Trait RenderSliverBoxChildManager

Source

pub trait RenderSliverBoxChildManager {
    // Required methods
    fn create_child(
        &self,
        app: &mut App,
        index: i32,
        after: Option<AnyRenderBox>,
    );
    fn remove_child(&self, app: &mut App, child: AnyRenderBox);
    fn estimate_max_scroll_offset(
        &self,
        app: &App,
        constraints: SliverConstraints,
        first_index: Option<i32>,
        last_index: Option<i32>,
        leading_scroll_offset: Option<f64>,
        trailing_scroll_offset: Option<f64>,
    ) -> f64;
    fn child_count(&self, app: &mut App) -> i32;
    fn did_adopt_child(&self, app: &mut App, child: AnyRenderBox);
    fn set_did_underflow(&self, app: &mut App, value: bool);

    // Provided methods
    fn estimated_child_count(&self, app: &App) -> Option<i32> { ... }
    fn did_start_layout(&self, app: &mut App) { ... }
    fn did_finish_layout(&self, app: &mut App) { ... }
    fn debug_assert_child_list_locked(&self, app: &App) -> bool { ... }
}

Expand description

A delegate used by RenderSliverMultiBoxAdaptor to manage its children.

RenderSliverMultiBoxAdaptor objects reify their children lazily to avoid spending resources on children that are not visible in the viewport. This delegate lets these objects create and remove children as well as estimate the total scroll offset extent occupied by the full child list.

The widget layer implements this; the render object holds it as an Rc<dyn RenderSliverBoxChildManager>.

Required Methods§

Source

fn create_child(&self, app: &mut App, index: i32, after: Option<AnyRenderBox>)

Called during layout when a new child is needed.

The child should be inserted into the child list in the appropriate position, after the after child (at the start of the list if after is None). Its index and scroll offsets will automatically be set appropriately.

The index argument gives the index of the child to show. It is possible for negative indices to be requested. For example: if the user scrolls from child 0 to child 10, and then those children get much smaller, and then the user scrolls back up again, this method will eventually be asked to produce a child for index -1.

If no child corresponds to index, then do nothing.

Which child is indicated by index zero depends on the crate::GrowthDirection specified in the constraints of the RenderSliverMultiBoxAdaptor.

During a call to this method it is valid to remove other children from the RenderSliverMultiBoxAdaptor object if they were not created during this frame and have not yet been updated during this frame. It is not valid to add any other children to this render object.

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fn remove_child(&self, app: &mut App, child: AnyRenderBox)

Remove the given child from the child list.

Called by RenderSliverMultiBoxAdaptor::collect_garbage, which itself is called from the adaptor’s perform_layout.

The index of the given child can be obtained using RenderSliverMultiBoxAdaptor::index_of.

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fn estimate_max_scroll_offset( &self, app: &App, constraints: SliverConstraints, first_index: Option<i32>, last_index: Option<i32>, leading_scroll_offset: Option<f64>, trailing_scroll_offset: Option<f64>, ) -> f64

Called to estimate the total scrollable extents of this object.

Must return the total distance from the start of the child with the earliest possible index to the end of the child with the last possible index.

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fn child_count(&self, app: &mut App) -> i32

Called to obtain a precise measure of the total number of children.

Must return the number that is one greater than the greatest index for which create_child will actually create a child.

This is used when create_child cannot add a child for a positive index, to determine the precise dimensions of the sliver. It will not be called if create_child is always able to create a child (e.g. for an infinite list).

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fn did_adopt_child(&self, app: &mut App, child: AnyRenderBox)

Called during RenderSliverMultiBoxAdaptor::insert or RenderSliverMultiBoxAdaptor::move_child.

Implementors must ensure that the SliverMultiBoxAdaptorParentData::index field of the child’s parent data accurately reflects the child’s index in the child list after this function returns.

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fn set_did_underflow(&self, app: &mut App, value: bool)

Called during layout to indicate whether this object provided insufficient children for the RenderSliverMultiBoxAdaptor to fill the SliverConstraints::remaining_paint_extent.

Typically called unconditionally at the start of layout with false and then later called with true when the adaptor fails to create a child required to fill the remaining paint extent.