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.

Methods
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C
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Classes and Modules
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
59:       def attach_to(namespace, log_subscriber=new, notifier=ActiveSupport::Notifications)
60:         log_subscribers << log_subscriber
61:         @@flushable_loggers = nil
62: 
63:         log_subscriber.public_methods(false).each do |event|
64:           next if 'call' == event.to_s
65: 
66:           notifier.subscribe("#{event}.#{namespace}", log_subscriber)
67:         end
68:       end
flush_all!()

Flush all log_subscribers’ logger.

Source: show | on GitHub 

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

Source: show | on GitHub 

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

Source: show | on GitHub 

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

Source: show | on GitHub 

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

Source: show | on GitHub 

    # File activesupport/lib/active_support/log_subscriber.rb, line 88
88:     def call(message, *args)
89:       return unless logger
90: 
91:       method = message.split('.').first
92:       begin
93:         send(method, ActiveSupport::Notifications::Event.new(message, *args))
94:       rescue Exception => e
95:         logger.error "Could not log #{message.inspect} event. #{e.class}: #{e.message}"
96:       end
97:     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 116
116:     def color(text, color, bold=false)
117:       return text unless colorize_logging
118:       color = self.class.const_get(color.to_s.upcase) if color.is_a?(Symbol)
119:       bold  = bold ? BOLD : ""
120:       "#{bold}#{color}#{text}#{CLEAR}"
121:     end