Active Support Core Extension (2)

Active Support
Core Extensions (2)
ROR lab. DD-1
- The 2nd round -
April 13, 2013
Hyoseong Choi

Ext. to Module
• alias_method_chain
active_support/core_ext/module/
aliasing.rb
• “alias chaining”? ➧ wrapping
http://erniemiller.org/2011/02/03/when-to-use-alias_method_chain/
ActionController::TestCase.class_eval do
  def process_with_stringified_params(...)
    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
    process_without_stringified_params(action,
params, session, flash, http_method)
  end
  alias_method_chain :process, :stringified_params
end

Ext. to Module
• alias_attribute
aliasing.rb
class User < ActiveRecord::Base
  # let me refer to the email column as "login",
  # possibly meaningful for authentication code
  alias_attribute :login, :email
end

Ext. to Module
• attr_accessor_with_default
attr_accessor_with_default.rb
class Url
  attr_accessor_with_default :port, 80
end

Url.new.port # => 80
class User
  attr_accessor :name, :surname
  attr_accessor_with_default(:full_name) do
    [name, surname].compact.join(" ")
  end
end

u = User.new
u.name = 'Xavier'
u.surname = 'Noria'
u.full_name # => "Xavier Noria"
not
cached

Ext. to Module
• attr_internal or attr_internal_accessor
attr_internal.rb
# library
class ThirdPartyLibrary::Crawler
attr_internal :log_level
end

# client code
class MyCrawler < ThirdPartyLibrary::Crawler
attr_accessor :log_level
end
@_log_level
Module.attr_internal_naming_format = “@_%s”
sprintf-like format

Ext. to Module
• attr_internal or attr_internal_accessor
attr_internal.rb
module ActionView
  class Base
    attr_internal :captures
    attr_internal :request, :layout
    attr_internal :controller, :template
  end
end

Ext. to Module
• mattr_accessor
attr_accessors.rb
module ActiveSupport
  module Dependencies
    mattr_accessor :warnings_on_first_load
    mattr_accessor :history
    mattr_accessor :loaded
    mattr_accessor :mechanism
    mattr_accessor :load_paths
    mattr_accessor :load_once_paths
    mattr_accessor :autoloaded_constants
    mattr_accessor :explicitly_unloadable_constants
    mattr_accessor :logger
    mattr_accessor :log_activity
    mattr_accessor :constant_watch_stack
    mattr_accessor :constant_watch_stack_mutex
  end
end
• cattr_accessor

Ext. to Module
• Parents
introspection.rb
•parent
•parent_name
•parents

Ext. to Module
• Parents
introspection.rb
module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parent # => X::Y
M.parent       # => X::Y
• parent

Ext. to Module
• Parents
introspection.rb
• parent_name
module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parent_name # => "X::Y"
M.parent_name       # => "X::Y"

Ext. to Module
• Parents
introspection.rb
• parents
module X
  module Y
    module Z
    end
  end
end
M = X::Y::Z

X::Y::Z.parents # => [X::Y, X, Object]
M.parents       # => [X::Y, X, Object]

Ext. to Module
• local_constants
module X
  X1 = 1
  X2 = 2
  module Y
    Y1 = :y1
    X1 = :overrides_X1_above
  end
end

X.local_constants    # => ["X2", "X1", "Y"], assumes Ruby 1.8
X::Y.local_constants # => ["X1", "Y1"], assumes Ruby 1.8
introspection.rb
• local_constant_names ⟶ always, strings
as symbols in Ruby 1.9
deprecated!!!

Ext. to Module
• const_deﬁned? /_get /_set
Math.const_defined? "PI" #=> true
IO.const_defined? :SYNC #=> true
IO.const_defined? :SYNC, false #=> false
introspection.rb
Math.const_get(:PI) #=> 3.14159265358979
Math.const_set("HIGH_SCHOOL_PI", 22.0/7.0)
#=> 3.14285714285714
Math::HIGH_SCHOOL_PI - Math::PI
#=> 0.00126448926734968
standard ruby methods
bare constant names

Ext. to Module
• const_deﬁned? /_get /_set
module Foo
class Bar
VAL = 10
end
class Baz < Bar; end
end
Object.const_get 'Foo::Baz::VAL' # => 10
Object.const_get 'Foo::Baz::VAL', false # => NameError
introspection.rb
standard ruby methods
bare constant names
flag for inherit

Ext. to Module
• qualiﬁed_const_deﬁned? /_get /_set
module M
  X = 1
end

module N
  class C
    include M
  end
end
qualified_const.rb
new extended methods
N.qualified_const_defined?("C::X", false)
N.qualified_const_defined?("C::X", true)
N.qualified_const_defined?("C::X")
relative to their receiver
qualified constant names

Ext. to Module
• synchronize
class Counter
  @@mutex = Mutex.new
  attr_reader :value

  def initialize
    @value = 0
  end

  def incr
    @value += 1 # non-atomic
  end
  synchronize :incr, :with => '@@mutex'
end
synchronization.rb
DEPRECATION WARNING: synchronize
is deprecated and will be
removed from Rails 4.0.

Ext. to Module
• reachable?
module M
end

M.reachable? # => true
orphan = Object.send(:remove_const, :M)

# The module object is orphan now but it still has a name.
orphan.name # => "M"

# You cannot reach it via the constant M because it does not even exist.
orphan.reachable? # => false

# Let's define a module called "M" again.
module M
end

# The constant M exists now again, and it stores a module
# object called "M", but it is a new instance.
orphan.reachable? # => false
reachable.rb
The constant M exists now,
and it stores a module object called "M"

Ext. to Module
• anonymous?
module M
end
M.name # => "M"

N = Module.new
N.name # => "N"

Module.new.name # => "" in 1.8, nil in 1.9
M.anonymous? # => false

Module.new.anonymous? # => true
m = Object.send(:remove_const, :M)

m.reachable? # => false
m.anonymous? # => false
anonymous.rb

Ext. to Module
• delegate
  has_one :profile
end
  has_one :profile

  def name
    profile.name
  end
end
  has_one :profile

  delegate :name, :to => :profile
end
delegate :name, :age, :address, :twitter, :to => :profile
delegate :name, :to => :profile, :allow_nil => true
delegate :street, :to => :address, :prefix => true
delegate :size, :to => :attachment, :prefix => :avatar
delegation.rb

Ext. to Module
• redeﬁne_method
redefine_method("#{reflection.name}=") do |new_value|
  association = association_instance_get(reflection.name)

  if association.nil? || association.target != new_value
    association = association_proxy_class.new(self, reflection)
  end

  association.replace(new_value)
  association_instance_set(reflection.name, new_value.nil? ?
nil : association)
end
remove_method.rb

Active Support Core Extension (2)

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