Macro amethyst_ui::define_widget

macro_rules! define_widget {
    ($t:ident =>
        entities: [$($field:tt),*]
        components: [$($component:tt),*]
    ) => { ... };
}

This macro allows you to define what components can be attached to a widget, and will automatically generate getters for all components you specify. For each component, you are required to specify which entity handle the component will be attached to.