offscreen_layout.dart

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

class OffscreenRenderView extends RenderView {
  OffscreenRenderView() : super(configuration: const ViewConfiguration(size: const Size(800.0, 600.0)));

  @override
  void compositeFrame() {
    // Don't draw to ui.window
  }
}

class OffscreenWidgetTree {
  OffscreenWidgetTree() {
    renderView.attach(pipelineOwner);
    renderView.scheduleInitialFrame();
  }

  final RenderView renderView = new OffscreenRenderView();
  final BuildOwner buildOwner = new BuildOwner();
  final PipelineOwner pipelineOwner = new PipelineOwner();
  RenderObjectToWidgetElement<RenderBox> root;

  void pumpWidget(Widget app) {
    root = new RenderObjectToWidgetAdapter<RenderBox>(
      container: renderView,
      debugShortDescription: '[root]',
      child: app
    ).attachToRenderTree(buildOwner, root);
    pumpFrame();
  }

  void pumpFrame() {
    buildOwner.buildScope(root);
    pipelineOwner.flushLayout();
    buildOwner.finalizeTree();
  }
}

void main() {
  runApp(new Center(child: new Text('Hello, world!')));

  BuildContext savedContext;
  final Widget container = new Container(width: 400.0, height: 300.0);
  final Widget builder = new Builder(
    builder: (BuildContext context) {
      savedContext = context;
      return container;
    },
  );
  new OffscreenWidgetTree().pumpWidget(new Center(child: builder));
  final double height = savedContext.size.height;

  runApp(new Center(child: new Text('height: $height')));
}