ImageGrabber.dart

/**
    This is a Widget extension that will automatically export an image of the widget when it is painted onscreen.
    We was using this to create symbols for MapBox from Widgets that are generated programmatically.
*/

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

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

class ImageGrabber extends SingleChildRenderObjectWidget {
  final Function(ByteData) onImageGrab;

  ImageGrabber({Key key, @required Widget child, @required this.onImageGrab})
      : super(key: key, child: child);

  @override
  RenderImageGrab createRenderObject(BuildContext context) =>
      RenderImageGrab(onImageGrab);

  // TODO implement @override updateRenderObject ???
}

class RenderImageGrab extends RenderProxyBox {
  final Function(ByteData) onImageGrab;
  bool imageGrabbed;

  RenderImageGrab(this.onImageGrab);

  @override
  bool get isRepaintBoundary => true;

  @override
  void paint(PaintingContext context, Offset offset) {
    print('=========== paint $size ===========');
    super.paint(context, offset);
    imageGrabbed ??= grabImage();
  }

  bool grabImage() {
    (layer as OffsetLayer)
        .toImage(Offset.zero & size)
        .then((image) => image.toByteData(format: ImageByteFormat.png))
        .then(onImageGrab);
    return true;
  }
}

/**
  USAGE:
*/
child: ImageGrab(
  child: SomeWidgetToGrab(),
  onImageGrab: (data) => print('png image length: ${data.lengthInBytes}'),
),