Add basic channel state.

lib/pleroma/application.ex

@@ -20,6 +20,7 @@ def start(_type, _args) do
                          limit: 2500
                        ]]),
       worker(Pleroma.Web.Federator, []),
+      worker(Pleroma.Web.ChatChannel.ChatChannelState, []),
     ]
     ++ if Mix.env == :test, do: [], else: [worker(Pleroma.Web.Streamer, [])]
 

lib/pleroma/web/chat_channel.ex

 defmodule Pleroma.Web.ChatChannel do
   use Phoenix.Channel
+  alias Pleroma.Web.ChatChannel.ChatChannelState
 
   def join("chat:public", _message, socket) do
+    send(self(), :after_join)
     {:ok, socket}
   end
 
+  def handle_info(:after_join, socket) do
+    push socket, "messages", %{messages: ChatChannelState.messages()}
+    {:noreply, socket}
+  end
+
   def handle_in("new_msg", %{"text" => text}, socket) do
     author = socket.assigns[:user]
     author = Pleroma.Web.MastodonAPI.AccountView.render("account.json", user: author)
-    broadcast! socket, "new_msg", %{text: text, author: author}
+    message= ChatChannelState.add_message(%{text: text, author: author})
+
+    broadcast! socket, "new_msg", message
     {:noreply, socket}
   end
 end
+
+defmodule Pleroma.Web.ChatChannel.ChatChannelState do
+  use Agent
+  @max_messages 20
+
+  def start_link do
+    Agent.start_link(fn -> %{max_id: 1, messages: []} end, name: __MODULE__)
+  end
+
+  def add_message(message) do
+    Agent.get_and_update(__MODULE__, fn state ->
+      id = state[:max_id] + 1
+      message = Map.put(message, "id", id)
+      messages = [message | state[:messages]] |> Enum.take(@max_messages)
+      {message, %{max_id: id, messages: messages}}
+    end)
+  end
+
+  def messages() do
+    Agent.get(__MODULE__, fn state -> state[:messages] |> Enum.reverse end)
+  end
+end