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@lukepighetti
Last active January 3, 2023 13:48
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Flutter PageView that loads the next/previous page off-screen
import 'package:flutter/gestures.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart' hide PageView;
/// This is copy-pasted from the Flutter framework with a support added for building
/// pages off screen using [Viewport.cacheExtents] and a [LayoutBuilder]
///
/// Based on commit 3932ffb1cd5dfa0c3891c60977ee4f9cd70ade66 on channel dev
// Having this global (mutable) page controller is a bit of a hack. We need it
// to plumb in the factory for _PagePosition, but it will end up accumulating
// a large list of scroll positions. As long as you don't try to actually
// control the scroll positions, everything should be fine.
final PageController _defaultPageController = PageController();
const PageScrollPhysics _kPagePhysics = PageScrollPhysics();
/// A scrollable list that works page by page.
///
/// Each child of a page view is forced to be the same size as the viewport.
///
/// You can use a [PageController] to control which page is visible in the view.
/// In addition to being able to control the pixel offset of the content inside
/// the [PageView], a [PageController] also lets you control the offset in terms
/// of pages, which are increments of the viewport size.
///
/// The [PageController] can also be used to control the
/// [PageController.initialPage], which determines which page is shown when the
/// [PageView] is first constructed, and the [PageController.viewportFraction],
/// which determines the size of the pages as a fraction of the viewport size.
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=J1gE9xvph-A}
///
/// See also:
///
/// * [PageController], which controls which page is visible in the view.
/// * [SingleChildScrollView], when you need to make a single child scrollable.
/// * [ListView], for a scrollable list of boxes.
/// * [GridView], for a scrollable grid of boxes.
/// * [ScrollNotification] and [NotificationListener], which can be used to watch
/// the scroll position without using a [ScrollController].
class ExtentsPageView extends StatefulWidget {
/// Creates a scrollable list that works page by page from an explicit [List]
/// of widgets.
///
/// This constructor is appropriate for page views with a small number of
/// children because constructing the [List] requires doing work for every
/// child that could possibly be displayed in the page view, instead of just
/// those children that are actually visible.
ExtentsPageView({
Key key,
this.scrollDirection = Axis.horizontal,
this.reverse = false,
PageController controller,
this.physics,
this.pageSnapping = true,
this.onPageChanged,
List<Widget> children = const <Widget>[],
this.dragStartBehavior = DragStartBehavior.start,
}) : controller = controller ?? _defaultPageController,
childrenDelegate = SliverChildListDelegate(children),
extents = 0,
super(key: key);
/// Creates a scrollable list that works page by page using widgets that are
/// created on demand.
///
/// This constructor is appropriate for page views with a large (or infinite)
/// number of children because the builder is called only for those children
/// that are actually visible.
///
/// Providing a non-null [itemCount] lets the [PageView] compute the maximum
/// scroll extent.
///
/// [itemBuilder] will be called only with indices greater than or equal to
/// zero and less than [itemCount].
///
/// [PageView.builder] by default does not support child reordering. If
/// you are planning to change child order at a later time, consider using
/// [PageView] or [PageView.custom].
ExtentsPageView.builder({
Key key,
this.scrollDirection = Axis.horizontal,
this.reverse = false,
PageController controller,
this.physics,
this.pageSnapping = true,
this.onPageChanged,
@required IndexedWidgetBuilder itemBuilder,
int itemCount,
this.dragStartBehavior = DragStartBehavior.start,
}) : controller = controller ?? _defaultPageController,
childrenDelegate = SliverChildBuilderDelegate(itemBuilder, childCount: itemCount),
extents = 0,
super(key: key);
ExtentsPageView.extents({
Key key,
this.extents = 1,
this.scrollDirection = Axis.horizontal,
this.reverse = false,
PageController controller,
this.physics,
