React Context pattern in Rails

I've recently found myself passing a lot of parameters down from controllers to service objects and then to jobs, etc.

This was one of the problems that were solved by the context pattern in React so I tried to do the same in the Rails app that I've been working on.

I had seen something a bit similar to in the I18n.with_locale function.

So I wrote this:

# frozen_string_literal: true

require "concurrent-ruby"

class RequestValueContext
  class << self
    # For the multi threaded environment
    @@request_value = Concurrent::ThreadLocalVar.new

    def with(request_value)
      if get.present?
        raise ContextAlreadyDefinedError,
          "Context already defined!"
      end

      begin
        @@request_value.value = request_value
        yield
      ensure
        @@request_value.value = nil
      end
    end

    def get
      @@request_value.value
    end
  end

  ContextAlreadyDefinedError = Class.new(StandardError)
end

And in the ApplicationController I've added this:

class ApplicationController < ActionController::Base
  around_action :with_context

  def with_context
    RequestValueContext.with("foo") do
      yield
    end
  end
end

Then I can access the value using RequestValueContext.get from any method that is called "within the controller stack".

A nice feature of this pattern is that the current context can be captured when the using ActiveJob::Callbacks.before_enqueue and then provided by ActiveJob::Callbacks.around_perform like so:

# frozen_string_literal: true

module WithContexts
  extend ActiveSupport::Concern

  REQUEST_VALUE_KEY = "request_value"

  included do
    attr_reader :request_value, :deserialize_called

    before_enqueue :capture_context
    around_perform :provide_context
  end

  def serialize
    super.merge(REQUEST_VALUE_KEY => request_value)
  end

  def deserialize(job_data)
    # "detects" when a job is called by *perform_now*
    @deserialize_called = true

    super

    @doorkeeper_application = request_value
  end

  def capture_context
    @doorkeeper_application = RequestValueContext.get
  end

  def provide_context
    if job_called_by_perform_now?
      # if the job is called by *perform_now* it will be executed inline
      # with the current context
      yield
    else
      RequestValueContext.with_application(request_value) do
        yield
      end
    end
  end

  def job_called_by_perform_now?
    !deserialize_called
  end
end

I believe something similar could be done for Proc/Block/Lambda.

I started writing Ruby less than a year ago and I found it to be quite a tricky language so if you have any feedback please let me know.

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React Context pattern in Rails

Vinícius Manjabosco

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