Drawing three overlapping circles with click detection

main.dart

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

void main() => runApp(HexagonGridDemo());

class HexagonGridDemo extends StatelessWidget {
  final GlobalKey blueCircle = new GlobalKey();
  final GlobalKey redCircle = new GlobalKey();
  final GlobalKey yellowCircle = new GlobalKey();
  final result = BoxHitTestResult();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Hexagon Grid Demo'),
        ),
        body: Listener(
          onPointerDown: (PointerEvent details) {
            handleClick(details);
          },
          child: Stack(
            children: <Widget>[
              CustomPaint(
                key: blueCircle,
                painter: CirclePainter(Offset(90, 120), 70, Colors.blue),
                child: Container(),
              ),
              CustomPaint(
                key: redCircle,
                painter: CirclePainter(Offset(60, 60), 50, Colors.red),
                child: Container(),
              ),
              CustomPaint(
                key: yellowCircle,
                painter: CirclePainter(Offset(140, 70), 40, Colors.yellow),
                child: Container(),
              )
            ],
          ),
        ),
      ),
    );
  }

  handleClick(PointerEvent details) {
    if (isClicked(details, redCircle)) {
      print("clicked the red circle");
    }
    if (isClicked(details, blueCircle)) {
      print("clicked the blue circle");
    }
    if (isClicked(details, yellowCircle)) {
      print("clicked the yellow circle");
    }
  }

  bool isClicked(final PointerEvent details, final GlobalKey key) {
    final RenderBox circleBox = key.currentContext.findRenderObject();
    Offset localClick = circleBox.globalToLocal(details.position);
    if (circleBox.hitTest(result, position: localClick)) {
      return true;
    }
    return false;
  }
}

class CirclePainter extends CustomPainter {
  final double radius;
  final Offset center;
  final Color color;

  CirclePainter(this.center, this.radius, this.color);

  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()..color = color;
    canvas.drawCircle(center, radius, paint);
  }

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

  @override
  bool hitTest(Offset position) {
    final Path path = new Path();
    path.addRect(Rect.fromCircle(center: center, radius: radius));
    return path.contains(position);
  }
}