this.pageSnapping = true,
this.onPageChanged,
@required IndexedWidgetBuilder itemBuilder,
int itemCount,
this.dragStartBehavior = DragStartBehavior.start,
}) : controller = controller ?? _defaultPageController,
childrenDelegate = SliverChildBuilderDelegate(
itemBuilder,
childCount: itemCount,
addAutomaticKeepAlives: false,
addRepaintBoundaries: false,
),
super(key: key);
/// Creates a scrollable list that works page by page with a custom child
/// model.
///
/// {@tool sample}
///
/// This [PageView] uses a custom [SliverChildBuilderDelegate] to support child
/// reordering.
///
/// ```dart
/// class MyPageView extends StatefulWidget {
/// @override
/// _MyPageViewState createState() => _MyPageViewState();
/// }
///
/// class _MyPageViewState extends State<MyPageView> {
/// List<String> items = <String>['1', '2', '3', '4', '5'];
///
/// void _reverse() {
/// setState(() {
/// items = items.reversed.toList();
/// });
/// }
///
/// @override
/// Widget build(BuildContext context) {
/// return Scaffold(
/// body: SafeArea(
/// child: PageView.custom(
/// childrenDelegate: SliverChildBuilderDelegate(
/// (BuildContext context, int index) {
/// return KeepAlive(
/// data: items[index],
/// key: ValueKey<String>(items[index]),
/// );
/// },
/// childCount: items.length,
/// findChildIndexCallback: (Key key) {
/// final ValueKey valueKey = key;
/// final String data = valueKey.value;
/// return items.indexOf(data);
/// }
/// ),
/// ),
/// ),
/// bottomNavigationBar: BottomAppBar(
/// child: Row(
/// mainAxisAlignment: MainAxisAlignment.center,
/// children: <Widget>[
/// FlatButton(
/// onPressed: () => _reverse(),
/// child: Text('Reverse items'),
/// ),
/// ],
/// ),
/// ),
/// );
/// }
/// }
///
/// class KeepAlive extends StatefulWidget {
/// const KeepAlive({Key key, this.data}) : super(key: key);
///
/// final String data;
///
/// @override
/// _KeepAliveState createState() => _KeepAliveState();
/// }
///
/// class _KeepAliveState extends State<KeepAlive> with AutomaticKeepAliveClientMixin{
/// @override
/// bool get wantKeepAlive => true;
///
/// @override
/// Widget build(BuildContext context) {
/// super.build(context);
/// return Text(widget.data);
/// }
/// }
/// ```
/// {@end-tool}
ExtentsPageView.custom({
Key key,
this.scrollDirection = Axis.horizontal,
this.reverse = false,
PageController controller,
this.physics,
this.pageSnapping = true,
this.onPageChanged,
@required this.childrenDelegate,
this.dragStartBehavior = DragStartBehavior.start,
}) : assert(childrenDelegate != null),
extents = 0,
controller = controller ?? _defaultPageController,
super(key: key);
/// The number of pages to build off screen.
///
/// For example, a value of `1` builds one page ahead and one page behind,
/// for a total of three built pages.
///
/// This is especially useful for making sure heavyweight widgets have a chance
/// to load off-screen before the user pulls it into the viewport.
final int extents;
/// The axis along which the page view scrolls.
///
/// Defaults to [Axis.horizontal].
final Axis scrollDirection;
/// Whether the page view scrolls in the reading direction.
///
/// For example, if the reading direction is left-to-right and
/// [scrollDirection] is [Axis.horizontal], then the page view scrolls from
/// left to right when [reverse] is false and from right to left when
/// [reverse] is true.
///
/// Similarly, if [scrollDirection] is [Axis.vertical], then the page view
/// scrolls from top to bottom when [reverse] is false and from bottom to top
/// when [reverse] is true.
///
/// Defaults to false.
final bool reverse;
/// An object that can be used to control the position to which this page
/// view is scrolled.
final PageController controller;
/// How the page view should respond to user input.
///
/// For example, determines how the page view continues to animate after the
/// user stops dragging the page view.
///
/// The physics are modified to snap to page boundaries using
/// [PageScrollPhysics] prior to being used.
///
/// Defaults to matching platform conventions.
final ScrollPhysics physics;
/// Set to false to disable page snapping, useful for custom scroll behavior.
final bool pageSnapping;
/// Called whenever the page in the center of the viewport changes.
final ValueChanged<int> onPageChanged;
/// A delegate that provides the children for the [PageView].
