Building an event-sourced game with Phoenix Liveview: Acting on the game from the views

Feb 22, 2021

In the last article, we’ve seen the GameServer implementation. It’s now time to look at the views, specifically how players interact with the game.

Starting the game from the LiveView

I decided to use LiveViews as they are convenient for real-time interactions without requiring any Javascript code.

In this article, we’ll focus on the LiveView used for players to register as it exhibits handling form, sending messages to the GameServer, and handling potential errors.

Let’s first create a new module for our LiveView. When the view is mounted, the game_id available in the route parameters is stored in the state.

defmodule DoctorPWeb.WaitingRoomLive do

def mount(%{game_id: game_id}, _session, socket) do

 state = %{game_id: game_id}

  socket = socket
  |> assign(state)

  {:ok, socket}
end

end

The simplest case to handle in this state is handling the click on the “Start game” button. The LiveView receives an event without any parameters and sends a message to the GameServer.

@impl true
def handle_event("start_playing", _params, socket) do
  GameServer.start_playing(socket.assigns.game_id)

  {:noreply, socket}
end

Thanks to the client function we’ve added to GameServer and being able to find it using the game ID, the implementation is easy. When a start_playing event is received by the LiveView process a call is issued to GameServer.start_playing(socket.assigns.game_id).

Registering a player from the LiveView

Registering a player is more complex because it involves a form for the player to specify her name.

Following ideas from this article, I managed to use Ecto as a validation library to create a form that is not tied to a database.

We declare a module describing the form:

defmodule RegisterPlayer do
  use Ecto.Schema
  import Ecto.Changeset

  embedded_schema do
    field(:player_name, :string)
  end

  alias __MODULE__

  def changeset(register_player, params \\ %{}) do
    register_player
    |> cast(params, [:player_name])
    |> validate_required([:player_name])
    |> validate_length(:player_name, min: 1)
  end

  def apply_changes(changeset) do
    Ecto.Changeset.apply_changes(changeset)
  end

end

Once this module is created, we can use the form in the LiveView. In the mount function we need to add the form changeset.

state = %{game_id: game_id, changeset: RegisterPlayer.changeset(%RegisterPlayer{})}

And modify the template to display the form:

<%= form_for @changeset, "#", [phx_submit: :register_player, id: "player_registration_form"], fn f -> %>

<%= label f, :player_name %>
<%= text_input f, :player_name, autofocus: true, placeholder: "What's your name?" %>
<%= error_tag f, :player_name %>

<%= submit "Register", phx_disable_with: "Registering..." %>
<% end %>
</form>

With this in place, the next step is to handle the form by creating a clause for handle_event matching the register_player event.

def handle_event("register_player", %{"register_player" => register_player} = params, socket) do

  changeset =
    %RegisterPlayer{}
    |> RegisterPlayer.changeset(register_player)
    |> Map.put(:action, :insert)


 with true <- changeset.valid?,
                   registration <- RegisterPlayer.apply_changes(changeset),
                   :ok <- GameServer.add_player(socket.assigns.game_id, registration.player_name) do
    {:noreply, socket}
  else
    _ -> {:noreply, assign(socket, :changeset, changeset)}
  end

end

In the first part of the function, a new changeset for the form is created based on the form’s data.

If the changeset is valid, the changes are applied to an empty RegisterUser structure. We access data from that structure to call GameServer.add_player.

If anything is not going as expected, we enter the else part of the with structure. The changeset is given back to the view, allowing to display form validation errors to the player.

Name can't be blank error displayed

Displaying errors from the GameServer

Another type of error is the ones returned by the GameServer when it cannot handle a command.

Let’s take an example with the registration process. As seen in the article about errors, we disallow two players to share the same name. If a player tries to register with a name taken by another player, an :player_name_not_available is returned by GameServer.add_player.

Sure, we could add some logic in the WaitingRoomLive LiveView module to keep track of all registered players and prevent form submission if needed, but what if two players try to register with the same still available name at the exact same moment?

The GameServer, as the source of truth, will handle one command after the other and refuse the second one.

Let’s see how to handle this.

First, we add the add_player_name_already_taken_error to the RegisterPlayer module, the one responsible for the registration form.

defmodule RegisterPlayer do
	# ....
  @name_already_taken_error_message "This name is already taken by another player"

 def add_player_name_already_taken_error(changeset), do:
    changeset
    |> add_error(:player_name, @name_already_taken_error_message)
end

This function adds an error for the :player_name field to the changeset.

Then, we can modify the with structure we’ve seen previously to use that function when needed.

with true <- changeset.valid?,
                 registration <- RegisterPlayer.apply_changes(changeset),
                 :ok <- GameServer.add_player(socket.assigns.game_id, socket.assigns.player_id, registration.player_name) do
  {:noreply, socket}
else
  {:error, :player_name_not_available} -> {:noreply, assign(socket, :changeset, RegisterPlayer.add_player_name_already_taken_error(changeset))}
  _ -> {:noreply, assign(socket, :changeset, changeset)}
end

If GameServer.add_player returns the {:error, :player_name_not_available} error tuple, we assign a modified changeset with the error to the socket.

Name already taken error displayed

That’s it for this article. We’ve covered all needed for players to act against the game. In the next article, we’ll see how the view constructs its state from the game’s state.