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class Module
# Provides a delegate class method to easily expose contained objects' public methods
# as your own. Pass one or more methods (specified as symbols or strings)
# and the name of the target object via the <tt>:to</tt> option (also a symbol
# or string). At least one method and the <tt>:to</tt> option are required.
#
# 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'
#
# If the delegate object is +nil+ an exception is raised, and that happens
# no matter whether +nil+ responds to the delegated method. You can get a
# +nil+ instead with the +:allow_nil+ option.
#
# class Foo
# attr_accessor :bar
# def initialize(bar = nil)
# @bar = bar
# end
# delegate :zoo, to: :bar
# end
#
# Foo.new.zoo # raises NoMethodError exception (you called nil.zoo)
#
# class Foo
# attr_accessor :bar
# def initialize(bar = nil)
# @bar = bar
# end
# delegate :zoo, to: :bar, allow_nil: true
# end
#
# Foo.new.zoo # returns nil
def delegate(*methods)
options = methods.pop
unless options.is_a?(Hash) && to = options[: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
prefix, allow_nil = options.values_at(:prefix, :allow_nil)
if prefix == true && to =~ /^[^a-z_]/
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
file, line = caller.first.split(':', 2)
line = line.to_i
to = to.to_s
to = 'self.class' if to == 'class'
methods.each do |method|
# Attribute writer methods only accept one argument. Makes sure []=
# methods still accept two arguments.
definition = (method =~ /[^\]]=$/) ? 'arg' : '*args, &block'
if allow_nil
module_eval(<<-EOS, file, line - 2)
def #{method_prefix}#{method}(#{definition}) # def customer_name(*args, &block)
if #{to} || #{to}.respond_to?(:#{method}) # if client || client.respond_to?(:name)
#{to}.#{method}(#{definition}) # client.name(*args, &block)
end # end
end # end
EOS
else
exception = %(raise "#{self}##{method_prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")
module_eval(<<-EOS, file, line - 1)
def #{method_prefix}#{method}(#{definition}) # def customer_name(*args, &block)
#{to}.#{method}(#{definition}) # client.name(*args, &block)
rescue NoMethodError # rescue NoMethodError
if #{to}.nil? # if client.nil?
#{exception} # # add helpful message to the exception
else # else
raise # raise
end # end
end # end
EOS
end
end
end
end
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