///
/// The [PageView.custom] constructor lets you specify this delegate
/// explicitly. The [PageView] and [PageView.builder] constructors create a
/// [childrenDelegate] that wraps the given [List] and [IndexedWidgetBuilder],
/// respectively.
final SliverChildDelegate childrenDelegate;
/// {@macro flutter.widgets.scrollable.dragStartBehavior}
final DragStartBehavior dragStartBehavior;
@override
_PageViewState createState() => _PageViewState();
}
class _PageViewState extends State<ExtentsPageView> {
int _lastReportedPage = 0;
@override
void initState() {
super.initState();
_lastReportedPage = widget.controller.initialPage;
}
AxisDirection _getDirection(BuildContext context) {
switch (widget.scrollDirection) {
case Axis.horizontal:
assert(debugCheckHasDirectionality(context));
final TextDirection textDirection = Directionality.of(context);
final AxisDirection axisDirection = textDirectionToAxisDirection(textDirection);
return widget.reverse ? flipAxisDirection(axisDirection) : axisDirection;
case Axis.vertical:
return widget.reverse ? AxisDirection.up : AxisDirection.down;
}
return null;
}
@override
Widget build(BuildContext context) {
final AxisDirection axisDirection = _getDirection(context);
final ScrollPhysics physics = widget.pageSnapping ? _kPagePhysics.applyTo(widget.physics) : widget.physics;
return NotificationListener<ScrollNotification>(
onNotification: (ScrollNotification notification) {
if (notification.depth == 0 && widget.onPageChanged != null && notification is ScrollUpdateNotification) {
final PageMetrics metrics = notification.metrics;
final int currentPage = metrics.page.round();
if (currentPage != _lastReportedPage) {
_lastReportedPage = currentPage;
widget.onPageChanged(currentPage);
}
}
return false;
},
child: Scrollable(
dragStartBehavior: widget.dragStartBehavior,
axisDirection: axisDirection,
controller: widget.controller,
physics: physics,
viewportBuilder: (BuildContext context, ViewportOffset position) {
return LayoutBuilder(
builder: (context, constraints) {
assert(constraints.hasBoundedHeight);
assert(constraints.hasBoundedWidth);
double cacheExtent;
switch (widget.scrollDirection) {
case Axis.vertical:
cacheExtent = constraints.maxHeight * widget.extents;
break;
case Axis.horizontal:
default:
cacheExtent = constraints.maxWidth * widget.extents;
break;
}
return Viewport(
cacheExtent: cacheExtent,
axisDirection: axisDirection,
offset: position,
slivers: <Widget>[
SliverFillViewport(
viewportFraction: widget.controller.viewportFraction,
delegate: widget.childrenDelegate,
),
],
);
},
);
},
),
);
}
@override
void debugFillProperties(DiagnosticPropertiesBuilder description) {
super.debugFillProperties(description);
description.add(EnumProperty<Axis>('scrollDirection', widget.scrollDirection));
description.add(FlagProperty('reverse', value: widget.reverse, ifTrue: 'reversed'));
description.add(DiagnosticsProperty<PageController>('controller', widget.controller, showName: false));
description.add(DiagnosticsProperty<ScrollPhysics>('physics', widget.physics, showName: false));
description.add(FlagProperty('pageSnapping', value: widget.pageSnapping, ifFalse: 'snapping disabled'));
}
}
@ABINASH56
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how can I add share option with next and previous page

@lukepighetti
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make a gist and post it here I guess?

@vishnukvmd
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Hey Luke, thanks for this. It's unfortunate that flutter/flutter#42107 faced blockers.

I'm new to Flutter and am curious to know if you have any suggestions for dealing with additions/deletions of pages within the ViewPort that is cached by an ExtentsPageView.

Thanks again.

@pishguy
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pishguy commented Dec 3, 2020

what does mean pages off screen?

@lukepighetti
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It means pages that are outside the bounds of the normal viewport.

@yashwant1999
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code is giving errors if you don't mind refractor it.

@lukepighetti
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Look at the revisions to see the changes you'd need to make to the current PageView provided by flutter

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