Switching Streams in Turbo Rails

Basecamp recently announced Hotwire and it seemed like a perfect replacement to complicated React code I was using to build a browser-based, role-playing game. While still a little rough around the edges, it’s impressive how a few HTML elements can change how interactive a website is.

Hotwire is an alternative approach to building modern web applications without using much JavaScript by sending HTML instead of JSON over the wire.

Subscribing to Streams

The game is based around the concept of individual rooms which isolate communication, items, enemies, etc. Characters stream from the room they’re in to receive messages.

<div id="streams">
  <%= turbo_stream_from character.room %>
</div>

Behind the seasons it creates a custom <turbo-cable-stream-source> element, which triggers a channel subscription via Action Cable. Now we can trigger the standard stream events to append, prepend, replace, and remove elements from the target.

Unsubscribing from Streams

When a character in the game leaves a room, they need to unsubscribe from the room their leaving and subscribe to the room their joining. The new subscription is done the same as before, but unsubscribing from a stream wasn’t clear to me from the documentation.

After digging into the turbo-rails code, I noticed that when a stream element is disconnected it’ll automatically unsubscribe.

class TurboCableStreamSourceElement extends HTMLElement {
  // ...

  disconnectedCallback() {
    disconnectStreamSource(this)
    if (this.subscription) this.subscription.unsubscribe()
  }

  // ...
}

So now that we know if we remove the element the subscription is unsubscribed, how do we remove the stream element?

As mentioned before, the Turbo stream has a remove event that can remove any DOM identifier, not necessarily specific to Turbo connected elements. We can set an ID attribute for the stream to allow us to identify it.

<div id="streams">
  <%= turbo_stream_from character.room, id: dom_id(character.room) %>
</div>

With the stream identified we can remove and add streams as needed, which works perfectly for us when a character is moving between two rooms.

<%= turbo_stream.remove previous_room %>
<%= turbo_stream.append("streams") do %>
  <%= turbo_stream_from current_room, id: dom_id(current_room) %>
<% end %>

While we can append the streams to any element, we’re appending to an element identified by streams. This is simply to allow a central location to see what streams the character is connected to.

While Turbo requires thinking about implementation a bit differently, it’s incredibly easy to use and should be much easier to maintain than a full React application. I’m excited to see how Turbo is implemented over the next year.