https://stackoverflow.com/questions/54000912/how-to-have-dynamic-height-in-custommultichildlayout

custom_layout.dart

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

class CustomLayout extends MultiChildRenderObjectWidget {
  
  CustomLayout({
    Key key,
    List<Widget> children = const <Widget>[],
  }) : super(key: key, children: children);
  
  @override
  RenderCustomLayoutBox createRenderObject(BuildContext context) {
    return RenderCustomLayoutBox();
  }

}

class RenderCustomLayoutBox extends RenderBox
    with ContainerRenderObjectMixin<RenderBox, CustomLayoutParentData>,
        RenderBoxContainerDefaultsMixin<RenderBox, CustomLayoutParentData> {

  RenderCustomLayoutBox({
    List<RenderBox> children,
  }) {
    addAll(children);
  }
  
  @override
  void setupParentData(RenderBox child) {
    if (child.parentData is! CustomLayoutParentData) {
      child.parentData = CustomLayoutParentData();
    }
  }

  double _getIntrinsicHeight(double childSize(RenderBox child)) {
    double inflexibleSpace = 0.0;
    RenderBox child = firstChild;
    while (child != null) {
      if (child == lastChild)
        break;
      inflexibleSpace += childSize(child);
      final FlexParentData childParentData = child.parentData;
      child = childParentData.nextSibling;
    }
    return inflexibleSpace;
  }

  double _getIntrinsicWidth(double childSize(RenderBox child)) {
    double maxSpace = 0.0;
    RenderBox child = firstChild;
    while (child != null) {
      if (child == lastChild)
        break;
      maxSpace = math.max(maxSpace, childSize(child));
      final FlexParentData childParentData = child.parentData;
      child = childParentData.nextSibling;
    }
    return maxSpace;
  }

  @override
  double computeMinIntrinsicWidth(double height) {
    return _getIntrinsicWidth((RenderBox child) => child.getMinIntrinsicWidth(height));
  }

  @override
  double computeMaxIntrinsicWidth(double height) {
    return _getIntrinsicWidth((RenderBox child) => child.getMaxIntrinsicWidth(height));
  }

  @override
  double computeMinIntrinsicHeight(double width) {
    return _getIntrinsicHeight((RenderBox child) => child.getMinIntrinsicHeight(width));
  }

  @override
  double computeMaxIntrinsicHeight(double width) {
    return _getIntrinsicHeight((RenderBox child) => child.getMaxIntrinsicHeight(width));
  }

  @override
  void performLayout() {
    if (childCount == 0) {
      size = constraints.biggest;
      assert(size.isFinite);
      return;
    }

    double width = constraints.maxWidth;
    double height = 0;

    RenderBox child = firstChild;
    while (child != null) {
      if (child == lastChild)
        break;

      final CustomLayoutParentData childParentData = child.parentData;

      child.layout(BoxConstraints.tightFor(width: width), parentUsesSize: true);
      childParentData.offset = Offset(0, height);

      final Size childSize = child.size;
      width = math.max(width, childSize.width);
      height += childSize.height;

      child = childParentData.nextSibling;
    }

    size = Size(width, height);

    lastChild.layout(BoxConstraints(maxWidth: width, maxHeight: height), parentUsesSize: true);
    final CustomLayoutParentData childParentData = lastChild.parentData;
    final double margin = 20;
    final double x = size.width - lastChild.size.width - margin;
    final double y = height - childParentData.previousSibling.size.height - lastChild.size.height / 2;
    childParentData.offset = Offset(x, y);
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    defaultPaint(context, offset);
  }

  @override
  bool hitTestChildren(HitTestResult result, { Offset position }) {
    return defaultHitTestChildren(result, position: position);
  }
}

class CustomLayoutParentData extends ContainerBoxParentData<RenderBox> {

}

Thanks for this pattern. Any advice for someone who would like to enforce the meaning of the children rather than depend on position and such? Think of your example, but I would like to pass the children in using properties like top, bottom, overlay. If I do this, is it important to call addAll([top, bottom, overlay]) or maintain a children list? I wouldn't necessarily use firstChild to return a RenderBox, but I don't know how to layout if I have a reference to the Widget, and not the RenderBox.

