The slivers to place inside the viewport.
What is a sliver?
sliver (noun): a small, thin piece of something.
This is as distinct from those widgets that are backed by RenderBox subclasses, which use BoxConstraints and Size respectively, and are known as box widgets. (Widgets like Container, Row, and SizedBox are box widgets.)
While boxes are much more straightforward (implementing a simple two-dimensional Cartesian layout system), slivers are much more powerful, and are optimized for one-axis scrolling environments.
Slivers are hosted in viewports, also known as scroll views, most notably CustomScrollView.
Examples of slivers
The Flutter framework has many built-in sliver widgets, and custom widgets
can be created in the same manner. By convention, sliver widgets always
start with the prefix
Sliver and are always used in properties called
slivers (as opposed to
children which are used
for box widgets).
Examples of widgets unique to the sliver world include:
- SliverList, a lazily-loading list of variably-sized box widgets.
- SliverFixedExtentList, a lazily-loading list of box widgets that are all forced to the same height.
- SliverPrototypeExtentList, a lazily-loading list of box widgets that are all forced to the same height as a given prototype widget.
- SliverGrid, a lazily-loading grid of box widgets.
- SliverAnimatedList and SliverAnimatedGrid, animated variants of SliverList and SliverGrid.
- SliverFillRemaining, a widget that fills all remaining space in a scroll view, and lays a box widget out inside that space.
- SliverFillViewport, a widget that lays a list of boxes out, each being sized to fit the whole viewport.
- SliverPersistentHeader, a sliver that implements pinned and floating headers, e.g. used to implement SliverAppBar.
- SliverToBoxAdapter, a sliver that wraps a box widget.
Examples of sliver variants of common box widgets include:
- SliverOpacity, SliverAnimatedOpacity, and SliverFadeTransition, sliver versions of Opacity, AnimatedOpacity, and FadeTransition.
- SliverIgnorePointer, a sliver version of IgnorePointer.
- SliverLayoutBuilder, a sliver version of LayoutBuilder.
- SliverOffstage, a sliver version of Offstage.
- SliverPadding, a sliver version of Padding.
- SliverReorderableList, a sliver version of ReorderableList
- SliverSafeArea, a sliver version of SafeArea.
- SliverVisibility, a sliver version of Visibility.
Benefits of slivers over boxes
The sliver protocol (SliverConstraints and SliverGeometry) enables scroll effects, such as floating app bars, widgets that expand and shrink during scroll, section headers that are pinned only while the section's children are visible, etc.
Mixing slivers and boxes
In general, slivers always wrap box widgets to actually render anything (for example, there is no sliver equivalent of Text or Container); the sliver part of the equation is mostly about how these boxes should be laid out in a viewport (i.e. when scrolling).
Typically, the simplest way to combine boxes into a sliver environment is to use a SliverList (maybe using a [ListView, which is a convenient combination of a CustomScrollView and a SliverList). In rare cases, e.g. if a single Divider widget is needed between two SliverGrids, a SliverToBoxAdapter can be used to wrap the box widgets.
Because the purpose of scroll views is to, well, scroll, it is common for scroll views to contain more contents than are rendered on the screen at any particular time.
To improve the performance of scroll views, the content can be rendered in lazy widgets, notably SliverList and SliverGrid (and their variants, such as SliverFixedExtentList and SliverAnimatedGrid). These widgets ensure that only the portion of their child lists that are actually visible get built, laid out, and painted.
final List<Widget> slivers;