How can I paint a Widget on a Canvas in Flutter?

Question

Is there any way to paint a Widget at a given position on a Canvas?

More specifically, I want to paint the child widgets of Marker's related to a FlutterMap on a separate Canvas in front of the actual FlutterMap widget. Here's an attempt at creating a CustomPainter that would do that, but I can't figure out how to actually paint the widgets on the canvas. Using the RenderObject requires a PaintingContext, which I don't know how to create/retrieve:

class MarkerPainter extends CustomPainter {
  MapController mc;
  BuildContext context;
  List<Marker> markers;

  MarkerPainter(this.context, this.mc, this.markers);

  @override
  void paint(Canvas canvas, Size size) {
    if( markers != null && markers.isNotEmpty ){
      for(int i=0; i<markers.length; i++){
        Marker marker = markers[i];
        Offset o = myCalculateOffsetFromLatLng(marker.point, mc, context);

        // Won't work, this needs a PaintingContext...
        marker.builder(context).createElement().renderObject.paint(context, o); 
      }
    }
  }

  @override
  bool shouldRepaint(MarkerPainter oldDelegate) => oldDelegate.markers != markers;
}

Solution

You cannot do this with a CustomPainter.

This class is only a simplification of the real deal: RenderObject, which has access to everything related to painter (and layout+more).

What you should do is, instead of a CustomPainter, create a RenderBox (a 2d RenderObject)

In your case, what you want is to draw a list of widgets. In that situation, you will need to create:

• a MultiChildRenderObjectWidget, a widget that takes a children to draw
• a RenderBox which mix-in RenderBoxContainerDefaultsMixin, which adds the necessary to RenderBox for handling a list of children.

Wrapping up, a widget used this way:

MyExample(
  children: [
    Text('foo'),
    Text('bar'),
  ],
),

would be written this way:

import 'package:flutter/rendering.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class MyExample extends MultiChildRenderObjectWidget {
  MyExample({
    Key? key,
    required List<Widget> children,
  }) : super(key: key, children: children);

  @override
  RenderMyExample createRenderObject(BuildContext context) {
    return RenderMyExample();
  }
}

class MyExampleParentData extends ContainerBoxParentData<RenderBox> {}

class RenderMyExample extends RenderBox
    with ContainerRenderObjectMixin<RenderBox, MyExampleParentData> {
  @override
  void setupParentData(RenderObject child) {
    if (child.parentData is! MyExampleParentData) {
      child.parentData = MyExampleParentData();
    }
  }

  @override
  void performLayout() {
    size = constraints.biggest;

    for (var child = firstChild; child != null; child = childAfter(child)) {
      child.layout(
        // limit children to a max height of 50
        constraints.copyWith(maxHeight: 50),
      );
    }
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    // Paints all children in order vertically, separated by 50px

    var verticalOffset = .0;
    for (var child = firstChild; child != null; child = childAfter(child)) {
      context.paintChild(child, offset + Offset(0, verticalOffset));

      verticalOffset += 50;
    }
  }
}