How to Create a Basic PubSub Application

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In LiveView

   def mount(_params, _session, socket) do
     Testbeds.subscribe()
     {:ok, assign(socket, testbeds: Testbeds.list_testbeds())}
   end

   
  def handle_info({TestBeds, [:testbed | _], _}, socket) do
     {:noreply, assign(socket, testbeds: TestBeds.list_testbeds())}
  end

App/test_beds.ex

@topic inspect(__MODULE__)

def subscribe do
  Phoenix.PubSub.subscribe(App.PubSub, @topic)
end



defp broadcast_change({:ok, result}, event) do
  Phoenix.PubSub.broadcast(App.PubSub, @topic, {__MODULE__, event, result})

  {:ok, result}
end



# ....

  def create_testbed(attrs \\ %{}) do
    %Testbed{}
    |> Testbed.changeset(attrs)
    |> Repo.insert()
    |> broadcast_change([:testbed, :created])
  end

Example Without Database

defmodule AppWeb.PageLive do
  use AppWeb, :live_view
  
  def mount(_params, _session, socket) do
    if connected?(socket) do
      AppWeb.Endpoint.subscribe(topic)
    end
    {:ok, assign(socket, username: username, messages: [])}
  end
  
  def handle_info(%{event: "message", payload: message}, socket) do
    {:noreply, assign(socket, messages: socket.assigns.messages ++ [message])}
  end
  
  def handle_event("send", %{"text" => text}, socket) do
    AppWeb.Endpoint.broadcast(topic, "message", %{text: text, name: socket.assigns.username})
    {:noreply, socket}
  end
  
  defp username do
    "User #{:rand.uniform(100)}"
  end
  
  defp topic do
    "chat"
  end

  def render(assigns)do   
   ~H"""
<div>
  <h1>ChatLive</h1>
  <div class="messages" style="border: 1px solid #eee; height: 400px; overflow: scroll; margin-bottom: 8px;">
    <%= for m <- @messages do %>
      <p style="margin: 2px;"><b><%= m.name %></b>: <%= m.text %></p>
    <% end %>
  </div>
  <form phx-submit="send">
    <input type="text" name="text" />
    <button type="submit">Send</button>
  </form>
</div>

"""
  end
  
end

# lib/app/counter.ex
defmodule Counter do
  use Agent

  def start_link(initial_value) do
    Agent.start_link(fn -> initial_value end, name: __MODULE__)
  end

  def value do
    Agent.get(__MODULE__, & &1)
  end

  def increment do
    Agent.update(__MODULE__, &(&1 + 1))
  end
end


# lib/app_web/live/page_live.ex
defmodule AppWeb.PageLive do
  use AppWeb, :live_view

  def mount(_params, _session, socket) do
    Phoenix.PubSub.subscribe(App.PubSub, "the-topic")
    
    # Get current value from the supervised Counter
    data = Counter.value()
    
    {:ok, assign(socket, data: data)}
  end

  def handle_event("increment", _params, socket) do
    Counter.increment()
    new_data = Counter.value()
    Phoenix.PubSub.broadcast(App.PubSub, "the-topic", new_data)
    {:noreply, assign(socket, data: new_data)}
  end

  def handle_info(new_data_from_broadcast, socket) do  # usually set with pattern match like {:data,thedata}
    IO.inspect new_data_from_broadcast
    {:noreply, assign(socket, data: new_data_from_broadcast)}
  end



  def render(assigns) do
    ~H"""
    <div>
      <.button phx-click="increment">click me </.button>

      <div>{@data}</div>
    </div>
    """
  end
end