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# frozen_string_literal: true
require "set"
require "active_support/core_ext/regexp"
class Module
# Error generated by +delegate+ when a method is called on +nil+ and +allow_nil+
# option is not used.
class DelegationError < NoMethodError; end
RUBY_RESERVED_KEYWORDS = %w(alias and BEGIN begin break case class def defined? do
else elsif END end ensure false for if in module next nil not or redo rescue retry
return self super then true undef unless until when while yield)
DELEGATION_RESERVED_KEYWORDS = %w(_ arg args block)
DELEGATION_RESERVED_METHOD_NAMES = Set.new(
RUBY_RESERVED_KEYWORDS + DELEGATION_RESERVED_KEYWORDS
).freeze
# Provides a +delegate+ class method to easily expose contained objects'
# public methods as your own.
#
# ==== Options
# * <tt>:to</tt> - Specifies the target object
# * <tt>:prefix</tt> - Prefixes the new method with the target name or a custom prefix
# * <tt>:allow_nil</tt> - If set to true, prevents a +Module::DelegationError+
# from being raised
# * <tt>:private</tt> - If set to true, changes method visibility to private
#
# The macro receives one or more method names (specified as symbols or
# strings) and the name of the target object via the <tt>:to</tt> option
# (also a symbol or string).
#
# Delegation is particularly useful with Active Record associations:
#
# class Greeter < ActiveRecord::Base
# def hello
# 'hello'
# end
#
# def goodbye
# 'goodbye'
# end
# end
#
# class Foo < ActiveRecord::Base
# belongs_to :greeter
# delegate :hello, to: :greeter
# end
#
# Foo.new.hello # => "hello"
# Foo.new.goodbye # => NoMethodError: undefined method `goodbye' for #<Foo:0x1af30c>
#
# Multiple delegates to the same target are allowed:
#
# class Foo < ActiveRecord::Base
# belongs_to :greeter
# delegate :hello, :goodbye, to: :greeter
# end
#
# Foo.new.goodbye # => "goodbye"
#
# Methods can be delegated to instance variables, class variables, or constants
# by providing them as a symbols:
#
# class Foo
# CONSTANT_ARRAY = [0,1,2,3]
# @@class_array = [4,5,6,7]
#
# def initialize
# @instance_array = [8,9,10,11]
# end
# delegate :sum, to: :CONSTANT_ARRAY
# delegate :min, to: :@@class_array
# delegate :max, to: :@instance_array
# end
#
# Foo.new.sum # => 6
# Foo.new.min # => 4
# Foo.new.max # => 11
#
# It's also possible to delegate a method to the class by using +:class+:
#
# class Foo
# def self.hello
# "world"
# end
#
# delegate :hello, to: :class
# end
#
# Foo.new.hello # => "world"
#
# Delegates can optionally be prefixed using the <tt>:prefix</tt> option. If the value
# is <tt>true</tt>, the delegate methods are prefixed with the name of the object being
# delegated to.
#
# Person = Struct.new(:name, :address)
#
# class Invoice < Struct.new(:client)
# delegate :name, :address, to: :client, prefix: true
# end
#
# john_doe = Person.new('John Doe', 'Vimmersvej 13')
# invoice = Invoice.new(john_doe)
# invoice.client_name # => "John Doe"
# invoice.client_address # => "Vimmersvej 13"
#
# It is also possible to supply a custom prefix.
#
# class Invoice < Struct.new(:client)
# delegate :name, :address, to: :client, prefix: :customer
# end
#
# invoice = Invoice.new(john_doe)
# invoice.customer_name # => 'John Doe'
# invoice.customer_address # => 'Vimmersvej 13'
#
# The delegated methods are public by default.
# Pass <tt>private: true</tt> to change that.
#
# class User < ActiveRecord::Base
# has_one :profile
# delegate :first_name, to: :profile
# delegate :date_of_birth, to: :profile, private: true
#
# def age
# Date.today.year - date_of_birth.year
# end
# end
#
# User.new.first_name # => "Tomas"
# User.new.date_of_birth # => NoMethodError: private method `date_of_birth' called for #<User:0x00000008221340>
# User.new.age # => 2
#
# If the target is +nil+ and does not respond to the delegated method a
# +Module::DelegationError+ is raised. If you wish to instead return +nil+,
# use the <tt>:allow_nil</tt> option.
