Twitch OAuth2 strategy for Überauth.

1. Setup your application in Twitch under your profile [applications menu][twitch-apps]

2. Add :ueberauth_twitch to your list of dependencies in mix.exs:

   def deps do
     [{:ueberauth_twitch, "~> 0.1.0"}]
   end

3. Add Twitch to your Überauth configuration to your config.exs:

   config :ueberauth, Ueberauth,
     providers: [
       twitch: {Ueberauth.Strategy.Twitch, [default_scope: "user:read:email"]}
     ]

4. Add your provider configuration to your config.exs next, all this information should mirror what you have for your Twitch app:

   config :ueberauth, Ueberauth.Strategy.Twitch.OAuth,
     client_id: System.get_env("TWITCH_CLIENT_ID"),
     client_secret: System.get_env("TWITCH_CLIENT_SECRET"),
     redirect_uri: System.get_env("TWITCH_REDIRECT_URI")

5. Include the Üeberauth plug in your router.exs in the browser or custom pipeline:

   defmodule TwitchWeb.Router do
      use TwitchWeb, :router
   
      pipeline :browser do
        plug Ueberauth
      ...
     end
   end

6. Define the request and callback routes in your router.exs:

   scope "/auth", TwitchWeb do
     pipe_through :browser
   
     get "/:provider", AuthController, :request
     get "/:provider/callback", AuthController, :callback
   end

7. Create a new controller or use an existing controller that implements callbacks to deal with Ueberauth.Auth and Ueberauth.Failure responses from Twitch.

     defmodule TwitchWeb.AuthController do
       use TwitchWeb, :controller
   
       def callback(%{assigns: %{ueberauth_failure: _fails}} = conn, _params) do
         Logger.debug(_fails)
         conn
         |> put_flash(:error, "Failed to authenticate.")
         |> redirect(to: "/")
       end
   
       def callback(%{assigns: %{ueberauth_auth: auth}} = conn, _params) do
         # This is an example of how you can pass the auth information to
         # a function that you implement that will register or login a user
         case UserFromAuth.find_or_create(auth) do
           {:ok, user} ->
             conn
             |> put_flash(:info, "Successfully authenticated.")
             |> put_session(:current_user, user)
             |> configure_session(renew: true)
             |> redirect(to: "/")
   
           {:error, reason} ->
             conn
             |> put_flash(:error, reason)
             |> redirect(to: "/")
         end
       end
     end

Once your setup, you can initiate auth using the following URL, unless you changed the routes from the guide:

/auth/twitch

Twitch allows a user to sign up without providing an email address (they can use a phone verify their account). As a result instead of providing an empty string when an email address is not present you will follow the following format: id+login@no-email-provided.twitch.tv