Elixir Phoenix PubSub Tutorial

This page is in progress

---

This tutorial explains the basics of building a Phoenix PubSub chat app.

The goal of this writing is to explain how to write code used to build PubSub applications in Phoenix. I tried to write examples that keep code writing to a minimum. I believe minimalist examples are the best approach toward comprehension.

The tutorial is divided into three parts, each part building on the completion of the previous one.

Project Description

• Part 1: We create a two route live view app with each route assigned its own page. The two routes are /send and /receive.

The page at /send contains an html form and form submission code. When data is submitted, the /receive page receives the data, and renders it to its page. The code we write for this exercise uses pubsub functions to connect the two LiveView pages.

When complete, you will have two browser tabs open, one opened to /send and one opened to /receive. The /send route sends data to /receive and you view the update in real time.

• Part 2: We convert the previous app into a single LiveView page that performs both actions on a single route. The end result is the creation of a single page chat application containing real time updates that you view across browser tabs.

• Part 3: We write code to update your single page app to store chat data in a database.

Part 1

Setup

To begin we will start with an new Phoenix instance. In your terminal create a new phoenix app by typing:

mix phx.new app

When the console prompts you to Fetch and install dependencies, choose yes.

When you see the following instructions configure your database, run mix ecto.create.

We are almost there! The following steps are missing:

    $ cd app

Then configure your database in config/dev.exs and run:

    $ mix ecto.create

Start your Phoenix app with:

    $ mix phx.server

You can also run your app inside IEx (Interactive Elixir) as:

    $ iex -S mix phx.server

If you are new to Elixir and are having trouble with database setup, please fix those issues before proceeding with this tutorial.

When the app is created, open your web browser to localhost:4000. The app will launch. If it doesn't, fix the errors before moving forward.

Create Routes

Go to the router and update it as shown.

directory: app/app_web/router.ex

  scope "/", AppWeb do
    pipe_through :browser
    live "/send", SendLive, :home
    live "/receive", ReceiveLive, :home
    get "/", PageController, :home
  end

Create the LiveView Pages

Create a new folder named live in this directory: App/lib/app_web. The end result will look like this: App/lib/app_web/live.

In the live directory, create two files and name them send_live.ex and receive_live.ex.

Edit Send Live.ex

Open the file send_live.ex in your text editor. Paste the following code into it.

defmodule AppWeb.SendLive do
  use AppWeb, :live_view
  
  def mount(_params, _session, socket) do
    {:ok, socket}
  end
  
  def handle_event("send", %{"text" => text}, socket) do
    IO.inspect text

    Phoenix.PubSub.broadcast(App.PubSub, "message", {:text_stuff, text}) 

    {:noreply, socket}
  end
  
  defp topic do #Topic
    "message"
  end

  def render(assigns)do   
   ~H"""
    <div>
      <h1>Send Message</h1>
      <form phx-submit="send">
        <input type="text" name="text" />
        <button type="submit">Send</button>
      </form>
    </div>

    """
  end
end

Open receive_live.ex and copy the following code to it.

defmodule AppWeb.SendLive do
  use AppWeb, :live_view
  
  def mount(_params, _session, socket) do
    {:ok, socket}
  end
  
  def handle_event("send", %{"text" => text}, socket) do
    IO.inspect text

    Phoenix.PubSub.broadcast(App.PubSub, "message", {:text_stuff, text}) 

    {:noreply, socket}
  end
  
  defp topic do #Topic
    "message"
  end

  def render(assigns)do   
   ~H"""
    <div>
      <h1>Send Message</h1>
      <form phx-submit="send">
        <input type="text" name="text" />
        <button type="submit">Send</button>
      </form>
    </div>

    """
  end
end