Shared via Rust Playground

playground.rs

use std::any::Any;
use std::rc::Rc;

trait AsAny {
    fn as_any(&self) -> &Any;
}

impl<T: Any> AsAny for T {
    fn as_any(&self) -> &Any { self }
}

type Key = usize;
type ChildWidgetIter<'a> = Box<Iterator<Item=(Option<Key>, Rc<Widget>)> + 'a>;

trait Widget: Any + AsAny {
    fn children(&self) -> ChildWidgetIter {
        Box::new(::std::iter::empty())
    }
    fn child_count(&self) -> usize {
        0
    }
    fn allows_children(&self) -> bool {
        false
    }
    fn child(&self, index: usize) -> Option<Rc<Widget>> {
        None
    }
    fn insert_child(&self, index: usize, key: Option<Key>, widget: Rc<Widget>) {
        panic!("This widget cannot be a parent.")
    }
    fn push_child(&self, key: Option<Key>, widget: Rc<Widget>) {
        panic!("This widget cannot be a parent.")
    }
    fn remove_child(&self, index: usize) {
        panic!("This widget has no children.")
    }
}

impl Widget {
    fn downcast_ref<T: Widget>(&self) -> Option<&T> {
        self.as_any().downcast_ref::<T>()
    }
}

struct Foo;

impl Widget for Foo {
}

fn main() {
    let x: Rc<Widget> = Rc::new(Foo);
    for child in x.children() {
    
    }
    //let x: &Foo = x.downcast_ref().unwrap();
}