Trait Widget 

pub trait Widget: Any {
    // Provided methods
    fn measure(
        &mut self,
        cx: &mut LayoutCx<'_>,
        constraints: Constraints,
    ) -> Size { ... }
    fn layout(&mut self, cx: &mut LayoutCx<'_>, size: Size) { ... }
    fn paint(&mut self, cx: &PaintCx<'_>, canvas: &mut Canvas<'_>) { ... }
    fn paint_over(&mut self, cx: &PaintCx<'_>, canvas: &mut Canvas<'_>) { ... }
    fn event(&mut self, cx: &mut EventCx<'_>, event: &Event<'_>) -> Handled { ... }
    fn accepts_focus(&self) -> bool { ... }
    fn focus_on_press(&self) -> bool { ... }
    fn accessibility(&self, cx: &PaintCx<'_>, node: &mut AccessNode) { ... }
    fn access_action(
        &mut self,
        cx: &mut EventCx<'_>,
        request: &AccessRequest<'_>,
    ) -> Handled { ... }
}

A part of the interface: it can measure itself, paint, and handle events.

A Ui owns its widgets in a tree; their places come from each widget’s Style and, for widgets without children, from measure. Painting and event handling run every frame, so they must not allocate once a widget’s content is set up; measuring and layout run only when sizes change and may.

Every method has a default: a plain container needs to implement nothing.

Provided Methods

fn measure(&mut self, cx: &mut LayoutCx<'_>, constraints: Constraints) -> Size

The size the widget wants within constraints, for a widget without children (one with children is sized by its style and its children). Physical pixels.

fn layout(&mut self, cx: &mut LayoutCx<'_>, size: Size)

Layout decided the widget’s size: re-wrap text or recompute anything that depends on it. Called after every layout pass that changed the size.

fn paint(&mut self, cx: &PaintCx<'_>, canvas: &mut Canvas<'_>)

Paints the widget below its children.

fn paint_over(&mut self, cx: &PaintCx<'_>, canvas: &mut Canvas<'_>)

Paints the widget over its children: scroll bars, focus rings, overlays.

fn event(&mut self, cx: &mut EventCx<'_>, event: &Event<'_>) -> Handled

Handles event. An event the widget does not handle goes on to its parent.

fn accepts_focus(&self) -> bool

Whether the widget takes keyboard focus, by click or by Tab.

fn focus_on_press(&self) -> bool

Whether a press on the widget, or on a widget inside it that takes no focus, focuses it (if it accepts focus). A container reachable by Tab, such as a dock’s tab list, says no, so clicks on its contents do not focus it; it can still ask for focus itself.

fn accessibility(&self, cx: &PaintCx<'_>, node: &mut AccessNode)

Describes the widget to assistive technology: fills node, which starts as a Role::Group with the widget’s rectangle. Called only while assistive technology is listening; it may allocate.

fn access_action( &mut self, cx: &mut EventCx<'_>, request: &AccessRequest<'_>, ) -> Handled

Does what assistive technology asks (only actions the widget’s node lists are asked of it). Focus and scrolling into view are done for every widget; a request the widget does not handle goes on to its parent if it is a scroll.

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors