trait Element {
    fn handle(&mut self, ui: &mut UI) {

    }
}

struct UI {
    elements: Vec<Box<Element>>
}

impl UI {
    fn handle(&mut self, index: usize) {
        let mut element = self.elements.get_mut(index).unwrap();
        element.handle(self);
    }
}

struct Thing;

impl Element for Thing {
    fn handle(&mut self, ui: &mut UI) {
        println!("test");
    }
}

fn main() {
    let mut ui = UI { elements: Vec::new() };
    ui.elements.push(Box::new(Thing));
    ui.handle(0);
}