Expand

Expand — a layout modifier that claims slack space in a stack and stretches its child to fill the allocated bounds.

Inside an HStack or VStack, Expand participates in the flex distribution pass by reporting a non-zero flex weight (default 1.0). The parent stack distributes leftover space proportionally to each child's flex weight. Expand::new() competes on both axes; Expand::horizontal() and Expand::vertical() restrict competition to the named axis so they do not accidentally steal slack from orthogonal siblings. By default the wrapped child is stretched to the full allocated rectangle; call .align_child(alignment) to keep the child at its natural size and align it within the slot instead.

The default flex basis is zero (CSS flex-basis: 0), giving exact proportional ratios. Call .respect_intrinsic() to switch to auto basis where the child's natural size acts as a floor before flex slack is added.

#![allow(unused)]
fn main() {
use teksilo_widgets::primitives::{HStack, Expand, RectWidget};
// Two panels sharing horizontal space in a 1:2 ratio
let _row = HStack::new()
    .child(Expand::new().flex(1.0).child(RectWidget::new()))
    .child(Expand::new().flex(2.0).child(RectWidget::new()));
}

Builder methods at a glance

horizontal, vertical, flex, align_child, respect_intrinsic, child_id, child

API reference

📖 Full rustdoc API for this module

`pub struct Expand`

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.

#![allow(unused)]
fn main() {
pub struct Expand { /* fields */ }
}

Methods

`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(mut 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(mut 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(mut 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(mut self, id: WidgetId) -> Self`

Set child by pre-registered ID.

Set an inline child widget (deferred insertion).

Teksilo Documentation