mirrored_row_boxy.dart

import 'dart:math';

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

const darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: MyWidget(),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  const MyWidget({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MirroredRow(
      body: Container(color: Colors.red, height: 50),
      button: Container(color: Colors.purple, width: 150, height: 50),
    );
  }
}

class MirroredRow extends StatelessWidget {
  const MirroredRow({
    Key? key,
    required this.body,
    required this.button,
  }) : super(key: key);

  final Widget body;
  final Widget button;

  @override
  Widget build(BuildContext context) {
    return CustomBoxy(
      delegate: MirroredRowBoxy(),
      children: [body, button],
    );
  }
}

class MirroredRowBoxy extends BoxyDelegate {
  @override
  Size layout() {
    final body = children[0];
    final button = children[1];
    final width = constraints.maxWidth;

    // Lay out button first to get its size
    final buttonSize = button.layout(BoxConstraints(
      maxWidth: width,
      minHeight: constraints.minHeight,
      maxHeight: constraints.maxHeight,
    ));

    // Lay out the body by subtracting the button's size from available width,
    // also clamp it to 0 because it can go negative
    final bodyWidth = max(0.0, width - buttonSize.width * 2);
    final bodySize = body.layout(BoxConstraints(
      maxWidth: bodyWidth,
      minWidth: bodyWidth,
      minHeight: constraints.minHeight,
      maxHeight: constraints.maxHeight,
    ));

    // Position the button to the right side
    // (or omit this line to put it on the left side)
    button.position(Offset(width - buttonSize.width, 0));

    // Position the body in the center
    body.position(Offset(buttonSize.width, 0));

    return Size(
      width,
      max(buttonSize.height, bodySize.height),
    );
  }
}