@immutable
abstract class Widget extends DiagnosticableTree {
  /// Initializes [key] for subclasses.
  const Widget({ this.key });
  final Key key;

  /// ...

  @protected
  Element createElement();

  /// ...

  static bool canUpdate(Widget oldWidget, Widget newWidget) {
    return oldWidget.runtimeType == newWidget.runtimeType
      && oldWidget.key == newWidget.key;
  }
}

abstract class Element extends DiagnosticableTree implements BuildContext {
  /// Creates an element that uses the given widget as its configuration.
  ///
  /// Typically called by an override of [Widget.createElement].
  Element(Widget widget)
    : assert(widget != null),
      _widget = widget;

  /// Change the widget used to configure this element.
  ///
  /// The framework calls this function when the parent wishes to use a
  /// different widget to configure this element. The new widget is guaranteed
  /// to have the same [runtimeType] as the old widget.
  ///
  /// This function is called only during the "active" lifecycle state.
  @mustCallSuper
  void update(covariant Widget newWidget) {
    /// ...
  }

  /// Creates an instance of the [RenderObject] class that this
  /// [RenderObjectWidget] represents, using the configuration described by this
  /// [RenderObjectWidget].
  ///
  /// This method should not do anything with the children of the render object.
  /// That should instead be handled by the method that overrides
  /// [RenderObjectElement.mount] in the object rendered by this object's
  /// [createElement] method. See, for example,
  /// [SingleChildRenderObjectElement.mount].
  @protected
  RenderObject createRenderObject(BuildContext context);
}

abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin implements HitTestTarget {
  /// Initializes internal fields for subclasses.
  RenderObject() {
    _needsCompositing = isRepaintBoundary || alwaysNeedsCompositing;
  }

  /// The render object at (or below) this location in the tree.
  ///
  /// If this object is a [RenderObjectElement], the render object is the one at
  /// this location in the tree. Otherwise, this getter will walk down the tree
  /// until it finds a [RenderObjectElement].
  RenderObject get renderObject {
    RenderObject result;
    void visit(Element element) {
      assert(result == null); // this verifies that there's only one child
      if (element is RenderObjectElement)
        result = element.renderObject;
      else
        element.visitChildren(visit);
    }
    visit(this);
    return result;
  }

  void layout(Constraints constraints, { bool parentUsesSize = false }) {
    /// ...
  }

  /// ...

  void paint(PaintingContext context, Offset offset) {
    /// ...
  }

}

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      color: Colors.white,
      debugShowCheckedModeBanner: false,
      builder: (context, child) => HomePage(),
    ),
  );
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  bool _isWorld = true;

  Widget _buildWorld() {
    return RichText(
      text: TextSpan(
        text: 'Hello world',
        style: TextStyle(color: Colors.black),
      ),
    );
  }

  Widget _buildFlutter() {
    return RichText(
      text: TextSpan(
        text: 'Hello flutter',
        style: TextStyle(color: Colors.black),
      ),
    );
  }

  void changeText() {
    setState(() {
      _isWorld = !_isWorld;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Center(
            child: _isWorld ? _buildWorld() : _buildFlutter(),
          ),
          SizedBox(height: 20.0),
          // Padding(padding: EdgeInsets.only(top: 20.0)),
          IconButton(icon: Icon(Icons.refresh), onPressed: changeText)
        ],
      ),
    );
  }
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    body: Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Center(
          child: RichText(
            text: TextSpan(
              text: 'Hello $text',
              style: TextStyle(color: Colors.black),
            ),
          ),
        ),
        SizedBox(height: 20.0),
        // Padding(padding: EdgeInsets.only(top: 20.0)),
        IconButton(icon: Icon(Icons.refresh), onPressed: changeText)
      ],
    ),
  );
}