[Cable] Support the stream_for and broadcast_to methods

[rsamoilov]

Description

Currently, Rage only supports creating streams using the stream_from method. Implementing stream_for would allow users to create streams based on the (channel, object) pair.

Example:

class CommentsChannel < Rage::Cable::Channel
  def subscribed
    user = User.find(params[:id])
    stream_for user
  end
end

The following code will create a stream with the name based on the name of the channel and the object identifier.

Later, broadcast_to can be used to post messages to this stream:

CommentsChannel.broadcast_to(user, { comment: { body: "" } })

How to build object identifiers

To automatically create a stream based on the (channel, object) pair, we need to generate an identifier for object. The process of creating object identifiers should be performant and consistent across different servers (which is why we can’t use object_id). Additionally, stream_for should accept any Ruby objects, not just ActiveRecord models.

Using #id for ActiveRecord models and #as_json.to_s for non-ActiveRecord objects should satisfy the requirements, but I’m also open to other suggestions.

Tips

• We want to make sure the object identifier is never nil as this is unlikely something users will ever do intentionally.
• See how subscriptions are currently implemented in lib/rage/cable/channel.rb and lib/rage/cable/protocol/actioncable_v1_json.rb.
• Check the overview doc that shows how Rage's core components interact with each other.
• Feel free to ask any questions or request help in the comments below!