RenderSliver class Null safety

Base class for the render objects that implement scroll effects in viewports.

A RenderViewport has a list of child slivers. Each sliver — literally a slice of the viewport's contents — is laid out in turn, covering the viewport in the process. (Every sliver is laid out each time, including those that have zero extent because they are "scrolled off" or are beyond the end of the viewport.)

Slivers participate in the sliver protocol, wherein during layout each sliver receives a SliverConstraints object and computes a corresponding SliverGeometry that describes where it fits in the viewport. This is analogous to the box protocol used by RenderBox, which gets a BoxConstraints as input and computes a Size.

Slivers have a leading edge, which is where the position described by SliverConstraints.scrollOffset for this sliver begins. Slivers have several dimensions, the primary of which is SliverGeometry.paintExtent, which describes the extent of the sliver along the main axis, starting from the leading edge, reaching either the end of the viewport or the end of the sliver, whichever comes first.

Slivers can change dimensions based on the changing constraints in a non-linear fashion, to achieve various scroll effects. For example, the various RenderSliverPersistentHeader subclasses, on which SliverAppBar is based, achieve effects such as staying visible despite the scroll offset, or reappearing at different offsets based on the user's scroll direction (SliverConstraints.userScrollDirection).

Writing a RenderSliver subclass

Slivers can have sliver children, or children from another coordinate system, typically box children. (For details on the box protocol, see RenderBox.) Slivers can also have different child models, typically having either one child, or a list of children.

Examples of slivers

A good example of a sliver with a single child that is also itself a sliver is RenderSliverPadding, which indents its child. A sliver-to-sliver render object such as this must construct a SliverConstraints object for its child, then must take its child's SliverGeometry and use it to form its own geometry.

The other common kind of one-child sliver is a sliver that has a single RenderBox child. An example of that would be RenderSliverToBoxAdapter, which lays out a single box and sizes itself around the box. Such a sliver must use its SliverConstraints to create a BoxConstraints for the child, lay the child out (using the child's layout method), and then use the child's RenderBox.size to generate the sliver's SliverGeometry.

The most common kind of sliver though is one with multiple children. The most straight-forward example of this is RenderSliverList, which arranges its children one after the other in the main axis direction. As with the one-box-child sliver case, it uses its constraints to create a BoxConstraints for the children, and then it uses the aggregate information from all its children to generate its geometry. Unlike the one-child cases, however, it is judicious in which children it actually lays out (and later paints). If the scroll offset is 1000 pixels, and it previously determined that the first three children are each 400 pixels tall, then it will skip the first two and start the layout with its third child.

Layout

As they are laid out, slivers decide their geometry, which includes their size (SliverGeometry.paintExtent) and the position of the next sliver (SliverGeometry.layoutExtent), as well as the position of each of their children, based on the input constraints from the viewport such as the scroll offset (SliverConstraints.scrollOffset).

For example, a sliver that just paints a box 100 pixels high would say its SliverGeometry.paintExtent was 100 pixels when the scroll offset was zero, but would say its SliverGeometry.paintExtent was 25 pixels when the scroll offset was 75 pixels, and would say it was zero when the scroll offset was 100 pixels or more. (This is assuming that SliverConstraints.remainingPaintExtent was more than 100 pixels.)

The various dimensions that are provided as input to this system are in the constraints. They are described in detail in the documentation for the SliverConstraints class.

The performLayout function must take these constraints and create a SliverGeometry object that it must then assign to the geometry property. The different dimensions of the geometry that can be configured are described in detail in the documentation for the SliverGeometry class.

Painting

In addition to implementing layout, a sliver must also implement painting. This is achieved by overriding the paint method.

The paint method is called with an Offset from the Canvas origin to the top-left corner of the sliver, regardless of the axis direction.

Subclasses should also override applyPaintTransform to provide the Matrix4 describing the position of each child relative to the sliver. (This is used by, among other things, the accessibility layer, to determine the bounds of the child.)

Hit testing

To implement hit testing, either override the hitTestSelf and hitTestChildren methods, or, for more complex cases, instead override the hitTest method directly.

To actually react to pointer events, the handleEvent method may be implemented. By default it does nothing. (Typically gestures are handled by widgets in the box protocol, not by slivers directly.)

Helper methods

There are a number of methods that a sliver should implement which will make the other methods easier to implement. Each method listed below has detailed documentation. In addition, the RenderSliverHelpers class can be used to mix in some helpful methods.

childScrollOffset

If the subclass positions children anywhere other than at scroll offset zero, it should override childScrollOffset. For example, RenderSliverList and RenderSliverGrid override this method, but RenderSliverToBoxAdapter does not.

This is used by, among other things, Scrollable.ensureVisible.

childMainAxisPosition

Subclasses should implement childMainAxisPosition to describe where their children are positioned.

childCrossAxisPosition

If the subclass positions children in the cross-axis at a position other than zero, then it should override childCrossAxisPosition. For example RenderSliverGrid overrides this method.