drawing watermelon with Flutter Painter.

main.dart

import 'dart:ui';

import 'package:flutter/material.dart';
import 'dart:math' as math;

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Watermelon',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  double r = 0;
  double dx = 0;
  double dy = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Flutter Watermelon Painter"),
      ),
      body: Column(
        children: [
          Expanded(
            child: Center(
              child: Wrap(
                children: [
                  CustomPaint(
                    size: Size(250, 250),
                    painter: MyWaterMelonPainter(radiant: r,dx: dx,dy: dy),
                  ),
                ],
              ),
            ),
          ),
          Text(
            "Radians = ${r.toStringAsFixed(2)} π",
            style: TextStyle(fontSize: 22),
          ),
          Text(
            "Dx = ${dx.floor()}",
            style: TextStyle(fontSize: 22),
          ),
          Text(
            "Dy = ${dy.floor()}",
            style: TextStyle(fontSize: 22),
          ),
          SizedBox(
            height: 50,
          ),
          Slider(
            value: r,
            min: - 2 * math.pi,
            max: 2 * math.pi,
            onChanged: (v) {
              setState(() {
                r = v;
              });
            },
          ),
          Slider(
            value: dx,
            min: -1000,
            max: 1000,
            onChanged: (v) {
              setState(() {
                dx = v;
              });
            },
          ),
          Slider(
            value: dy,
            min: -1000,
            max: 1000,
            onChanged: (v) {
              setState(() {
                dy = v;
              });
            },
          ),
        ],
      ),
    );
  }
}


class MyWaterMelonPainter extends CustomPainter {
  double radiant;
  double dx;
  double dy;

  MyWaterMelonPainter({this.radiant = 0, this.dx = 0, this.dy = 0});


  @override
  void paint(Canvas canvas, Size size) {
    double W = size.width;
    double H = size.height;

    canvas.save();
    canvas.rotate(radiant);
    canvas.translate(dx, dy);

    canvas.save();
    canvas.clipRect(Rect.fromLTWH(0, 0, W, H / 2));
    canvas.drawPaint(Paint()..color = Colors.green[100]);
    Paint paintWaterMelonBody = Paint()..color = Color(0xff78bd53);
    canvas.drawCircle(Offset(W / 2, H / 2), W / 2, paintWaterMelonBody);
    Paint paintWaterMelonRind = Paint()..color = Color(0xffe23342);
    canvas.drawCircle(Offset(W / 2, H / 2), (W / 2) - W / 10, paintWaterMelonRind);
    canvas.restore();

    canvas.save();
    Paint paintSeed = Paint()..color = Color(0xff3c3b41);
    double sizeSeed = W / 40;
    canvas.drawCircle(Offset((W / 2) - (W / 10), (H / 2) - (H / 10)), sizeSeed, paintSeed);
    canvas.drawCircle(Offset((W / 2) - (W / 8), (H / 2) - (H / 4)), sizeSeed, paintSeed);
    canvas.drawCircle(Offset((W / 2) - (W / 4), (H / 2) - (H / 8)), sizeSeed, paintSeed);
    canvas.drawCircle(Offset((W / 2) + (W / 4), (H / 2) - (H / 8)), sizeSeed, paintSeed);
    canvas.drawCircle(Offset((W / 2) + (W / 6), (H / 2) - (H / 5)), sizeSeed, paintSeed);
    canvas.drawCircle(Offset((W / 2) + (W / 14), (H / 2) - (H / 7)), sizeSeed, paintSeed);
    canvas.drawCircle(Offset((W / 2) + (W / 28), (H / 2) - (H / 3.5)), sizeSeed, paintSeed);
    canvas.drawCircle(Offset((W / 2) + (W / 32), (H / 2) - (H / 18)), sizeSeed, paintSeed);
    canvas.restore();
    canvas.restore();
  }

  @override
  bool shouldRepaint(MyWaterMelonPainter oldDelegate) {
    return true;
  }
}