class Module
# Provides a delegate class method to easily expose contained objects' methods
# as your own. Pass one or more methods (specified as symbols or strings)
# and the name of the target object as the final <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
#
# 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 object to which you delegate can be nil, you may want to use the
# :allow_nil option. In that case, it returns nil instead of raising a
# NoMethodError exception:
#
# 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
if options[:prefix] == true && options[:to].to_s =~ /^[^a-z_]/
raise ArgumentError, "Can only automatically set the delegation prefix when delegating to a method."
end
prefix = options[:prefix] && "#{options[:prefix] == true ? to : options[:prefix]}_"
file, line = caller.first.split(':', 2)
line = line.to_i
methods.each do |method|
on_nil =
if options[:allow_nil]
'return'
else
%(raise "#{self}##{prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")
end
module_eval(<<-EOS, file, line)
def #{prefix}#{method}(*args, &block) # def customer_name(*args, &block)
#{on_nil} if #{to}.nil?
#{to}.__send__(#{method.inspect}, *args, &block) # client && client.__send__(:name, *args, &block)
end # end
EOS
end
end
end
