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Description:

value layout builder

value_layout_builder #

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

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

Getting started #
In the pubspec.yaml of your flutter project, add the following dependency:
dependencies:
...
value_layout_builder:
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In your library add the following import:
import 'package:value_layout_builder/value_layout_builder.dart';
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For help getting started with Flutter, view the online documentation.
Usage #
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;

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

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

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

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

@override
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,
);
child.layout(constraints);
size = constraints.biggest;
}

@override
void paint(PaintingContext context, Offset offset) {
context.paintChild(child, offset);
}
}
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We can then use _Tester<T> like this:
_Tester<String>(
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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Sponsoring #
I'm working on my packages on my free-time, but I don't have as much time as I would. If this package or any other package I created is helping you, please consider to sponsor me. By doing so, I will prioritize your issues or your pull-requests before the others.
Changelog #
Please see the Changelog page to know what's recently changed.
Contributions #
Feel free to contribute to this project.
If you find a bug or want a feature, but don't know how to fix/implement it, please fill an issue.
If you fixed a bug or implemented a feature, please send a pull request.