Struct Lazy 

pub struct Lazy { /* private fields */ }

A subtree that is not built until the first time it would be shown — lazy when:$cond { … } in .rsx.

This is the general form of what a NavHost does per route: pay for a screen when the user first reaches it, not at startup. Use it for anything expensive behind a condition the user may never satisfy — a settings panel, an inspector, a tab body, a chart that only some accounts see.

It is deliberately not what a reactive if $cond does. That builds its branch whenever the condition turns true and disposes it when it turns false, so a repeatedly toggled panel is rebuilt every time and loses whatever state it held. This builds once, on the first true, and from then on only shows or hides the same subtree — so scroll position, form entry and in-flight work survive being closed and reopened. The cost is symmetric: a subtree shown once is held until the whole block is dropped.

Implementations

impl Lazy

pub fn new( container_style: LayoutStyle, visible: impl Fn() -> bool + 'static, build: impl FnOnce() -> Result<Vec<Box<dyn LayoutItem>>, LayoutError> + 'static, ) -> Result<Self, LayoutError>

visible is the reactive condition; build constructs the children the first time it holds, against the live layout tree from inside the tracking effect — the same way a reactive list builds its items.

pub fn is_built(&self) -> bool

Whether the subtree has been built yet — false until the condition first holds.

Trait Implementations

impl Component for Lazy

fn view(&self) -> RenderNode

fn on_event(&mut self, event: &Event) -> EventResult

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

Human-readable widget type name for the devtools tree inspector.

impl LayoutItem for Lazy

fn layout_node(&self) -> NodeId