Ruby allows defining methods for a single instance using its singleton class. This allows overriding existing methods or defining new ones for that specific instance. Everything in Ruby is an object, including classes, and singleton classes are used to define per-instance behavior. Singleton classes can be used in testing to mock or stub specific instances.

1. Ruby’s Singleton Class

2. You can define methods for a single instance
a = Array.new
b = Array.new
def a.hello
puts "Hello Devclub!"
end
a.hello
# => "Hello Devclub!"
b.hello
# => NoMethodError: undefined method 'hello' for []:Array

3. Or override a method on a single instance
class Dog
def speak
puts "bark"
end
end
muki = Dog.new
def muki.speak
puts "auh"
end
muki.speak
# => "auh"
puff = Dog.new
puff.speak
# => "bark"

4. Enters the Singleton class
class
puff Dog
class muki super super
muki Dog
Singleton class
muki.singleton_methods
# => ["speak"]
puff.singleton_methods
# => []

5. class A
def self.hello
puts "Hello from class method!"
end
end
A.hello
# => "Hello from class method!"
A.singleton_methods
# => ["hello"]

6. A.class
# => Class
A.superclass
# => Object
class
A Class
super
A Object
method dispatch
A 'A Object

7. a_meta = class << A; self; end;
puts a_meta
# => #<Class:A>
meta_instance = a_meta.new
# => TypeError: can't create instance of virtual
class

8. Everything in Ruby is an object
1.class
# => Fixnum
nil.class
# => NilClass
NilClass.class
# => Class
Class.class
# => Class
class A
puts self.class
end
# => Class

9. Practical uses
class ApiClient
attr_accessor :session
def get_session
# http request
end
def user
session.user
end
end

10. Practical uses
class ApiClientTest < Test::Unit::TestCase
def setup
@api_client = ApiClient.new
def @api_client.get_session
@api_client.session = Session.new(:user => "Mati")
end
end
def test_user_mati
@api_client.get_session
assert(@api_client.user == 'Mati')
end
def test_user_nil
assert(@api_client.user.nil?)
end
end

11. Questions?

12. ESTYLE comp
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