Type Alias RootBuilder 

pub type RootBuilder = Box<dyn FnOnce(&mut WidgetTree, WindowState) -> WidgetId>;

Signature of a window’s root-builder closure.

Receives a mutable WidgetTree and a cloned WindowState so the builder can register widgets that bind against window-level signals (placement, title, size, …).

Aliased Type

pub struct RootBuilder(/* private fields */);