Or would you recommend, I pass in the widgets as children, and then interrogate the RenderBox during performLayout to determine which widget it represents?

Here's my update for a nullable version:

class CustomLayout extends MultiChildRenderObjectWidget {
  const CustomLayout({
    super.key,
    super.children = const <Widget>[],
  });

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

class RenderCustomLayoutBox extends RenderBox
    with
        ContainerRenderObjectMixin<RenderBox, CustomLayoutParentData>,
        RenderBoxContainerDefaultsMixin<RenderBox, CustomLayoutParentData> {
  RenderCustomLayoutBox({
    List<RenderBox>? children,
  }) {
    addAll(children);
  }

  @override
  void setupParentData(RenderBox child) {
    if (child.parentData is! CustomLayoutParentData) {
      child.parentData = CustomLayoutParentData();
    }
  }

  double _getIntrinsicHeight(double Function(RenderBox child) childSize) {
    double inflexibleSpace = 0.0;
    RenderBox? child = firstChild;
    while (child != null) {
      if (child == lastChild) break;
      inflexibleSpace += childSize(child);
      final FlexParentData childParentData = child.parentData as FlexParentData;
      child = childParentData.nextSibling;
    }
    return inflexibleSpace;
  }

  double _getIntrinsicWidth(double Function(RenderBox child) childSize) {
    double maxSpace = 0.0;
    RenderBox? child = firstChild;
    while (child != null) {
      if (child == lastChild) break;
      maxSpace = math.max(maxSpace, childSize(child));
      final FlexParentData childParentData = child.parentData as FlexParentData;
      child = childParentData.nextSibling;
    }
    return maxSpace;
  }

  @override
  double computeMinIntrinsicWidth(double height) {
    return _getIntrinsicWidth(
      (RenderBox child) => child.getMinIntrinsicWidth(height),
    );
  }

  @override
  double computeMaxIntrinsicWidth(double height) {
    return _getIntrinsicWidth(
      (RenderBox child) => child.getMaxIntrinsicWidth(height),
    );
  }

  @override
  double computeMinIntrinsicHeight(double width) {
    return _getIntrinsicHeight(
      (RenderBox child) => child.getMinIntrinsicHeight(width),
    );
  }

  @override
  double computeMaxIntrinsicHeight(double width) {
    return _getIntrinsicHeight(
      (RenderBox child) => child.getMaxIntrinsicHeight(width),
    );
  }

  @override
  void performLayout() {
    if (childCount == 0) {
      size = constraints.biggest;
      assert(size.isFinite);
      return;
    }

    double width = constraints.maxWidth;
    double height = 0;

    RenderBox? child = firstChild;
    while (child != null) {
      if (child == lastChild) break;

      final CustomLayoutParentData childParentData =
          child.parentData as CustomLayoutParentData;

      child.layout(BoxConstraints.tightFor(width: width), parentUsesSize: true);
      childParentData.offset = Offset(0, height);

      final Size childSize = child.size;
      width = math.max(width, childSize.width);
      height += childSize.height;

      child = childParentData.nextSibling;
    }

    size = Size(width, height);

    lastChild?.layout(
      BoxConstraints(maxWidth: width, maxHeight: height),
      parentUsesSize: true,
    );

    final CustomLayoutParentData? childParentData =
        lastChild?.parentData as CustomLayoutParentData?;

    const double margin = 20;

    final double x = size.width - (lastChild?.size.width ?? 0) - margin;
    final double y = height -
        (childParentData?.previousSibling?.size.height ?? 0) -
        (lastChild?.size.height ?? 0) / 2;
    childParentData?.offset = Offset(x, y);
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    defaultPaint(context, offset);
  }

  @override
  bool hitTestChildren(HitTestResult result, {required Offset position}) {
    return defaultHitTestChildren(
      result as BoxHitTestResult,
      position: position,
    );
  }
}

class CustomLayoutParentData extends ContainerBoxParentData<RenderBox> {}

https://pub.dev/packages/boxy