SmoothSize

SmoothSize — auto-sizes the slot to fit the child's intrinsic size, but tweens the change instead of jumping. The "empty panel that suddenly must grow gracefully to accept new content" pattern.

ctx.add(
    SmoothSize::new()
        .axes(SmoothSizeAxes::Both)
        .child(Panel::new().child(content_signal)),
);

For explicit size animation (target is a numeric signal you already drive, e.g. a sidebar width), use the existing FixedSize::new().width(animated_signal) + Signal::animate_to pattern instead — that path doesn't need to measure the child every frame.

Layout semantics

• The wrapper measures the child's natural size at the proposal each layout pass.
• When the natural size differs from the current animation target (above 0.5 px), kicks off a new tween.
• size_that_fits returns the current animated value — what the wrapper actually occupies right now, not the target.
• The child is always laid out at its full natural size and clipped to the wrapper's smaller animated bounds. Same trick as Collapse — the child's own internal layout doesn't reflow each frame, only the clip rect changes.

Reduced motion

Honours prefers-reduced-motion: snaps to the natural size each layout pass instead of tweening.

Builder methods at a glance

axes, duration, easing, child, child_id

API reference

📖 Full rustdoc API for this module

pub enum SmoothSizeAxes

Which axes participate in the size tween. Use Width or Height to leave the other axis tracking the child's natural size instantly.

#![allow(unused)]
fn main() {
pub enum SmoothSizeAxes { /* variants */ }
}

Variants

• Width — Animate width changes only; height snaps to natural immediately.
• Height — Animate height changes only; width snaps to natural immediately.
• Both — Animate both width and height changes. Default.

pub struct SmoothSize

Wraps a child widget and animates the wrapper's reported size toward the child's current natural size whenever that size changes.

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

Methods

pub fn new() -> Self

New wrapper. Both axes animate by default.

pub fn axes(mut self, axes: SmoothSizeAxes) -> Self

Restrict the tween to one axis (the other tracks the child's natural size instantly).

pub fn duration(mut self, duration: Duration) -> Self

Override the tween duration. Default: MotionTokens::duration_normal.

pub fn easing(mut self, easing: Easing) -> Self

Override the easing curve. Default: MotionTokens::easing_standard.

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

Inline child widget (deferred insertion).

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

Pre-registered child by WidgetId.