StackでWidgetを重ねる

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Custom ListTile Example',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: ListTileWithTriangle(),
    );
  }
}

class ListTileWithTriangle extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final List<Map<String, dynamic>> data = [
      {'title': 'Item 1', 'highlight': false},
      {'title': 'Item 2', 'highlight': true},
      {'title': 'Item 3', 'highlight': false},
      {'title': 'Item 4', 'highlight': true},
    ];

    return Scaffold(
      appBar: AppBar(title: Text('Custom ListTile Example')),
      body: ListView.builder(
        itemCount: data.length,
        itemBuilder: (context, index) {
          final item = data[index];
          return Stack(
            children: [
              ListTile(
                title: Text(item['title']),
                tileColor: item['highlight'] ? Colors.yellow[100] : Colors.white,
              ),
              if (item['highlight'])
                Positioned(
                  top: 0,
                  right: 0,
                  child: CustomPaint(
                    size: Size(30, 30), // 三角形のサイズ
                    painter: TrianglePainter(Colors.red),
                  ),
                ),
            ],
          );
        },
      ),
    );
  }
}

class TrianglePainter extends CustomPainter {
  final Color color;

  TrianglePainter(this.color);

  @override
  void paint(Canvas canvas, Size size) {
    final Paint paint = Paint()..color = color;

    final Path path = Path()
      ..moveTo(size.width, 0) // 右上
      ..lineTo(size.width, size.height) // 右下
      ..lineTo(size.width - size.height,0 ) // 左下
      ..close(); // 三角形を閉じる

    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => false;
}