ruby 49 lines · 9 steps

How a typing indicator works in Rails ActionCable

An ActionCable channel tracks who's typing using short-lived cache keys and periodic sweeps to broadcast live presence.

Explained by highlit
Rails Intermediate websockets presence cache-expiry pub-sub real-time
1class ChatChannel < ApplicationCable::Channel
2 periodically :sweep_stale_typists, every: 3.seconds
3 
4 def subscribed
5 @room = Room.find(params[:room_id])
6 stream_for @room
7 end
8 
9 def unsubscribed
10 return unless @room
11 
12 Rails.cache.delete(typing_key)
13 broadcast_typists
14 end
15 
16 def start_typing
17 Rails.cache.write(typing_key, current_user.name, expires_in: 5.seconds)
18 broadcast_typists
19 end
20 
21 def stop_typing
22 Rails.cache.delete(typing_key)
23 broadcast_typists
24 end
25 
26 private
27 
28 def sweep_stale_typists
29 broadcast_typists unless Rails.cache.exist?(typing_key)
30 end
31 
32 def broadcast_typists
33 names = active_typists.reject { |name| name == current_user.name }
34 
35 ChatChannel.broadcast_to @room, type: "typing", users: names
36 end
37 
38 def active_typists
39 Rails.cache.read_multi(*typing_keys_for_room).values.compact
40 end
41 
42 def typing_keys_for_room
43 @room.membership_user_ids.map { |id| "room:#{@room.id}:typing:#{id}" }
44 end
45 
46 def typing_key
47 "room:#{@room.id}:typing:#{current_user.id}"
48 end
49end
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Expiring cache keys give you self-cleaning presence state without a background job or database table.
  2. 2A periodic timer paired with a key existence check lets clients recover from missed stop events.
  3. 3Broadcasting the full recomputed list on every change keeps every subscriber's view consistent.

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