Viewport class

A widget through which a portion of larger content can be viewed, typically in combination with a Scrollable.

Viewport is the visual workhorse of the scrolling machinery. It displays a subset of its children according to its own dimensions and the given offset. As the offset varies, different children are visible through the viewport.

Viewport hosts a bidirectional list of slivers, anchored on a center sliver, which is placed at the zero scroll offset. The center widget is displayed in the viewport according to the anchor property.

Slivers that are earlier in the child list than center are displayed in reverse order in the reverse axisDirection starting from the center. For example, if the axisDirection is AxisDirection.down, the first sliver before center is placed above the center. The slivers that are later in the child list than center are placed in order in the axisDirection. For example, in the preceding scenario, the first sliver after center is placed below the center.

Viewport cannot contain box children directly. Instead, use a SliverList, SliverFixedExtentList, SliverGrid, or a SliverToBoxAdapter, for example.

See also:

• ListView, PageView, GridView, and CustomScrollView, which combine Scrollable and Viewport into widgets that are easier to use.
• SliverToBoxAdapter, which allows a box widget to be placed inside a sliver context (the opposite of this widget).
• ShrinkWrappingViewport, a variant of Viewport that shrink-wraps its contents along the main axis.
• ViewportElementMixin, which should be mixed in to the Element type used by viewport-like widgets to correctly handle scroll notifications.