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@override
void paintChildren(FlowPaintingContext context) {
// TODO: We'll add more to this later.
}
@override
bool shouldRepaint(covariant FlowDelegate oldDelegate) {
// TODO: We'll add more to this later.
return true;
}
}<code_end>
A FlowDelegate controls how its children are sized
and where those children are painted. In this case,
your Flow widget has only one child: the background image.
That image must be exactly as wide as the Flow widget.Return tight width constraints for your background image child.
<code_start>@override
BoxConstraints getConstraintsForChild(int i, BoxConstraints constraints) {
return BoxConstraints.tightFor(
width: constraints.maxWidth,
);
}<code_end>
Your background images are now sized appropriately,
but you still need to calculate the vertical position
of each background image based on its scroll
position, and then paint it.There are three critical pieces of information that
you need to compute the desired position of a
background image:To look up the bounds of the Scrollable,
you pass a ScrollableState into your FlowDelegate.To look up the bounds of your individual list item,
pass your list item’s BuildContext into your FlowDelegate.To look up the final size of your background image,
assign a GlobalKey to your Image widget,
and then you pass that GlobalKey into your
FlowDelegate.Make this information available to ParallaxFlowDelegate.
<code_start>@immutable
class LocationListItem extends StatelessWidget {
final GlobalKey _backgroundImageKey = GlobalKey();
Widget _buildParallaxBackground(BuildContext context) {
return Flow(
delegate: ParallaxFlowDelegate(
scrollable: Scrollable.of(context),
listItemContext: context,
backgroundImageKey: _backgroundImageKey,
),
children: [
Image.network(
imageUrl,
key: _backgroundImageKey,
fit: BoxFit.cover,
),
],
);
}
}<code_end>
<code_start>class ParallaxFlowDelegate extends FlowDelegate {
ParallaxFlowDelegate({
required this.scrollable,
required this.listItemContext,
required this.backgroundImageKey,
});
final ScrollableState scrollable;
final BuildContext listItemContext;
final GlobalKey backgroundImageKey;
}<code_end>
Having all the information needed to implement
parallax scrolling, implement the shouldRepaint() method.
<code_start>@override
bool shouldRepaint(ParallaxFlowDelegate oldDelegate) {
return scrollable != oldDelegate.scrollable ||
listItemContext != oldDelegate.listItemContext ||
backgroundImageKey != oldDelegate.backgroundImageKey;
}<code_end>
Now, implement the layout calculations for the parallax effect.First, calculate the pixel position of a list
item within its ancestor Scrollable.
<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);
}<code_end>
Use the pixel position of the list item to calculate its
percentage from the top of the Scrollable.
A list item at the top of the scrollable area should
produce 0%, and a list item at the bottom of the
scrollable area should produce 100%.
<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);<code_end>
Use the scroll percentage to calculate an Alignment.
At 0%, you want Alignment(0.0, -1.0),