positioned_sliver_list 0.1.0

Sliver list for very large lists with variable item extents: precise jump/animate to index, item position reporting, scroll anchoring, a real ScrollController, and a scrollable_positioned_list compatible API.

positioned_sliver_list

A fork of super_sliver_list (MIT) maintained by the Otzaria contributors. It is not affiliated with or endorsed by the authors of super_sliver_list or of scrollable_positioned_list.

On top of super_sliver_list it adds:

• exact jumps by index that are resolved during layout, before paint, and build only the items around the target, even when the extents in between are only estimated;
• animated scrolling to an index that returns a Future, can be canceled, and never builds the items in between;
• an initial index applied before the first frame;
• item positions published after layout, and scroll anchoring that keeps the item at the top of the viewport in place when items above it change extent or the width changes;
• package:positioned_sliver_list/positioned_list.dart, a replacement for the scrollable_positioned_list API with a real ScrollController.

The fixes from the Telosnex and ahey forks of super_sliver_list are included.

SliverList and ListView - Supercharged

SuperSliverList and SuperListView are drop in replacement widgets for SliverList and ListView with greatly improved performance and additional features:

Fast scrolling with large amount of items with variable extents

SliverList performance degrades heavily when quickly scrolling through a large amount of items with different extents, requiring workarounds such as using FixedExtentSliverList or prototype items. SuperSliverList uses different layout algorithm and can handle virtually unlimited number of items with variable extents without any slow-downs.

Ability to jump or animate to specific item

SliverList does not provide any way to jump or animate a particular index. There is a scrollable_positioned_list package that provides this functionality, but it comes at a cost, as it requires custom scroll view, does not seem to work properly with Scrollbars, can't be used with with other slivers (such as sticky headers) and ultimately is backed by a SliverList so it has the same performance issues as mentioned above.

SuperSliverList provides a way to reliably jump and animate to a specific item, even if the item is outside of the viewport and has not been built or laid out yet.

Smooth and predictable scrollbar behavior

SliverList is quite prone to scrollbar erraticaly jumping around when scrolling through a list of items with different extents. With SuperSliverList the scrollbar should behave more predictably. See the Advanced section for more details.

Basic Usage

SuperListView is a drop-in replacement for ListView, and as such you can use it same way you'd use ListView:

SuperListView.builder(
  itemCount: 1000,
  itemBuilder: (context, index) {
    return ListTile(title: Text('Item $index'));
  },
)

SuperSliverList is a drop-in replacement for SliverList and should work with any CustomScrollView configuration:

CustomScrollView(
  slivers: <Widget>[
    SliverPadding(
      padding: const EdgeInsets.all(20.0),
      sliver: SuperSliverList(
        delegate: SliverChildListDelegate(
          <Widget>[
            const Text("I'm dedicating every day to you"),
            const Text('Domestic life was never quite my style'),
            const Text('When you smile, you knock me out, I fall apart'),
            const Text('And I thought I was so smart'),
          ],
        ),
      ),
    ),
  ],
)

Jumping and animating to specific item

ListController can be provided to SuperSliverList/SuperListView and used to jump or animate to specific item:

class _MyState extends State<MyWidget> {
  final _listController = ListController();
  final _scrollController = ScrollController();

  @override
  Widget build(BuildContext context) {
    return SuperListView.builder(
      listController: _listController,
      controller: _scrollController,
      itemCount: 1000,
      itemBuilder: (context, index) {
        return ListTile(title: Text('Item $index'));
      },
    );
  }

  void jumpToItem(int index) {
    _listController.jumpToItem(
      index: index,
      scrollController: _scrollController,
      alignment: 0.5,
    );
  }

  void animateToItem(int index) {
    _listController.animateToItem(
      index: index,
      scrollController: _scrollController,
      alignment: 0.5,
      // You can provide duration and curve depending on the estimated
      // distance between currentPosition and the target item position.
      duration: (estimatedDistance) => Duration(milliseconds: 250),
      curve: (estimatedDistance) => Curves.easeInOut,
    );
  }
}

Index based positioning

jumpToIndex and scrollToIndex place the leading edge of an item at a fraction of the viewport (alignment: 0 is the leading edge, 0.5 the center, 1 the trailing edge, negative values allowed) plus offset pixels. They need no ScrollController:

// Exact, before the next paint. Completes after that layout.
await listController.jumpToIndex(12000, alignment: 0.2);

// Jumps to about one viewport before a distant item, then animates the rest.
// The handle is a Future and can be canceled; a drag cancels it too.
final handle = listController.scrollToIndex(
  40000,
  duration: const Duration(milliseconds: 400),
  curve: Curves.easeOut,
);
await handle;

SuperSliverList(initialIndex: ..., initialAlignment: ..., initialOffset: ...) applies such a position during the first layout.

By default a target is clamped to the scroll range: the last item cannot be moved to the top of the viewport. SuperSliverList(overscrollToTarget: ...) changes that:

• OverscrollToTarget.none (default): clamped to the content.

• OverscrollToTarget.matchScrollablePositionedList: clamped like scrollable_positioned_list. Content shorter than the viewport may be pushed down by the alignment, at most until its end reaches the trailing edge; otherwise the position is clamped to the content.

• OverscrollToTarget.always: a jump, an animation or the initial position adds blank space after the last item or before the first item, so the target always lands exactly where requested:

• The space is part of the scroll extent while it exists, so the ScrollController and a Scrollbar stay consistent.

