Challenge 1: A Notifications Channel — Solution

# app/channels/notifications_channel.rb
class NotificationsChannel < ApplicationCable::Channel
  def subscribed
    stream_for current_user
  end
end

=begin
Notes:
- current_user is available here because Connection#connect (from the
  chapter's example) set self.current_user during the WebSocket handshake,
  and identified_by :current_user makes it accessible to every channel
  built on that connection -- no params lookup needed, unlike
  CommentsChannel's post_id.
- stream_for current_user derives a stream identifier from the user object
  itself, so each user's browser tab(s) join a stream unique to them --
  two different logged-in users subscribing to NotificationsChannel end up
  on two separate streams automatically.
- Because the stream is tied to the authenticated user rather than a
  client-supplied parameter, there's no way for a client to subscribe to
  another user's notification stream by passing a different ID -- the
  scoping is enforced server-side, from data already verified during
  connect.
=end