Differential D351 Diff 810 web/channels/presence.ex

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web/channels/presence.ex

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				defmodule KolabChat.Presence do
				@moduledoc """
				Provides presence tracking to channels and processes.

				See the [`Phoenix.Presence`](http://hexdocs.pm/phoenix/Phoenix.Presence.html)
				docs for more details.

				## Usage

				Presences can be tracked in your channel after joining:

				defmodule KolabChat.MyChannel do
				use KolabChat.Web, :channel
				alias KolabChat.Presence

				def join("some:topic", _params, socket) do
				send(self, :after_join)
				{:ok, assign(socket, :user_id, ...)}
				end

				def handle_info(:after_join, socket) do
				{:ok, _} = Presence.track(socket, socket.assigns.user_id, %{
				online_at: inspect(System.system_time(:seconds))
				})
				push socket, "presence_state", Presence.list(socket)
				{:noreply, socket}
				end
				end

				In the example above, `Presence.track` is used to register this
				channel's process as a presence for the socket's user ID, with
				a map of metadata. Next, the current presence list for
				the socket's topic is pushed to the client as a `"presence_state"` event.

				Finally, a diff of presence join and leave events will be sent to the
				client as they happen in real-time with the "presence_diff" event.
				See `Phoenix.Presence.list/2` for details on the presence datastructure.

				## Fetching Presence Information

				The `fetch/2` callback is triggered when using `list/1`
				and serves as a mechanism to fetch presence information a single time,
				before broadcasting the information to all channel subscribers.
				This prevents N query problems and gives you a single place to group
				isolated data fetching to extend presence metadata.

				The function receives a topic and map of presences and must return a
				map of data matching the Presence datastructure:

				%{"123" => %{metas: [%{status: "away", phx_ref: ...}],
				"456" => %{metas: [%{status: "online", phx_ref: ...}]}

				The `:metas` key must be kept, but you can extend the map of information
				to include any additional information. For example:

				def fetch(_topic, entries) do
				query =
				from u in User,
				where: u.id in ^Map.keys(entries),
				select: {u.id, u}

				users = query \|> Repo.all \|> Enum.into(%{})

				for {key, %{metas: metas}} <- entries, into: %{} do
				{key, %{metas: metas, user: users[key]}}
				end
				end

				The function above fetches all users from the database who
				have registered presences for the given topic. The fetched
				information is then extended with a `:user` key of the user's
				information, while maintaining the required `:metas` field from the
				original presence data.
				"""
				use Phoenix.Presence, otp_app: :kolab_chat,
				pubsub_server: KolabChat.PubSub
				end