Very much not-production ready Event-bus based off `ActiveSupport::Instrumentation`.

dummy_event_bus.rb

require 'active_support'

class CustomEventBus
  def initialize()
    @subscribers = {}
  end

  def subscribe(pattern = nil, callback = nil, &block)
    puts "[#{self.class}] #subscribe TO: #{pattern.inspect}, with callback=#{callback.class.inspect}, block=#{block.class.inspect}"
    @subscribers[pattern] ||= callback || block
  end

  def monotonic_subscribe(pattern = nil, callback = nil, &block)
    subscribe(pattern, callback, &block)
  end

  def subscribed(callback, pattern = nil, monotonic: false, &block)
    puts "[#{self.class}] #subscribed TO: #{pattern.inspect}, with callback=#{callback.inspect}, block=#{block.inspect}, monotonic=#{monotonic.inspect}"
    subscriber = subscribe(pattern, callback)
    yield if block_given?
    unsubscribe(pattern)
  end

  def unsubscribe(subscriber_or_name)
    puts "[#{self.class}] #unsubscribed: #{subscriber_or_name.inspect}"
    @subscribers.find do
      _1 == subscriber_or_name || _2.is_a?(subscriber_or_name)
    end.tap { @subscribers.delete(_1) if _1 }
  end

  def publish(name, *args)
    puts "[#{self.class}] #publish: name=#{name.inspect} payload=#{args.inspect}"
    @subscribers[name]&.call(*args)
  end

  def publish_event(event)
    puts "[#{self.class}] #publish_event: name=#{event.name} payload=#{event.payload.inspect}"
    @subscribers[event.name]&.call(event)
  end

  def instrument(name, payload = {}, &block)
    puts "[#{self.class}] #instrument #{name.inspect} payload=#{payload.inspect} block=#{block.inspect}"

    start(name, 'id', payload)
    @event.record { yield payload if block_given? }
  rescue => e
    payload[:exception] = [e.class.name, e.message]
    payload[:exception_object] = e
  ensure
    finish(name, 'id', payload)
  end

  def start(name, id, payload)
    puts "[#{self.class}] #start #{name.inspect} id=#{id.inspect} payload=#{payload.inspect}"
    @event = ActiveSupport::Notifications::Event.new(name, nil, nil, id, payload)
  end

  def finish(name, id, payload, listeners = nil)
    puts "[#{self.class}] #finish #{name.inspect} id=#{id.inspect} payload=#{payload.inspect} listeners=#{listeners.inspect}"
    @event.payload = payload
    publish_event(@event)
  end
end

main_app.rb

def main
  # event_bus = ActiveSupport::Notifications 
  event_bus = CustomEventBus.new
  subscriber = MyLogSubscriber.new()
  subscriber.class.attach_to(:sample, subscriber)

  event_bus.instrument('app_event.sample', {data: :initial}) do |payload|
    puts 'My code started working'
    payload[:extra] = :info
    puts 'My code provides an extra piece of context to consumers'

    puts 'My code allocates something'
    event_bus.instrument('checkpoint_1.sample', {checkpoint_first: :data})
    alloc = Hash.new; # let's make an allocation just for fun
    puts 'My starts doing some heavy work here'
    sleep 0.1
    event_bus.instrument('checkpoint_2.sample', {checkpoint_first: :data})
    puts 'My starts doing more heavy work here'
    sleep 0.2
    puts 'Finish!'
  end

  puts ""
  puts "=== Now let's simulate an error ==="
  puts ""

  event_bus.instrument('app_event.sample', {data: :initial}) do |payload|
    puts 'My code started working'
    payload[:extra] = :info
    puts 'My code provides an extra piece of context to consumers'

    puts 'My code allocates something'
    event_bus.instrument('checkpoint_1.sample', {checkpoint_first: :data})
    alloc = Hash.new; # let's make an allocation just for fun
    puts 'My starts doing some heavy work here'
    sleep 0.1
    event_bus.instrument('checkpoint_2.sample', {checkpoint_first: :data})
    puts 'My starts doing more heavy work here'
    sleep 0.2
    puts 'Finish!'

    raise 'Finish with error!'
  end
end

puts "=== BEGIN ==="

main() rescue nil

puts "=== END ==="

my_log_subscriber.rb

class MyLogSubscriber < ActiveSupport::LogSubscriber
  def logger
    @logger ||= Logger.new($stdout).tap do |logger|
      logger.formatter = proc do |severity, datetime, progname, msg|
        "#{datetime} [#{severity}]: #{msg}\n"
      end
      logger.level = Logger::DEBUG
      logger
    end
  end
  def app_event(event)
    info "  Example Application Event (#{event.duration.round(2)}ms) - payload=#{event.payload.inspect} allocations=#{event.allocations}"
  end

  def checkpoint_1(event)
    info "  Checkpoint the first! (#{event.duration.round(2)}ms) - payload=#{event.payload.inspect}"
  end

  def checkpoint_2(event)
    info "  Checkpoint the second! (#{event.duration.round(2)}ms) - payload=#{event.payload.inspect}"
  end
end