ruby - create singleton with parameters?

Question

I've seen how to define a class as being a singleton (how to create a singleton in Ruby):

require 'singleton'
 
class Example
  include Singleton
end

But what if I want to pass some parameters to #new when initializing that single instance? Example should always have certain properties initialized. For example, say I had a class whose sole purpose is to log to a file but it requires a name of a file to log to before it can work.

class MyLogger
  def initialize(file_name)
    @file_name = file_name
  end
end

How can I make MyLogger a singleton but make sure it gets a file_name?

Answer 1

Here's another way to do it -- put the log file name in a class variable:

require 'singleton'
class MyLogger
  include Singleton
  @@file_name = ""
  def self.file_name= fn
    @@file_name = fn
  end
  def initialize
    @file_name = @@file_name
  end
end

Now you can use it this way:

MyLogger.file_name = "path/to/log/file"
log = MyLogger.instance  # => #<MyLogger:0x000.... @file_name="path/to/log/file">

Subsequent calls to instance will return the same object with the path name unchanged, even if you later change the value of the class variable. A nice further touch would be to use another class variable to keep track of whether an instance has already been created, and have the file_name= method raise an exception in that case. You could also have initialize raise an exception if @@file_name has not yet been set.

Answer 2

Singleton does not provide this functionality, but instead of using singleton you could write it by yourself

class MyLogger
  @@singleton__instance__ = nil
  @@singleton__mutex__    = Mutex.new

  def self.instance(file_name)
    return @@singleton__instance__ if @@singleton__instance__

    @@singleton__mutex__.synchronize do
      return @@singleton__instance__ if @@singleton__instance__

      @@singleton__instance__ = new(file_name)
    end
    @@singleton__instance__
  end

  private

  def initialize(file_name)
    @file_name = file_name
  end
  private_class_method :new
end

It should work, but I did not tested the code.

This code forces you to use MyLogger.instance <file_name> or at least at the first call if you know it will be first time calling.

Answer 3

Here is an approach I used to solve a similar problem, which I wanted to share in case you or other people find it suitable:

require 'singleton'

class Logger
  attr_reader :file_name

  def initialize file_name
    @file_name = file_name
  end
end


class MyLogger < Logger
  include Singleton

  def self.new
    super "path/to/file.log"
  end

  # You want to make {.new} private to maintain the {Singleton} approach;
  # otherwise other instances of {MyLogger} can be easily constructed.
  private_class_method :new
end

p MyLogger.instance.file_name
# => "path/to/file.log"

MyLogger.new "some/other/path"
# => ...private method `new' called for MyLogger:Class (NoMethodError)

I've tested the code in 2.3, 2.4 and 2.5; earlier versions may of course exhibit divergent behavior.

This allows you to have a general parametrized Logger class, which can be used to create additional instances for testing or future alternative configurations, while defining MyLogger as a single instance of it following to Ruby's standardized Singleton pattern. You can split instance methods across them as you find appropriate.

Ruby's Singleton constructs the instance automatically when first needed, so the Logger#initialize parameters must be available on-demand in MyLogger.new, but you can of course pull the values from the environment or set them up as MyLogger class instance variables during configuration before the singleton instance is ever used, which is consistent with the singleton instance being effectively global.