Fractional Flex Widget - by Simon Lightfoot - 08/05/2020

fractional_flex.dart

// MIT License
//
// Copyright (c) 2020 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//

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

class FractionalFlex extends SingleChildRenderObjectWidget {
  const FractionalFlex({
    Key key,
    this.minFraction = 0.5,
    this.maxFraction = 0.5,
    @required Widget child,
  })  : assert(minFraction != null),
        assert(maxFraction != null),
        assert(minFraction <= maxFraction),
        super(key: key, child: child);

  final double minFraction;
  final double maxFraction;

  @override
  RenderObject createRenderObject(BuildContext context) {
    return RenderFractionalFlex(this);
  }

  @override
  void updateRenderObject(BuildContext context, RenderFractionalFlex renderObject) {
    renderObject.widget = this;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DoubleProperty('minFraction', minFraction));
    properties.add(DoubleProperty('maxFraction', maxFraction));
  }
}

class RenderFractionalFlex extends RenderProxyBox {
  RenderFractionalFlex(
    this._widget, {
    RenderBox child,
  }) : super(child);

  FractionalFlex _widget;

  FractionalFlex get widget => _widget;

  set widget(FractionalFlex value) {
    if (_widget == value) return;
    _widget = value;
    markNeedsLayout();
  }

  @override
  void performLayout() {
    if (child != null) {
      final parent = this.parent as RenderBox;
      assert(parent is RenderFlex);
      assert(parent.constraints.hasBoundedWidth);
      final parentWidth = parent.constraints.maxWidth;
      final childWidth = child.getMaxIntrinsicWidth(constraints.maxHeight);
      child.layout(
        constraints.tighten(
          width: childWidth.clamp(
            parentWidth * _widget.minFraction,
            parentWidth * _widget.maxFraction,
          ),
        ),
        parentUsesSize: true,
      );
      size = child.size;
    } else {
      performResize();
    }
  }
}

You could cascade L54-55