary custom components, like a draggable rectangle that can be moved around a grid?
"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.