handler_ref.rs

use std::cell::{RefCell, RefMut, BorrowMutError};

enum Event {
  Click,
  Close,
  Keystroke
}

trait Child<'a> {
  fn handle_event(&'a mut self, event: Event, ctx: &'a RefCell<u64>) -> u64;
}

struct FooChild<'a> {
  value: Option<RefMut<'a, u64>>
}

impl<'a> Child<'a> for FooChild<'a> {
  fn handle_event(&'a mut self, event: Event, ctx: &'a RefCell<u64>) -> u64 {
    match event {
      Event::Click => {
        self.value = Some(ctx.try_borrow_mut().unwrap());
        0
      },
      Event::Keystroke => {
        if let Some(ref mut value) = self.value {
          **value = (**value) + 2;
          (**value) * (**value)
        }
        else {
          0
        }
      },
      Event::Close => {
        self.value = None;
        1
      },
      _ => 0
    }
  }
}

struct Main<C> {
  child: C,
  store: RefCell<u64>
}

impl<'a, C: Child<'a>> Main<C> {
  pub fn handle_event(&mut self, event: Event) -> u64 {
    self.child.handle_event(event, &self.store)
  }
}

fn main() {
  let m = Main {
    child: FooChild {value: None},
    store: RefCell::new(0)
  };
  println!("Click: {}", m.handle_event(Event::Click));
  println!("Keystroke: {}", m.handle_event(Event::Keystroke));
  println!("Keystroke: {}", m.handle_event(Event::Keystroke));
  println!("Close: {}", m.handle_event(Event::Close));
}

Fixed version: https://play.rust-lang.org/?gist=3be53b681c3960cdbda75238170d6183&version=stable