examples/api/lib/widgets/framework/build_owner.0.dart

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Template: dev/snippets/config/templates/freeform.tmpl
//
// Comment lines marked with "▼▼▼" and "▲▲▲" are used for authoring
// of samples, and may be ignored if you are just exploring the sample.

// Flutter code sample for BuildOwner
//
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// This example shows how to build an off-screen widget tree used to measure
// the layout size of the rendered tree. For some use cases, the simpler
// [Offstage] widget may be a better alternative to this approach.

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//***************************************************************************

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//* ▼▼▼▼▼▼▼▼ code-imports ▼▼▼▼▼▼▼▼ (do not modify or remove section marker)

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

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//* ▼▼▼▼▼▼▼▼ code ▼▼▼▼▼▼▼▼ (do not modify or remove section marker)

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  print(measureWidget(const SizedBox(width: 640, height: 480)));
}

Size measureWidget(Widget widget) {
  final PipelineOwner pipelineOwner = PipelineOwner();
  final MeasurementView rootView = pipelineOwner.rootNode = MeasurementView();
  final BuildOwner buildOwner = BuildOwner(focusManager: FocusManager());
  final RenderObjectToWidgetElement<RenderBox> element =
      RenderObjectToWidgetAdapter<RenderBox>(
    container: rootView,
    debugShortDescription: '[root]',
    child: widget,
  ).attachToRenderTree(buildOwner);
  try {
    rootView.scheduleInitialLayout();
    pipelineOwner.flushLayout();
    return rootView.size;
  } finally {
    // Clean up.
    element.update(RenderObjectToWidgetAdapter<RenderBox>(container: rootView));
    buildOwner.finalizeTree();
  }
}

class MeasurementView extends RenderBox
    with RenderObjectWithChildMixin<RenderBox> {
  @override
  void performLayout() {
    assert(child != null);
    child!.layout(const BoxConstraints(), parentUsesSize: true);
    size = child!.size;
  }

  @override
  void debugAssertDoesMeetConstraints() => true;
}

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