main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyPainter extends CustomPainter {
  final BuildContext context;
  final Path path;

  MyPainter({this.context, @required this.path});

  @override
  void paint(Canvas canvas, Size size) {
    final Paint paint = Paint();
    paint.color = Colors.red;
    paint.isAntiAlias = true;
    paint.style = PaintingStyle.stroke;
    paint.strokeWidth = 10;

    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return false;
  }
}


class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: SafeArea(
        child: LayoutBuilder(
          builder: (BuildContext context, BoxConstraints constraints) {
            final Path path = Path();
            path.moveTo(0, 0);
            path.lineTo(constraints.maxWidth, constraints.maxHeight);
            return CustomPaint(
              painter: MyPainter(
                context: context,
                path: path,
              ),
              child: Container(
                height: MediaQuery.of(context).size.height,
                width: MediaQuery.of(context).size.width,
              ),
            );
          },
        ),
      ),
    );
  }
}