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A foundation for building Elixir Phoenix apps — SaaS, social networks, ERP systems, marketplaces, and more
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Files
lib/phoenix_kit_web/live/modules/blogging/presence_helpers.ex
defmodule PhoenixKitWeb.Live.Modules.Blogging.PresenceHelpers do
@moduledoc """
Helper functions for collaborative blog post editing with Phoenix.Presence.
Provides utilities for tracking editing sessions, determining owner/spectator roles,
and syncing state between users.
"""
alias PhoenixKitWeb.Live.Modules.Blogging.Presence
@doc """
Tracks the current LiveView process in a Presence topic.
## Parameters
- `form_key`: The unique key for the post being edited
- `socket`: The LiveView socket
- `user`: The current user struct
## Examples
track_editing_session("blog:my-post/en.phk", socket, user)
# => {:ok, ref}
"""
def track_editing_session(form_key, socket, user) do
topic = editing_topic(form_key)
Presence.track(self(), topic, socket.id, %{
user_id: user.id,
user_email: user.email,
user: user,
joined_at: System.system_time(:millisecond),
phx_ref: socket.id,
pid: self(),
transport_pid: socket.transport_pid
})
end
@doc """
Determines if the current socket is the owner (first in the presence list).
Returns `{:owner, presences}` if this socket is the owner (or same user in different tab), or
`{:spectator, owner_meta, presences}` if a different user is the owner.
## Examples
case get_editing_role("blog:my-post", socket.id, current_user.id) do
{:owner, all_presences} ->
# I can edit!
{:spectator, owner_metadata, all_presences} ->
# I'm read-only, sync with owner's state
end
"""
def get_editing_role(form_key, socket_id, current_user_id) do
presences = get_sorted_presences(form_key)
case presences do
[] ->
# No one here (shouldn't happen since caller is here)
# But treat as owner to avoid blocking
{:owner, []}
[{^socket_id, _meta} | _rest] ->
# I'm first! I'm the owner
{:owner, presences}
[{_other_socket_id, owner_meta} | _rest] ->
# Check if same user (different tab) or different user
if owner_meta.user_id == current_user_id do
# Same user, different tab - treat as owner so both tabs can edit
{:owner, presences}
else
# Different user - spectator mode (FIFO locking)
{:spectator, owner_meta, presences}
end
end
end
@doc """
Gets all presences for a form, sorted by join time (FIFO).
Returns a list of tuples: `[{socket_id, metadata}, ...]`
"""
def get_sorted_presences(form_key) do
topic = editing_topic(form_key)
raw_presences = Presence.list(topic)
raw_presences
|> Enum.flat_map(fn {socket_id, %{metas: metas}} ->
# Filter out metas with dead PIDs
valid_metas =
Enum.filter(metas, fn meta ->
case Map.get(meta, :pid) do
pid when is_pid(pid) -> Process.alive?(pid)
# Keep metas without PID for backward compatibility
_ -> true
end
end)
# Take the first valid meta (most recent)
case valid_metas do
[meta | _] -> [{socket_id, meta}]
[] -> []
end
end)
|> Enum.sort_by(fn {_socket_id, meta} -> meta.joined_at end)
end
@doc """
Gets the lock owner's metadata, or nil if no one is editing.
"""
def get_lock_owner(form_key) do
case get_sorted_presences(form_key) do
[{_socket_id, meta} | _] -> meta
[] -> nil
end
end
@doc """
Gets all spectators (everyone except the first person).
Returns a list of metadata for spectators only.
"""
def get_spectators(form_key) do
case get_sorted_presences(form_key) do
[] -> []
[_owner | spectators] -> Enum.map(spectators, fn {_id, meta} -> meta end)
end
end
@doc """
Counts total number of people editing (owner + spectators).
"""
def count_editors(form_key) do
get_sorted_presences(form_key) |> length()
end
@doc """
Subscribes the current process to presence events for a form.
After subscribing, the process will receive:
- `%Phoenix.Socket.Broadcast{event: "presence_diff", ...}` when users join/leave
"""
def subscribe_to_editing(form_key) do
topic = editing_topic(form_key)
Phoenix.PubSub.subscribe(:phoenix_kit_internal_pubsub, topic)
end
@doc """
Generates the Presence topic name for a form.
## Examples
editing_topic("blog:my-post/en.phk")
# => "blog_edit:blog:my-post/en.phk"
"""
def editing_topic(form_key), do: "blog_edit:#{form_key}"
end