A widget to add gaps to Row and Column widgets

gap.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Container(
            width: 400,
            color: Colors.amber,
            child: Column(
              mainAxisSize: MainAxisSize.min,
              children: [
                Text('Here is a message'),
                Gap.large,
                Row(
                  mainAxisSize: MainAxisSize.min,
                  children: [
                    Spacer(),
                    RaisedButton(
                      onPressed: () => null,
                      child: Text('Cancel'),
                    ),
                    Gap.small,
                    RaisedButton(
                      onPressed: () => null,
                      child: Text('OK'),
                    ),
                  ],
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

class Gap extends SingleChildRenderObjectWidget {
  const Gap({Key key, this.size = 16})
      : assert(size != null),
        super(key: key);

  final double size;

  @override
  RenderGap createRenderObject(BuildContext context) {
    return RenderGap(size);
  }

  @override
  void updateRenderObject(BuildContext context, RenderGap renderObject) {
    renderObject.gapSize = size;
  }

  static const small = Gap(size: 8);
  static const medium = Gap(size: 16);
  static const large = Gap(size: 32);
}

class RenderGap extends RenderBox {
  RenderGap(this._gapSize);

  double _gapSize;
  double get gapSize => _gapSize;
  set gapSize(double gapSize) {
    if (_gapSize != gapSize) {
      _gapSize = gapSize;
      markNeedsLayout();
    }
  }

  @override
  void performLayout() {
    final parent = this.parent;
    if (parent is RenderFlex) {
      if (parent.direction == Axis.horizontal) {
        size = constraints.constrain(Size(_gapSize, 0));
      } else {
        size = constraints.constrain(Size(0, _gapSize));
      }
    } else {
      throw FlutterError('Gap must be used inside a Flex widget');
    }
  }

  @override
  double computeMinIntrinsicWidth(double width) {
    final parent = this.parent;
    if (parent is RenderFlex && parent.direction == Axis.horizontal) {
      return gapSize;
    }
    return super.computeMinIntrinsicWidth(width);
  }

  @override
  double computeMaxIntrinsicWidth(double width) {
    return computeMinIntrinsicWidth(width);
  }

  @override
  double computeMinIntrinsicHeight(double height) {
    final parent = this.parent;
    if (parent is RenderFlex && parent.direction == Axis.vertical) {
      return gapSize;
    }
    return super.computeMinIntrinsicHeight(height);
  }

  @override
  double computeMaxIntrinsicHeight(double height) {
    return computeMaxIntrinsicHeight(height);
  }
}