We will add an actor to resend messages to other actors. We need some basic types from the actix crate, along with a HashSet to keep addresses of actors. Import the NewComment struct, which we will clone and resend:

use actix::{Actor, Context, Handler, Message, Recipient};
use std::collections::HashSet;
use super::NewComment;

Add a RepeaterActor struct with a listeners field of the HashSet type that contains Recipient instances:

pub struct RepeaterActor {
    listeners: HashSet<Recipient<RepeaterUpdate>>,
}

You are familiar with the Addr type, but we haven't used Recipient before. Actually, you can convert any Addr instance into a Recipient using the recipient method call. The Recipient type is an address that supports only one type of Message.

Add a constructor that creates an empty HashSet:

impl RepeaterActor {
    pub fn new() -> Self {
        Self {
            listeners: HashSet::new(),
        }
    }
}

Next, implement an Actor trait for this struct:

impl Actor for RepeaterActor {
    type Context = Context<Self>;
}

It's enough to have a standard Context type as an associated context type of Actor, because it can work asynchronously.

Now, we have to add messages to this actor type.