impl Actor for Watcher {

    fn pre_start(&mut self, ctx: ActorContext) {

        // Registers target actor as watched
        ctx.system().watch(&ctx.self_, &self.target);
    }

    fn post_stop(&mut self, ctx: ActorContext) {

        // Unregisters target actor as watched
        ctx.system().unwatch(&ctx.self_, &self.target);
    }

    fn receive(self: &mut Self, msg: Message, ctx: ActorContext) -> HandleResult {
        match_downcast_ref!(msg.get(), {
            _m: Terminated => {
                // Sender of this message is the target actor. It indicates that the target actor was
                // stopped
                if ctx.sender.path() == self.target.path() {
                    println!("Target actor '{}' was terminated", ctx.sender);
                }
            },
            _ => return Ok(false)
        });

        Ok(true)
    }
}