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listItemBox.size.centerLeft(Offset.zero),
ancestor: scrollableBox);
// determine the percent position of this list item within the
// scrollable area.
final viewportDimension = scrollable.position.viewportDimension;
final scrollFraction =
(listitemoffset.dy / viewportDimension).clamp(0.0, 1.0);
// calculate the vertical alignment of the background
// based on the scroll percent.
final verticalAlignment = alignment(0.0, scrollFraction * 2 - 1);
// convert the background alignment into a pixel offset for
// painting purposes.
final backgroundSize =
(backgroundimagekey.currentcontext!.findrenderobject() as RenderBox)
.size;
final listItemSize = context.size;
final childRect =
verticalAlignment.inscribe(backgroundSize, offset.zero & listItemSize);
// paint the background.
context.paintChild(
0,
transform:
transform.translate(offset: offset(0.0, childRect.top)).transform,
);
}
@override
bool shouldRepaint(ParallaxFlowDelegate oldDelegate) {
return scrollable != oldDelegate.scrollable ||
listItemContext != oldDelegate.listItemContext ||
backgroundImageKey != oldDelegate.backgroundImageKey;
}
}
class parallax extends SingleChildRenderObjectWidget {
const parallax({
super.key,
required widget background,
}) : super(child: background);
@override
RenderObject createRenderObject(BuildContext context) {
return RenderParallax(scrollable: scrollable.of(context));
}
@override
void updateRenderObject(
BuildContext context, covariant RenderParallax renderObject) {
renderObject.scrollable = scrollable.of(context);
}
}
class ParallaxParentData extends ContainerBoxParentData<RenderBox> {}
class RenderParallax extends RenderBox
with RenderObjectWithChildMixin<RenderBox>, RenderProxyBoxMixin {
RenderParallax({
required ScrollableState scrollable,
}) : _scrollable = scrollable;
ScrollableState _scrollable;
ScrollableState get scrollable => _scrollable;
set scrollable(ScrollableState value) {
if (value != _scrollable) {
if (attached) {
_scrollable.position.removeListener(markNeedsLayout);
}
_scrollable = value;
if (attached) {
_scrollable.position.addListener(markNeedsLayout);
}
}
}
@override
void attach(covariant PipelineOwner owner) {
super.attach(owner);
_scrollable.position.addListener(markNeedsLayout);
}
@override
void detach() {
_scrollable.position.removeListener(markNeedsLayout);
super.detach();
}
@override
void setupParentData(covariant RenderObject child) {
if (child.parentdata is! ParallaxParentData) {
child.parentData = ParallaxParentData();
}
}
@override
void performLayout() {
size = constraints.biggest;
// force the background to take up all available width
// and then scale its height based on the image's aspect ratio.
final background = child!;
final backgroundImageConstraints =
BoxConstraints.tightFor(width: size.width);
background.layout(backgroundImageConstraints, parentUsesSize: true);
// set the background's local offset, which is zero.
(background.parentdata as ParallaxParentData).offset = offset.zero;
}
@override
void paint(PaintingContext context, offset offset) {
// get the size of the scrollable area.
final viewportDimension = scrollable.position.viewportDimension;
// calculate the global position of this list item.
final scrollableBox = scrollable.context.findRenderObject() as RenderBox;