[Rails] Ruby Method

Three different methods

1. Instance methods (accessible for only that instance)

2. Class methods (accessible for all instances)

3. Singleton methods (only that object has that method)

class Foo  
  def an_instance_method  
    puts "I am an instance method"  
  end  
  def self.a_class_method  
    puts "I am a class method"  
  end  
end

foo = Foo.new
def foo.a_singleton_method
  puts "I am a singleton method"
end

Singleton Method

What are they?

• Singleton class: a class that only has one instance

• Singleton method: a method that can be defined for a specific object only

Why do we need them?

Example from geeksforgeeks

# Ruby program to demonstrate the use 
# of singleton methods
class Vehicle
  def wheels
    puts "There are many wheels"
  end
end
  
# Object train
train = Vehicle.new 
  
# Object car
car = Vehicle.new   
  
# Singleton method for car object
def car.wheels   
  puts "There are four wheels"
end
  
puts "Singleton Method Example"
puts "Invoke from train object:"
train.wheels         
puts "Invoke from car object:"
car.wheels

# Singleton Method Example
# Invoke from train object:
# There are many wheels -> can not access car.wheels
# Invoke from car object:
# There are four wheels

“The real problem with Singletons is that they give you such a good excuse not to think carefully about the appropriate visibility of an object.” – Kent Beck

How to use them?

• Ruby has Singleton module

class Object
	include Singleton
	
	def singleton_method
	end
end

# create an object
Object.new

# access that instance
Object.instance
Object.instance.object_id
Object.instance.singleton_method

Another way to create singleton method

foo = Foo.new

class << foo
  def a_singleton_method
    puts "I am a singleton method"
  end
end

A class method is also a singleton method for itself

class Foo
  class << self
    def a_singleton_and_class_method
      puts "I am a singleton method for self and a class method for Foo"
    end
  end
end

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