Question 301476 in Flutter Question posted in
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Flutter – How do I paint child in renderbox derivative?

I want to paint Widget in a RenderBox derivative. I tried the obvious, but no painting occured. What am I doing wrong?

Ive got a vague feeling that I should subclass RenderObjectWidget and do some sort of a Element, but I’d rather shortcut that

import 'package:flutter/material.dart';

void main() {
  runApp(const K());
}

class K extends LeafRenderObjectWidget {
  const K({super.key});

  @override
  RenderObject createRenderObject(BuildContext context) {

    const w = SizedBox.square(
      dimension: 30,child: ColoredBox(color: Colors.green),
    );
    final row = w.createRenderObject(context);
    row.layout(const BoxConstraints());

    return KImpl(row);
  }
}

class KImpl extends RenderBox {

  KImpl(this.ro);

  RenderObject ro ;

  @override
  void performLayout() {
    final c = constraints.normalize();
    size = Size(c.maxWidth, c.maxHeight);
  }
  @override
  void paint(PaintingContext context, Offset offset) {
    context.canvas.drawColor(Colors.red, BlendMode.src);

    context.paintChild(ro, Offset.zero); // THIS HAS NO EFFECT !
  }
}
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2

Answers


  1. Chosen as BEST ANSWER
    0

    Seems like this is what I need


  2. 0

    Here’s an updated version of your code with a modification to the Offset passed to context.paintChild:

    import 'package:flutter/material.dart';

void main() {
  runApp(const K());
}

class K extends LeafRenderObjectWidget {
  const K({Key? key}) : super(key: key);

  @override
  RenderObject createRenderObject(BuildContext context) {
    const w = SizedBox.square(
      dimension: 30,
      child: ColoredBox(color: Colors.green),
    );
    final row = w.createRenderObject(context);
    row.layout(const BoxConstraints());

    return KImpl(row);
  }
}

class KImpl extends RenderBox {
  KImpl(this.ro);

  final RenderObject ro;

  @override
  void performLayout() {
    final c = constraints.normalize();
    size = Size(c.maxWidth, c.maxHeight);
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    context.canvas.drawColor(Colors.red, BlendMode.src);
    
    // Adjust the offset based on your requirements
    final childOffset = Offset(10, 10);
    
    context.paintChild(ro, offset + childOffset);
  }
}
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