ActiveSupport::LogSubscriber is an object set to consume ActiveSupport::Notifications with the sole purpose of logging them. The log subscriber dispatches notifications to a registered object based on its given namespace.

An example would be Active Record log subscriber responsible for logging queries:

module ActiveRecord
  class LogSubscriber < ActiveSupport::LogSubscriber
    def sql(event)
      "#{event.payload[:name]} (#{event.duration}) #{event.payload[:sql]}"
    end
  end
end

And it's finally registered as:

ActiveRecord::LogSubscriber.attach_to :active_record

Since we need to know all instance methods before attaching the log subscriber, the line above should be called after your ActiveRecord::LogSubscriber definition.

After configured, whenever a “sql.active_record” notification is published, it will properly dispatch the event (ActiveSupport::Notifications::Event) to the sql method.

Log subscriber also has some helpers to deal with logging and automatically flushes all logs when the request finishes (via action_dispatch.callback notification) in a Rails environment.

Namespace
Methods
A
C
F
L
Constants
CLEAR = "\e[0m"
 

Embed in a String to clear all previous ANSI sequences.
BOLD = "\e[1m"
 
BLACK = "\e[30m"
 

Colors
RED = "\e[31m"
 
GREEN = "\e[32m"
 
YELLOW = "\e[33m"
 
BLUE = "\e[34m"
 
MAGENTA = "\e[35m"
 
CYAN = "\e[36m"
 
WHITE = "\e[37m"
 
Class Public methods
attach_to(namespace, log_subscriber=new, notifier=ActiveSupport::Notifications)

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 59
def attach_to(namespace, log_subscriber=new, notifier=ActiveSupport::Notifications)
  log_subscribers << log_subscriber
  @@flushable_loggers = nil

  log_subscriber.public_methods(false).each do |event|
    next if 'call' == event.to_s

    notifier.subscribe("#{event}.#{namespace}", log_subscriber)
  end
end
flush_all!()

Flush all ::log_subscribers' logger.

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 83
def flush_all!
  flushable_loggers.each { |log| log.flush }
end
flushable_loggers()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 74
def flushable_loggers
  @@flushable_loggers ||= begin
    loggers = log_subscribers.map(&:logger)
    loggers.uniq!
    loggers.select { |l| l.respond_to?(:flush) }
  end
end
log_subscribers()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 70
def log_subscribers
  @@log_subscribers ||= []
end
logger()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 55
def logger
  @logger ||= Rails.logger if defined?(Rails)
end
Instance Public methods
call(message, *args)

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 88
def call(message, *args)
  return unless logger

  method = message.split('.').first
  begin
    send(method, ActiveSupport::Notifications::Event.new(message, *args))
  rescue Exception => e
    logger.error "Could not log #{message.inspect} event. #{e.class}: #{e.message}"
  end
end
Instance Protected methods
color(text, color, bold=false)

Set color by using a string or one of the defined constants. If a third option is set to true, it also adds bold to the string. This is based on the Highline implementation and will automatically append CLEAR to the end of the returned String.

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 115
def color(text, color, bold=false)
  return text unless colorize_logging
  color = self.class.const_get(color.to_s.upcase) if color.is_a?(Symbol)
  bold  = bold ? BOLD : ""
  "#{bold}#{color}#{text}#{CLEAR}"
end