class Module
Public Instance Methods
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 :to
option (also a symbol or string). At least one method and the :to
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 :prefix
option. If the value is true
, 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
# File lib/docx/core_ext/module.rb, line 116 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