Design Patterns in Ruby: Singleton

The concept of the Singleton pattern is fairly straightforward: only a single instance of a class can exist. This can be useful when an application allows only one object to be instantiated for a given class.

Even though there are mixed feelings amongst developers about this pattern, it is often used in other languages, such as Java and C-based languages. In Ruby, the Singleton module in the standard library can be used to implement this pattern.

When to use Singleton?

Let’s say that we need to design a class to hold configuration data for our application, and there can only ever exist one instance of this configuration. Sure, we could simulate a Singleton by creating a module, but we would have to make sure that it could not be duplicated or cloned, otherwise it would lose its purpose.

Integration

The first step in creating a Singleton class is to require and include the Singleton module in a class:

require 'singleton'
 
class SomeClassName
  include Singleton
end

If you try to instantiate this class as you normally would a regular class, a NoMethodError exception is raised. The constructor is made private to prevent other instances from being accidentally created:

SomeClassName.new

#=>NoMethodError: private method 'new' called for AppConfig:Class

To access the instance of this class, we need to use the instance() method provided by the Singleton module. When this method is first called, an instance of the class is created, and all subsequent calls return the created instance.

a, b = SomeClassName.instance, SomeClassName.instance
a == b
#=>true

e have t

Let’s modify the SomeClassName class and add a few things to see how it works:

require 'singleton'
 
class SomeClassName
  include Singleton
  attr_accessor :var_name
   
  def some_function_name
   puts "Hello I am inside the method"
  end
end

Now let’s run these following lines:

SomeClassName.instance.var_name = {version: '1.0.1'}
#=>{version: '1.0.1'}
SomeClassName.instance.some_function_name
#=>Hello I am inside the method
 
second = SomeClassName.instance
second.var_name = {version: '1.0.2'}
#=>{version: '1.0.2'}
SomeClassName.instance.var_name
#=>{version: '1.0.2'}