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 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 # # # 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 :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 # 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, to, allow_nil = options[:prefix], options[:to], options[:allow_nil] if prefix == true && to.to_s =~ /^[^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 methods.each do |method| method = method.to_s if allow_nil module_eval(<<-EOS, file, line - 2) def #{method_prefix}#{method}(*args, &block) # def customer_name(*args, &block) if #{to} || #{to}.respond_to?(:#{method}) # if client || client.respond_to?(:name) #{to}.__send__(:#{method}, *args, &block) # client.__send__(: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}(*args, &block) # def customer_name(*args, &block) #{to}.__send__(:#{method}, *args, &block) # client.__send__(: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