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and at 100%, you want Alignment(0.0, 1.0).
These coordinates correspond to top and bottom
alignment, respectively.
<code_start>@override
void paintChildren(FlowPaintingContext context) {
// Calculate the position of this list item within the viewport.
final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
final listItemBox = listItemContext.findRenderObject() as RenderBox;
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);<code_end>
Use verticalAlignment, along with the size of the
list item and the size of the background image,
to produce a Rect that determines where the
background image should be positioned.
<code_start>@override
void paintChildren(FlowPaintingContext context) {
// Calculate the position of this list item within the viewport.
final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
final listItemBox = listItemContext.findRenderObject() as RenderBox;
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);<code_end>
Using childRect, paint the background image with
the desired translation transformation.
It’s this transformation over time that gives you the
parallax effect.
<code_start>@override
void paintChildren(FlowPaintingContext context) {
// Calculate the position of this list item within the viewport.
final scrollableBox = scrollable.context.findRenderObject() as RenderBox;
final listItemBox = listItemContext.findRenderObject() as RenderBox;
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: