Struct Expand 

pub struct Expand { /* private fields */ }

Layout modifier that claims space along one or both axes from its parent and stretches its child to fill it.

In an HStack / VStack, Expand participates in flex slack distribution: it returns a LayoutResponse with flex (default 1.0), so the parent stack hands it a share of the leftover space proportional to flex. Default basis is zero — the wrapped child’s natural size does NOT count in the rigid pool, which gives clean ratio layouts. Call Expand::respect_intrinsic to switch to auto basis (CSS flex-basis: auto), where the child’s natural size acts as a floor and flex adds slack on top.

Expand::new() is the common case: claim space, fill the child. Use .flex(n) to change the ratio (e.g. 1:2 by pairing flex(1) with flex(2)). Use .align_child(...) to opt out of fill and align the child at its natural size within the claimed bounds.

horizontal() / vertical() semantics. The named axis is the one the wrapper competes for slack on. Both sizing and flex behavior follow from that:

• Sizing: when the parent binds an axis (proposal.{axis} = Some), the wrapper claims that axis regardless of its name. So Expand::vertical(child) inside a VStack (which binds width and leaves height open) fills the VStack’s full width AND distributes vertical slack via flex. Cross-axis collapse to child intrinsic only happens when the parent left that axis open too.

• Flex contribution: the wrapper reports its flex weight only on axes the parent is distributing (i.e. left open). Expand::horizontal() inside a VStack reports flex = 0 on the open vertical axis, so it does NOT compete for vertical slack with siblings — it just claims the cross-axis width and sits at its child’s intrinsic height. Symmetric for Expand::vertical() inside an HStack.

Implementations

impl Expand

pub fn new() -> Self

Expand on both axes. Default flex(1), child fills bounds.

pub fn horizontal() -> Self

Compete for slack on the horizontal axis only. Inside an HStack, distributes flex on width while claiming bound height as-is. Inside a VStack (which binds width and distributes height), claims the VStack’s full width but reports flex = 0 so it doesn’t steal vertical slack from siblings — height stays at child intrinsic.

pub fn vertical() -> Self

Compete for slack on the vertical axis only. Inside a VStack, distributes flex on height while claiming bound width as-is. Inside an HStack (which binds height and distributes width), claims the HStack’s full height but reports flex = 0 so it doesn’t steal horizontal slack from siblings — width stays at child intrinsic.

pub fn flex(self, flex: f32) -> Self

Override the flex weight reported to a parent stack. flex(0) opts out of slack distribution (the wrapper still claims any offered proposal, useful inside non-stack containers). Default: 1.0.

pub fn align_child(self, alignment: Alignment) -> Self

Opt out of stretching the child. The child is laid out at its natural size and positioned within the Expand’s bounds according to alignment.

pub fn respect_intrinsic(self) -> Self

Switch to auto flex basis — the wrapped child’s natural size acts as a floor on each flex axis, and the parent stack adds slack on top via the flex weight. Useful when the wrapper sits inside an unconstrained parent (e.g. an outer VStack with height = None), where the default zero-basis would let the child overflow because the parent has no bound to share.

Trade-off: with respect_intrinsic, exact ratios bend by content width — [Expand::flex(1).child(60), Expand::flex(2).child(40)] in 300 px gives 60 + 66 = 126 and 40 + 133 = 173 rather than 100 / 200. Without it (the default), the same layout splits exactly 100 / 200.

Do not use this inside a bounded parent

The floor is a hard one: Expand reports shrink = 0, so if the child’s natural size exceeds what the parent can offer, the resulting over-constraint deficit cannot be absorbed and later siblings are pushed outside the bounds.

This bites hardest with children whose natural size is large and content-driven. A vertical TabBar answers an unbounded height query with its stacked height — every tab, one below another. So:

// 21 tabs => the bar's natural height is ~1050 dp.
VStack::new()
    .child(Expand::vertical().respect_intrinsic().child(tab_widget))
    .child(status_bar)

makes the VStack want 1050 + status_bar, at every window size. The status bar is placed at y=1050 and stays below the fold until the window is grown past it — the bar never scrolls, because it was never asked to fit. Dropping respect_intrinsic() fixes it: the bar takes the slack left after the status bar and scrolls its tabs internally.

Rule of thumb: reach for this only when the parent genuinely has no bound to share (height = None). When the parent is bounded — a window root, a sized pane — the default zero basis is what you want.

pub fn child_id(self, id: WidgetId) -> Self

Set child by pre-registered ID.

pub fn child(self, widget: impl Widget + 'static) -> Self

Set an inline child widget (deferred insertion).

Trait Implementations

impl Debug for Expand

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Expand

fn default() -> Self

Returns the “default value” for a type.

impl Widget for Expand

fn build(&mut self, ctx: &mut BuildContext<'_>) -> Vec<WidgetId>

Compose child widgets. Called once after the widget is placed in the arena, and again on environment change (theme switch, locale switch). Takes &mut self — store child IDs, signal handles, any state needed later. Returns the list of root child IDs (empty for leaf widgets).

fn layout_response( &self, proposal: SizeProposal, ctx: &LayoutContext<'_>, ) -> LayoutResponse

Respond to the parent’s size proposal with this widget’s wanted size, grow/shrink weights, and compression floor (see [LayoutResponse]).

fn place_children( &self, bounds: Rect, _proposal: SizeProposal, children: &mut [WidgetPlacement], ctx: &LayoutContext<'_>, )

Position children within the allocated bounds.

fn paint(&self, _bounds: Rect, _canvas: &mut Canvas, _ctx: &PaintContext<'_>)

Draw the widget’s visual representation.

fn children(&self) -> Vec<WidgetId>

Return the child widget IDs that this widget manages.

fn type_name(&self) -> &'static str

Concrete type name of this widget (e.g. "teksilo_widgets::button::Button"). The default implementation resolves at the impl site via std::any::type_name::<Self>(), so calls through &dyn Widget correctly dispatch to the monomorphized fn for the concrete type — getting the concrete name through the vtable without per-impl boilerplate.

fn cacheable_layout(&self) -> bool

Whether this widget’s layout_response may be memoized by the per-pass layout cache. Defaults to true.

fn wants_after_paint(&self) -> bool

Whether this widget wants its after_paint hook to fire each frame. Returning false (the default) saves a virtual call per widget per frame for the vast majority of widgets that don’t aggregate descendant geometry.

fn after_paint(&self, _view: &WidgetTreeView<'_>, _ctx: &PaintContext<'_>)

Called once per frame after this widget’s subtree has finished painting. Receives a read-only view of the layout-resolved arena so a parent can read its descendants’ final bounds — e.g. TitleBar aggregates its drag region and control-button rects into a single HitRegions payload for the Windows backend’s WM_NCHITTEST.

fn wants_post_paint(&self) -> bool

Whether this widget wants its post_paint hook to fire each frame. Returning false (the default) saves a virtual call per widget per frame for the vast majority of widgets that don’t draw a foreground over their children.

fn post_paint( &self, _bounds: Rect, _canvas: &mut Canvas, _ctx: &PaintContext<'_>, )

Draw a foreground layer over this widget’s children.

fn accessibility(&self, _builder: &mut AccessNodeBuilder)

Declare this widget’s accessibility identity.

fn wants_descendant_redirects(&self) -> bool

Whether this widget wants the AT walker to consult its a11y_redirect_descendant hook for every descendant during AT tree emission, not just its direct arena children.

fn a11y_redirect_descendant( &self, _self_id: WidgetId, _descendant: WidgetId, ) -> Option<NodeId>

Optional redirection hook for AT-tree placement of a child.

fn accessible_title_hint(&self) -> Option<String>

Suggest an accessible title to an enclosing container that wraps this widget as content — typically a modal / dialog shell that wants to propagate the inner content’s visible title as the shell’s own accessible name.

fn initial_focus_hint(&self) -> Option<WidgetId>

Optional hint that directs initial focus to a specific descendant when this widget is the root of a deferred-built modal surface.

fn accessibility_children(&self) -> Option<Vec<WidgetId>>

Optional override for the child ORDER presented to assistive technology, when it must differ from the paint / z-order child order returned by children.

fn as_any(&self) -> Option<&(dyn Any + 'static)>

Downcast hook. Default implementation returns None; concrete widgets override with Some(self) when they want to expose their concrete type to test-level introspection or reflection. The trait already bounds on std::any::Any so concrete types satisfy the 'static requirement.

fn as_any_mut(&mut self) -> Option<&mut (dyn Any + 'static)>

Mutable counterpart of as_any. Default returns None; widgets that want to expose mutable state to tests (e.g. so a test can mutate a Scene inside a SceneView post-layout) override with Some(self). Should follow the same opt-in pattern as as_any: only widgets that opt into & introspection should opt into &mut.

fn clips_children(&self) -> bool

Whether this widget clips its children to its bounds.

fn focus_reveal_rect(&self, _bounds: Rect) -> Option<Rect>

The rectangle (in absolute tree coordinates) that best represents this widget when the framework reveals it into an ancestor scroll area on focus gain. Returning None (the default) reveals the widget’s whole bounds — correct for most controls.

fn hit_shape(&self, _local_point: Point, _bounds: Rect) -> bool

Whether the point lies inside this widget’s actual shape, not just its rectangular bounds. Consulted by hit-testing right after the bounds check: returning false for a point that is inside the bounding box makes the widget transparent to the click there, so it falls through to whatever sibling is painted underneath (the same machinery as a fully pass-through node, but shape-aware).

fn preserves_children_on_rebuild(&self) -> bool

How rebuild_single_widget treats this widget’s existing children when re-running its build().

fn tooltip_has_content(&self) -> bool

Whether this widget, used as tooltip content, currently has anything worth showing.

fn declare_shortcuts(&self) -> Vec<Shortcut>

Declare the rebindable keyboard shortcuts this widget exposes, without installing handlers. The framework calls this at arena insertion time (before build()) and at certain lazy boundaries (e.g. Switcher walks declarations on its not-yet-mounted Pending slots), so settings UIs and the ShortcutRegistry see the keystrokes the moment the owning container mounts — even if build() hasn’t run.

fn take_handler_set(&mut self) -> Option<HandlerSet>

Extract attached handler set from a WidgetWithHandlers wrapper. Called during arena insertion to transfer handlers to the WidgetNode. Default: returns None (no attached handlers).