Ensuring Elixir Application Performance with Testing and ExUnit/

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Testing a Channel

Testing a Channel

Learn how to write test cases for phoenix channels.

We'll cover the following...

The Phoenix Channel example we’ve provided in the NotSkull sample app, LobbyChannel, has limited functionality. We won’t exhaustively show how to use all of the test macros included in Phoenix as we test ChannelTest, but we’ll cover enough so that we’ll understand how to use all of them.

Understanding the channel

Let’s take a look at the file:

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#file path -> lib/not_skull_web/channels/lobby_channel.ex
#add this code at the indicated place mentioned in comments of lib/not_skull_web/
#channels/lobby_channel.ex in the playground widget
alias NotSkull.GameEngine.Game
@impl true
  def join("lobby:lobby", _payload, socket) do 
    {:ok, socket}
  end
  @spec broadcast_new_game(Game.t()) :: :ok | :error
  def broadcast_new_game(%Game{current_phase: :joining} = game) do
    NotSkullWeb.Endpoint.broadcast!("lobby:lobby", "new_game_created", %{ 
      game_id: game.id
    })
  end
  def broadcast_new_game(_) do
    :error
  end

This file provides just two functions, join/3 and broadcast_new_game/1. In the join/3 function, found on line 7 is a required callback for the Channel framework and simply returns a socket, assuming it’s been called with valid parameters. As a result, we’ll cover this functionality in our test as part of our setup. On line 13, there is broadcast_new_game/1, which will broadcast a map containing the ID of a new game to any client connected to the Channel. There’s one more clause for broadcast_new_game/1 that returns an error if either something other than a %Game{} struct is passed in or if the value of the game struct’s current_state is something other than :joining.

Defining the test

We’ll need a happy path test, and error tests for each state that falls into the error clause. Create a new test file at test/not_skull_web/channels/lobby_channel_test.exs and populate it with the following code for the test file:

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#file path -> test/not_skull_web/channels/lobby_channel_test.exs
#add this code at the indicated place mentioned in comments of test/not_skull_web/
#channels/lobby_channel_test.exs in the playground widget
defmodule NotSkullWeb.LobbyChannelTest do
  use NotSkullWeb.ChannelCase
  alias NotSkullWeb.{LobbyChannel, UserSocket}
  describe "broadcast_new_game/1" do
    setup do
      user_id = Factory.uuid() 
      {:ok, _, socket} =
        UserSocket
        |> socket("user_socket:#{user_id}", %{user_id: user_id}) 
        |> subscribe_and_join(LobbyChannel, "lobby:lobby") 
      %{socket: socket}
    end

Typical tests for a Phoenix Channel will start with the assumption that the user has already connected to the WebSocket through the code in ...