widget_rebuild_evaluation

main.dart

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

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

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  int buildIndex = 0;

  ColoredBox2 get colorboxConst {
    print('method const: $hashCode');
    return const ColoredBox2(key: ValueKey('method const'), color: Colors.red);
  }

  ColoredBox2 get colorboxConst2 {
    print('method not const: $hashCode');
    return ColoredBox2(key: ValueKey('method not const'), color: Colors.red);
  }

  @override
  Widget build(BuildContext context) {
    print('========= $buildIndex ==========');
    print('Parent: $hashCode');
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Column(children: [
          colorboxConst,
          colorboxConst2,
          ColoredBox2(key: ValueKey('not const'), color: Colors.red),
          const ColoredBox2(key: ValueKey('const'), color: Colors.red),
          RepaintBoundary(
            child: TextButton(
              onPressed: () {
                setState(() {
                  buildIndex = buildIndex + 1;
                });
              },
              child: Text('Update'),
            ),
          ),
        ]),
      ),
    );
  }
}

class ColoredBox2 extends SingleChildRenderObjectWidget {
  /// Creates a widget that paints its area with the specified [Color].
  const ColoredBox2({required this.color, super.child, super.key});

  /// The color to paint the background area with.
  final Color color;

  @override
  RenderObject createRenderObject(BuildContext context) {
    print('Create $key ${context.widget.hashCode}');
    return _RenderColoredBox(color: color);
  }

  @override
  void updateRenderObject(BuildContext context, RenderObject renderObject) {
    print('update $key ${context.widget.hashCode}');
    (renderObject as _RenderColoredBox).color = color;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);

    properties.add(DiagnosticsProperty<Color>('color', color));
  }
}

class _RenderColoredBox extends RenderProxyBoxWithHitTestBehavior {
  _RenderColoredBox({required Color color})
      : _color = color,
        super(behavior: HitTestBehavior.opaque);

  /// The fill color for this render object.
  Color get color => _color;
  Color _color;
  set color(Color value) {
    if (value == _color) {
      return;
    }
    _color = value;
    markNeedsPaint();
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    print('paint: ${context.hashCode}');
    // It's tempting to want to optimize out this `drawRect()` call if the
    // color is transparent (alpha==0), but doing so would be incorrect. See
    // https://github.com/flutter/flutter/pull/72526#issuecomment-749185938 for
    // a good description of why.
    if (size > Size.zero) {
      context.canvas.drawRect(offset & size, Paint()..color = color);
    }
    if (child != null) {
      context.paintChild(child!, offset);
    }
  }
}