Is it possible to export hidden widgets as an Image?

I know how to save visible widgets to an image using RepaintBoundary

What I want is a way to save a widget that is not visible to the user as an image.

Solution

/// Creates an image from the given widget by first spinning up a element and render tree,
/// then waiting for the given [wait] amount of time and then creating an image via a [RepaintBoundary].
/// 
/// The final image will be of size [imageSize] and the the widget will be layout, ... with the given [logicalSize].
Future<Uint8List> createImageFromWidget(Widget widget, {Duration wait, Size logicalSize, Size imageSize}) async {
  final RenderRepaintBoundary repaintBoundary = RenderRepaintBoundary();

  logicalSize ??= ui.window.physicalSize / ui.window.devicePixelRatio;
  imageSize ??= ui.window.physicalSize;

  assert(logicalSize.aspectRatio == imageSize.aspectRatio);

  final RenderView renderView = RenderView(
    window: null,
    child: RenderPositionedBox(alignment: Alignment.center, child: repaintBoundary),
    configuration: ViewConfiguration(
      size: logicalSize,
      devicePixelRatio: 1.0,
    ),
  );

  final PipelineOwner pipelineOwner = PipelineOwner();
  final BuildOwner buildOwner = BuildOwner();

  pipelineOwner.rootNode = renderView;
  renderView.prepareInitialFrame();

  final RenderObjectToWidgetElement<RenderBox> rootElement = RenderObjectToWidgetAdapter<RenderBox>(
    container: repaintBoundary,
    child: widget,
  ).attachToRenderTree(buildOwner);

  buildOwner.buildScope(rootElement);

  if (wait != null) {
    await Future.delayed(wait);
  }

  buildOwner.buildScope(rootElement);
  buildOwner.finalizeTree();

  pipelineOwner.flushLayout();
  pipelineOwner.flushCompositingBits();
  pipelineOwner.flushPaint();

  final ui.Image image = await repaintBoundary.toImage(pixelRatio: imageSize.width / logicalSize.width);
  final ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);

  return byteData.buffer.asUint8List();
}