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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