Keep a presence list up to date using broadcasted presence_diff data.

The functionality in this package is closely modelled after what the syncState and syncDiff functions on Presence in phoenix.js provide. Just like you would use syncState and syncDiff in the browser to keep the presence list inside the browser up to date, you can use the supplied PhoenixPresenceList module to keep a presence list inside a BEAM process up to date.

Say you have a GenServer process that you would like to subscribe to presence changes on a given topic and have that process keep track of the presence list.

Your GenServer module is named FooWeb.Bar, phoenix presence module is named FooWeb.Presence, your endpoint is named FooWeb.Endpoint and you have a channel with the topic lobby where you are tracking presence.

Then you could do something like this in order to keep a list of presences inside FooWeb.Bar:

defmodule FooWeb.Bar do
  use GenServer
  import PhoenixPresenceList, only: [apply_diff: 2]
  alias Phoenix.Socket.Broadcast

  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts)
  end

  def list(server) do
    GenServer.call(server, :list)
  end

  def init(opts) do
    FooWeb.Endpoint.subscribe("lobby")
    initial_state = FooWeb.Presence.list("lobby")
    {:ok, initial_state}
  end

  def handle_call(:list, _from, state) do
    {:reply, Map.keys(state) |> MapSet.new(), state}
  end

  def handle_info(%Broadcast{event: "presence_diff", payload: payload}, state) do
    {state, _joined, _left} = apply_diff(state, payload)
    {:noreply, state}
  end
end

For a more complete example have a look at this test.

This package can be installed by adding phoenix_presence_list to your list of dependencies in mix.exs:

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

This project is licensed under the MIT License - see the LICENSE.txt file for details