flutter/examples/layers/widgets/custom_render_box.dart

// Copyright 2015 The Chromium Authors. 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/rendering.dart';
import 'package:flutter/widgets.dart';

class RenderDots extends RenderConstrainedBox {
  RenderDots() : super(additionalConstraints: const BoxConstraints.expand());

  // Makes this render box hittable so that we'll get pointer events.
  bool hitTestSelf(Point position) => true;

  final Map<int, Point> _dots = <int, Point>{};

  void handleEvent(PointerEvent event, BoxHitTestEntry entry) {
    if (event is PointerDownEvent || event is PointerMoveEvent) {
      _dots[event.pointer] = event.position;
      markNeedsPaint();
    } else if (event is PointerUpEvent || event is PointerCancelEvent) {
      _dots.remove(event.pointer);
      markNeedsPaint();
    }
  }

  void paint(PaintingContext context, Offset offset) {
    final Canvas canvas = context.canvas;
    canvas.drawRect(offset & size, new Paint()..color = new Color(0xFF0000FF));

    Paint paint = new Paint()..color = new Color(0xFF00FF00);
    for (Point point in _dots.values)
      canvas.drawCircle(point, 50.0, paint);

    super.paint(context, offset);
  }
}

class Dots extends OneChildRenderObjectWidget {
  Dots({ Key key, Widget child }) : super(key: key, child: child);
  RenderDots createRenderObject() => new RenderDots();
}

void main() {
  runApp(new Dots(child: new Center(child: new Text('Touch me!'))));
}