elixirphoenix-live-view

How to get app.html.heex to use assigns properly with LiveView


Background

I have a Phoenix application, where all pages (expect the login page) have a menu at the top. This menu will therefore only appear if the user has already logged in.

I am trying to replicate this behaviour, by incorporating said menu in the app.html.heex so I don't have to repeat it constantly.

However, no matter what I do, the menu is never displayed.

Code

I am trying to change my app.html and verify if the user logged in using assigns[:user].

app.html.heex:

<header>

 <%= if assigns[:user] do %>
   <h1> Cool Menu here </h1>
 <% end %>

</header>

<main class="">
  <div class="mx-auto max-w-2xl">
    <.flash_group flash={@flash} />
    <%= @inner_content %>
  </div>
</main>

The login process is async, as shown here. Basically I send a login request to a Manager, and when it feels like replying back, I update the socket with assigns[:user] and redirect to a cool page.

user_login_live.ex:

defmodule WebInterface.MyAppLive do
  use WebInterface, :live_view

  @impl true
  def mount(_params, _session, socket), do: {:ok, socket}

  @impl true
  def handle_event("login", params, socket) do
    IO.puts("Seding async request")
    :ok = Manager.login(params)
    {:noreply, socket}
  end

  @impl true
  def handle_info({:login,  user, :done}, socket) do
    IO.puts("Authentication succeeded for user #{inspect(user)}")

    updated_socket =
      socket
      |> assign(:user, user)
      |> redirect(to: ~p"/cool_page")

    {:noreply, updated_socket}
  end

end

I would expect the cool page to have the <h1> menu, but that is not the case.

Questions

  • What am I doing wrong here?
  • Isn't the assigns updated automatically?
  • Does app.html.heex work differently from other files?

Solution

  • For the sake of completeness, I have decided to post my final answer. This curated answer is mostly a resume of this huge thread that focuses on the most relevant issues and explores other options I also found. I hope future readers find it interesting.


    Why this happens

    So, after a lot of investigation and help from the community I found out what is happening.

    Turns out that session data (data that needs to travel between multiple pages) cannot be shared over websockets. LiveViews use Websockets and therefore suffer from this limitation (https://elixirforum.com/t/how-to-get-app-html-heex-to-use-assigns-properly-with-liveview/57481/18?u=fl4m3ph03n1x):

    Yes, if you are logging in a user you have to store a successful login id in a session. The thing is, you can’t set session data over a websocket. This is a websocket limitation, not a LiveView one ( ... )

    This also means you cant use cookies/alter them (https://elixirforum.com/t/persisting-data-across-liveview-navigation/53971/4?u=fl4m3ph03n1x):

    ( ... ) because LV is all over a websocket you cant alter the cookies so your stuck with local storage or another system.

    Possible solutions

    However, this is not the end. There are other options if you need to share data between websockets. I was able to identify 4:

    1. Add the session data to the URL of the target page. An example of this is using GET HTTTP method with session data as parameters (or perhaps using post): https://stackoverflow.com/a/76857225/1337392
    2. Save the data inside a global ETS table in the server: https://elixirforum.com/t/persisting-data-across-liveview-navigation/53971/4?u=fl4m3ph03n1x
    3. Save the data in an Agent. You could have an Agent per websocket, thus avoiding a global state and improving efficiency: https://elixirforum.com/t/persisting-data-across-liveview-navigation/53971/4?u=fl4m3ph03n1x
    4. Store the data in a Phoenix Session: https://elixirforum.com/t/how-to-get-app-html-heex-to-use-assigns-properly-with-liveview/57481/18?u=fl4m3ph03n1x

    Here you have to weight the risks/benefits of every solution. If you have your own website, solution 4 would probably be the most secure, while solution 1 would also be OK provided you encrypt the data before putting it in the url parameters or post body request.

    However, in my specific case, a Windows desktop application for a single user, these considerations are not important. I have therefore opted for solution 2, since it is simpler than solution 3 and the data there is mostly read only.

    Final code

    So now I am using a temporal persistence layer to store session information:

    defmodule WebInterface.Persistence do
      @moduledoc """
      Responsible for temporary persistence in WebInterface. Uses ETS beneath the scenes.
      """
    
      alias ETS
      alias Shared.Data.User
    
      @table_name :data
    
      @spec init :: :ok | {:error, any}
      def init do
        with {:ok, _table} <- ETS.KeyValueSet.new(name: @table_name, protection: :public) do
          :ok
        end
      end
    
      @spec set_user(User.t) :: :ok | {:error, any}
      def set_user(%User{} = user) do
        with {:ok, table} <- ETS.KeyValueSet.wrap_existing(@table_name),
          {:ok, _updated_table} <- ETS.KeyValueSet.put(table, :user, user) do
            :ok
          end
      end
    
      @spec get_user :: {:ok, User.t} | {:error, any}
      def get_user do
        with {:ok, table} <- ETS.KeyValueSet.wrap_existing(@table_name) do
          ETS.KeyValueSet.get(table, :user)
        end
      end
    
      @spec has_user? :: boolean
      def has_user? do
        case get_user() do
          {:ok, nil} -> false
          {:ok, _user} -> true
          _ -> false
        end
      end
    end
    

    Now in my app file, I simply ask the persistence module if I have the data I want. This is quite transparent:

    app.html.heex:

    <header>
     <%= if WebInterface.Persistence.has_user?() do %>
      <h1> <%= @user.name %> is awesome !</h1>
      <% end %>
    
    </header>
    <main class="">
      <div class="mx-auto max-w-2xl">
        <.flash_group flash={@flash} />
        <%= @inner_content %>
      </div>
    </main>
    

    And I set/get the data I want via the Persistence API.

    user_login_live.ex:

    defmodule WebInterface.UserLoginLive do
      use WebInterface, :live_view
    
      alias Manager
      alias Shared.Data.{Credentials, User}
      alias WebInterface.Persistence
    
      @impl true
      def mount(_params, _session, socket), do: {:ok, socket}
    
      @impl true
      def handle_event("login", %{"email" => email, "password" => password} = params, socket) do
       # this is an async request for authentication
       # here we simply start it 
       :ok =
          email
          |> Credentials.new(password)
          |> Manager.login(Map.has_key?(params, "remember-me"))
    
          {:noreply, socket}
      end
    
    
      @impl true
      def handle_info({:login, %User{} = user, :done}, socket) do
        # when we receive this message, we know authentication is done.
         
        # and in other places we use get_user/0 to retrieve session data
        :ok = Persistence.set_user(user)
    
        {:noreply,  socket |> redirect(to: ~p"/other_page")}
      end
    end
    

    For now this solution works wonderfully.