• It shrinks as the user scrolls or flings back towards the content and never grows again from user scrolling; it is gone once the viewport is back within the content. The next programmatic jump computes it again.

• While it exists, the item that was jumped to keeps its place when items above it change extent or the width changes, and items appended at the end use the space up.

• ListController.overscrollSpace reports the current space. It is meant for a list that is the only sliver of its viewport (SliverPadding around it is fine).

listController.itemPositions is a ValueListenable<List<ItemPosition>> with the visible items and their leading and trailing edges as fractions of the viewport, updated after every frame in which they changed (only while it has listeners). listController.anchor is the first visible item and how far the viewport edge is inside it; jumpToIndex(anchor.index, offset: -anchor.offsetInItem) restores it.

animateToItem keeps its getOffsetToReveal alignment and now returns an AnimationHandle, which is also a Future<void>. When you do not await it, wrap it in unawaited(...) to satisfy the unawaited_futures lint.

Index based positioning is supported for a list that grows forward from the viewport's center, which is the usual case; for a sliver placed before the center of a CustomScrollView a jump is ignored with a logged warning.

Each layout pass may correct the scroll offset a limited number of times (a viewport throws after 10 layout cycles per sliver). When a pass reaches that limit, the remaining correction is applied in the next frame.

scrollable_positioned_list compatibility

package:positioned_sliver_list/positioned_list.dart provides ScrollablePositionedList (.builder, .separated), ItemScrollController, ItemPositionsListener, ItemPosition, ScrollOffsetController and ScrollOffsetListener with the API of scrollable_positioned_list, implemented as one CustomScrollView around a SuperSliverList:

import 'package:positioned_sliver_list/positioned_list.dart';

final itemScrollController = ItemScrollController();
final itemPositionsListener = ItemPositionsListener.create();
final scrollController = ScrollController();

Scrollbar(
  controller: scrollController,
  child: ScrollablePositionedList.builder(
    itemCount: paragraphs.length,
    itemBuilder: (context, index) => Paragraph(paragraphs[index]),
    itemScrollController: itemScrollController,
    itemPositionsListener: itemPositionsListener,
    scrollController: scrollController, // optional
    estimateExtent: (index, width) => paragraphs[index].length * 900 / width,
    initialScrollIndex: 120,
  ),
);

itemScrollController.jumpTo(index: 500, alignment: 0.1);
await itemScrollController.scrollTo(index: 9000, duration: const Duration(milliseconds: 500));

Provide estimateExtent when you can: items that were never laid out are estimated at 100 pixels otherwise, which makes the scrollbar less steady (jumps are exact either way).

Additions to the original API: scrollController, estimateExtent, estimatedSeparatorExtent, initialOffset, restorePosition, overscrollToTarget, an offset for jumpTo/scrollTo, ItemScrollController.scrollController/listController/anchor, and ScrollOffsetController.jumpBy/isAttached.

Behavior that differs from scrollable_positioned_list:

• There is a single list. scrollTo to a distant item jumps to about one viewport before it and animates from there; there is no cross-fade, and opacityAnimationWeights is ignored. The trajectory of the animation therefore differs.
• overscrollToTarget defaults to OverscrollToTarget.matchScrollablePositionedList, which reproduces how the original clamps at the edges: content shorter than the viewport may start below the leading edge, at most until its end reaches the trailing edge, and otherwise the last item cannot be moved above the trailing edge nor the first below the leading edge. OverscrollToTarget.always lets every target land exactly, for example the last item at the top of the viewport, with blank space that shrinks as the user scrolls back.
• Saving the position to PageStorage is opt-in (restorePosition: true).
• The cache area around a jump target is filled one frame after the jump.
• ScrollOffsetListener reports scroll movements only; layout corrections (exact landing, anchoring) are not reported.
• jumpToDistantItems is accepted and ignored.

Advanced

Very roughtly speaking SuperSliverList works by estimating the extent of items that are outside of viewport and when these items are scrolled into the viewport cache area the scroll position is transparently adjusted to account of the difference between estimated and actual extents. On small lists this difference may result in scrollbar movement not being perfectly aligned with list movement. SuperSliverList provides two ways to rectify this:

Improve extent estimation

You can register custom callback that will be used to estimate extent of estimated items. This can be useful if you have an idea, atleast approximately, how large the extent of each item is.

SuperSliverList(
    delegate: /*...*/,
    // Called with a null index first: return a positive value if all items
    // have the same estimate, or 0 to be asked for each index.
    extentEstimation: (index, crossAxisExtent) => 100.0,
)

Precalculate extents for items

SuperSliverList can, if needed, asynchronously precalculate extents for items. To enfore this, subclass ExtentPrecalculationPolicy and provide it to SuperSliverList:

In this example the extents are eagerly precalculated for lists with less than 100 items:

class MyPrecalculationPolicy extends ExtentPrecalculationPolicy {
  @override
  bool shouldPrecaculateExtents(ExtentPrecalculationContext context)  {
    return context.numberOfItems < 100;
  }
}

return SuperSliverList(
    delegate: /*...*/,
    extentPrecalculationPolicy: myPolicy,
)

The threshold is arbitrary, but in general there are diminishing returns for precalculating extents for large lists, as the extent estimation error for each item has much smaller impact on the scrollbar position if there are many items.

Example

See the example folder for a complete sample app using SuperSliverList.