#
# class User < ActiveRecord::Base
# has_one :profile
# delegate :age, to: :profile
# end
#
# User.new.age
# # => Module::DelegationError: User#age delegated to profile.age, but profile is nil
#
# But if not having a profile yet is fine and should not be an error
# condition:
#
# class User < ActiveRecord::Base
# has_one :profile
# delegate :age, to: :profile, allow_nil: true
# end
#
# User.new.age # nil
#
# Note that if the target is not +nil+ then the call is attempted regardless of the
# <tt>:allow_nil</tt> option, and thus an exception is still raised if said object
# does not respond to the method:
#
# class Foo
# def initialize(bar)
# @bar = bar
# end
#
# delegate :name, to: :@bar, allow_nil: true
# end
#
# Foo.new("Bar").name # raises NoMethodError: undefined method `name'
#
# The target method must be public, otherwise it will raise +NoMethodError+.
def delegate(*methods, to: nil, prefix: nil, allow_nil: nil, private: nil)
unless to
raise ArgumentError, "Delegation needs a target. Supply an options hash with a :to key as the last argument (e.g. delegate :hello, to: :greeter)."
end
if prefix == true && /^[^a-z_]/.match?(to)
raise ArgumentError, "Can only automatically set the delegation prefix when delegating to a method."
end
method_prefix = \
if prefix
"#{prefix == true ? to : prefix}_"
else
""
end
location = caller_locations(1, 1).first
file, line = location.path, location.lineno
to = to.to_s
to = "self.#{to}" if DELEGATION_RESERVED_METHOD_NAMES.include?(to)
method_names = methods.map do |method|
# Attribute writer methods only accept one argument. Makes sure []=
# methods still accept two arguments.
definition = /[^\]]=$/.match?(method) ? "arg" : "*args, &block"
# The following generated method calls the target exactly once, storing
# the returned value in a dummy variable.
#
# Reason is twofold: On one hand doing less calls is in general better.
# On the other hand it could be that the target has side-effects,
# whereas conceptually, from the user point of view, the delegator should
# be doing one call.
if allow_nil
method_def = [
"def #{method_prefix}#{method}(#{definition})",
"_ = #{to}",
"if !_.nil? || nil.respond_to?(:#{method})",
" _.#{method}(#{definition})",
"end",
"end"
].join ";"
else
exception = %(raise DelegationError, "#{self}##{method_prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")
method_def = [
"def #{method_prefix}#{method}(#{definition})",
" _ = #{to}",
" _.#{method}(#{definition})",
"rescue NoMethodError => e",
" if _.nil? && e.name == :#{method}",
" #{exception}",
" else",
" raise",
" end",
"end"
].join ";"
end
module_eval(method_def, file, line)
end
private(*method_names) if private
method_names
end
# When building decorators, a common pattern may emerge:
#
# class Partition
# def initialize(event)
# @event = event
# end
#
# def person
# @event.detail.person || @event.creator
# end
#
# private
# def respond_to_missing?(name, include_private = false)
# @event.respond_to?(name, include_private)
# end
#
# def method_missing(method, *args, &block)
# @event.send(method, *args, &block)
# end
# end
#
# With <tt>Module#delegate_missing_to</tt>, the above is condensed to:
#
# class Partition
# delegate_missing_to :@event
#
# def initialize(event)
# @event = event
# end
#
# def person
# @event.detail.person || @event.creator
# end
# end
#
# The target can be anything callable within the object, e.g. instance
# variables, methods, constants, etc.
#
# The delegated method must be public on the target, otherwise it will
# raise +NoMethodError+.
def delegate_missing_to(target)
target = target.to_s
target = "self.#{target}" if DELEGATION_RESERVED_METHOD_NAMES.include?(target)
module_eval <<-RUBY, __FILE__, __LINE__ + 1
def respond_to_missing?(name, include_private = false)
# It may look like an oversight, but we deliberately do not pass
# +include_private+, because they do not get delegated.
#{target}.respond_to?(name) || super
end
def method_missing(method, *args, &block)
if #{target}.respond_to?(method)
#{target}.public_send(method, *args, &block)
else
begin
super
rescue NoMethodError
if #{target}.nil?
raise DelegationError, "\#{method} delegated to #{target}, but #{target} is nil"
else
raise
end
end
end
end
RUBY
end
end