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
C
F
L
S
Constants
BLACK = "\e[30m"
 

Colors
BLUE = "\e[34m"
 
BOLD = "\e[1m"
 
CLEAR = "\e[0m"
 

Embed in a String to clear all previous ANSI sequences.
CYAN = "\e[36m"
 
GREEN = "\e[32m"
 
MAGENTA = "\e[35m"
 
RED = "\e[31m"
 
WHITE = "\e[37m"
 
YELLOW = "\e[33m"
 
Attributes
[W] logger
Class Public methods
flush_all!()

Flush all ::log_subscribers' logger.

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 70
def flush_all!
  logger.flush if logger.respond_to?(:flush)
end
log_subscribers()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 65
def log_subscribers
  subscribers
end
logger()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 57
def logger
  @logger ||= if defined?(Rails) && Rails.respond_to?(:logger)
    Rails.logger
  end
end
Instance Public methods
finish(name, id, payload)

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 83
def finish(name, id, payload)
  super if logger
rescue => e
  if logger
    logger.error "Could not log #{name.inspect} event. #{e.class}: #{e.message} #{e.backtrace}"
  end
end
logger()

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 75
def logger
  LogSubscriber.logger
end
start(name, id, payload)

Source: show | on GitHub 

# File activesupport/lib/active_support/log_subscriber.rb, line 79
def start(name, id, payload)
  super if logger
end
Instance Private methods
color(text, color, bold = false)

Set color by using a symbol 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 105
def color(text, color, bold = false) # :doc:
  return text unless colorize_logging
  color = self.class.const_get(color.upcase) if color.is_a?(Symbol)
  bold  = bold ? BOLD : ""
  "#{bold}#{color}#{text}#{CLEAR}"
end