my_center.dart

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

class MyCenter extends SingleChildRenderObjectWidget {

  MyCenter({Key key, Widget child}): super(key: key, child: child);

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

class _RenderMyCenter extends RenderBox with RenderObjectWithChildMixin<RenderBox> {


  @override
  void performLayout() {
    if(child != null) {
      size = constraints.biggest;
      child.layout(constraints.loosen(), parentUsesSize: true);
      double dx = (constraints.maxWidth - child.size.width) / 2;
      double dy = (constraints.maxHeight - child.size.height) / 2;

      BoxParentData childParentData = child.parentData;
      childParentData.offset = Offset(dx, dy);

    } else {
      size = Size(0,0);
    }
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if(child != null) {
      var parentData = child.parentData as BoxParentData;
      context.paintChild(child, offset + parentData.offset);
    }
  }

}