A LayoutBuilder with an extra value. It's useful when you want to build a widget with a value computed during layout.


  • ValueLayoutBuilder - The RenderBox version.
  • SliverValueLayoutBuilder - The RenderSliver version.

Getting started

In the pubspec.yaml of your flutter project, add the following dependency:


In your library add the following import:

import 'package:value_layout_builder/value_layout_builder.dart';

For help getting started with Flutter, view the online documentation.


ValueLayoutBuilder and SliverValueLayoutBuilder will pair with a custom RenderObject which accepts a one of them as a child.

For example it is used in flutter_sticky_header in order to know how much the header is hidden.

The typical usage is to expose a builder in the parameter of your widget's constructor which takes a BuildContext and the value you want to get from layout, and to pass it from your custom RenderObject.

The following code is a dumb example to show you how to use it:

class _Tester<T> extends SingleChildRenderObjectWidget {
  const _Tester({
    Key? key,
    required this.value,
    required Widget child,
  }) : super(key: key, child: child);

  final T value;

  _RenderTester<T> createRenderObject(BuildContext context) {
    return _RenderTester<T>(value: value);

  void updateRenderObject(
    BuildContext context,
    _RenderTester<T> renderObject,
  ) {
    renderObject.value = value;

class _RenderTester<T> extends RenderBox
    with RenderObjectWithChildMixin<RenderBox> {
    required T value,
  }) : _value = value;

  T get value => _value;
  T _value;
  set value(T value) {
    if (_value != value) {
      _value = value;

  void performLayout() {
    // We create a specific constraints with the value we want to pass to the builder.
    BoxValueConstraints<T> constraints = BoxValueConstraints<T>(
      constraints: this.constraints,
      value: value,
    size = constraints.biggest;

  void paint(PaintingContext context, Offset offset) {
    context.paintChild(child, offset);

We can then use _Tester<T> like this:

  value: 'test',
  child: ValueLayoutBuilder<String>(
    builder: (context, constraints) {
      // The constraints holds the String value.
      final String value = constraints.value;
      return Text(value);


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