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final listItemOffset = listItemBox.localToGlobal(
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,
);
<code_end>
you need one final detail to achieve the parallax effect.
the ParallaxFlowDelegate repaints when the inputs change,
but the ParallaxFlowDelegate doesn’t repaint every time
the scroll position changes.
pass the ScrollableState’s ScrollPosition to
the FlowDelegate superclass so that the FlowDelegate
repaints every time the ScrollPosition changes.
<code_start>
class ParallaxFlowDelegate extends FlowDelegate {
ParallaxFlowDelegate({
required this.scrollable,
required this.listItemContext,
required this.backgroundImageKey,
}) : super(repaint: scrollable.position);
}
<code_end>
congratulations!
you now have a list of cards with parallax,
scrolling background images.
<topic_end>
<topic_start>
interactive example
run the app:
<code_start>
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
const color darkBlue = Color.fromARGB(255, 18, 32, 47);
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
widget build(BuildContext context) {
return MaterialApp(
theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
debugShowCheckedModeBanner: false,
home: const scaffold(
body: center(
child: ExampleParallax(),
),
),
);
}
}
class ExampleParallax extends StatelessWidget {
const ExampleParallax({
super.key,
});
@override
widget build(BuildContext context) {
return SingleChildScrollView(
child: column(
children: [
for (final location in locations)
LocationListItem(
imageUrl: location.imageUrl,
name: location.name,
country: location.place,
),
],
),
);
}
}
class LocationListItem extends StatelessWidget {
LocationListItem({
super.key,
required this.imageUrl,
required this.name,
required this.country,
});
final string imageUrl;
final string name;