Masjing two Images with custom painter

import 'dart:typed_data';
import 'dart:ui' as ui;

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart' show rootBundle;

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  ui.Image mask;
  ui.Image image;

  @override
  void initState() {
    super.initState();
    load('images/squircle.png').then((i) {
      setState(() {
        mask = i;
      });
    });
    load('images/noodlejpg.jpg').then((i) {
      setState(() {
        image = i;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(backgroundColor: Colors.blue, title: Text('I am a title')),
      body: SafeArea(
        child: SizedBox(
          width: 200.0,
          height: 200.0,
          child: CustomPaint(painter: OverlayPainter(mask, image)),
        ),
      ),
    );
  }

  Future<ui.Image> load(String asset) async {
    ByteData data = await rootBundle.load(asset);
    ui.Codec codec = await ui.instantiateImageCodec(data.buffer.asUint8List());
    ui.FrameInfo fi = await codec.getNextFrame();
    return fi.image;
  }
}

class OverlayPainter extends CustomPainter {
  ui.Image mask;
  ui.Image image;

  OverlayPainter(this.mask, this.image);

  @override
  void paint(Canvas canvas, Size size) {
    var paint = new Paint();
    paint.isAntiAlias = true;

    if (image != null) {
      var rect = Rect.fromLTRB(0, 0, 200, 200);
      Size outputSize = rect.size;
      Size inputSize = Size(image.width.toDouble(), image.height.toDouble());
      final FittedSizes fittedSizes =
          applyBoxFit(BoxFit.cover, inputSize, outputSize);
      final Size sourceSize = fittedSizes.source;

      canvas.save();
      final Rect sourceRect = Alignment.center.inscribe(
        sourceSize,
        Offset.zero & inputSize,
      );
      canvas.drawImageRect(image, sourceRect, rect, paint);
      canvas.restore();
    }

    if (mask != null) {
      var rect = Rect.fromLTRB(0, 0, 200, 200);
      Size outputSize = rect.size;
      Size inputSize = Size(mask.width.toDouble(), mask.height.toDouble());
      final FittedSizes fittedSizes =
          applyBoxFit(BoxFit.cover, inputSize, outputSize);
      final Size sourceSize = fittedSizes.source;

      canvas.saveLayer(rect, Paint()..blendMode = BlendMode.dstIn);

      final Rect sourceRect = Alignment.center.inscribe(
        sourceSize,
        Offset.zero & inputSize,
      );
      canvas.drawImageRect(mask, sourceRect, rect, paint);
      canvas.restore();
    }
  }

  @override
  bool shouldRepaint(OverlayPainter oldDelegate) {
    return mask != oldDelegate.mask || image != oldDelegate.image;
  }
}