vendor/rails/activesupport/lib/active_support/core_ext/module/delegation.rb

   1 class Module
   2   # Provides a delegate class method to easily expose contained objects' methods
   3   # as your own. Pass one or more methods (specified as symbols or strings)
   4   # and the name of the target object as the final <tt>:to</tt> option (also a symbol
   5   # or string).  At least one method and the <tt>:to</tt> option are required.
   6   #
   7   # Delegation is particularly useful with Active Record associations:
   8   #
   9   #   class Greeter < ActiveRecord::Base
  10   #     def hello()   "hello"   end
  11   #     def goodbye() "goodbye" end
  12   #   end
  13   #
  14   #   class Foo < ActiveRecord::Base
  15   #     belongs_to :greeter
  16   #     delegate :hello, :to => :greeter
  17   #   end
  18   #
  19   #   Foo.new.hello   # => "hello"
  20   #   Foo.new.goodbye # => NoMethodError: undefined method `goodbye' for #<Foo:0x1af30c>
  21   #
  22   # Multiple delegates to the same target are allowed:
  23   #
  24   #   class Foo < ActiveRecord::Base
  25   #     belongs_to :greeter
  26   #     delegate :hello, :goodbye, :to => :greeter
  27   #   end
  28   #
  29   #   Foo.new.goodbye # => "goodbye"
  30   #
  31   # Methods can be delegated to instance variables, class variables, or constants
  32   # by providing them as a symbols:
  33   #
  34   #   class Foo
  35   #     CONSTANT_ARRAY = [0,1,2,3]
  36   #     @@class_array  = [4,5,6,7]
  37   #
  38   #     def initialize
  39   #       @instance_array = [8,9,10,11]
  40   #     end
  41   #     delegate :sum, :to => :CONSTANT_ARRAY
  42   #     delegate :min, :to => :@@class_array
  43   #     delegate :max, :to => :@instance_array
  44   #   end
  45   #
  46   #   Foo.new.sum # => 6
  47   #   Foo.new.min # => 4
  48   #   Foo.new.max # => 11
  49   #
  50   # Delegates can optionally be prefixed using the <tt>:prefix</tt> option. If the value
  51   # is <tt>true</tt>, the delegate methods are prefixed with the name of the object being
  52   # delegated to.
  53   #
  54   #   Person = Struct.new(:name, :address)
  55   #
  56   #   class Invoice < Struct.new(:client)
  57   #     delegate :name, :address, :to => :client, :prefix => true
  58   #   end
  59   #
  60   #   john_doe = Person.new("John Doe", "Vimmersvej 13")
  61   #   invoice = Invoice.new(john_doe)
  62   #   invoice.client_name    # => "John Doe"
  63   #   invoice.client_address # => "Vimmersvej 13"
  64   #
  65   # It is also possible to supply a custom prefix.
  66   #
  67   #   class Invoice < Struct.new(:client)
  68   #     delegate :name, :address, :to => :client, :prefix => :customer
  69   #   end
  70   #
  71   #   invoice = Invoice.new(john_doe)
  72   #   invoice.customer_name    # => "John Doe"
  73   #   invoice.customer_address # => "Vimmersvej 13"
  74   #
  75   # If the object to which you delegate can be nil, you may want to use the
  76   # :allow_nil option. In that case, it returns nil instead of raising a
  77   # NoMethodError exception:
  78   #
  79   #  class Foo
  80   #    attr_accessor :bar
  81   #    def initialize(bar = nil)
  82   #      @bar = bar
  83   #    end
  84   #    delegate :zoo, :to => :bar
  85   #  end
  86   #
  87   #  Foo.new.zoo   # raises NoMethodError exception (you called nil.zoo)
  88   #
  89   #  class Foo
  90   #    attr_accessor :bar
  91   #    def initialize(bar = nil)
  92   #      @bar = bar
  93   #    end
  94   #    delegate :zoo, :to => :bar, :allow_nil => true
  95   #  end
  96   #
  97   #  Foo.new.zoo   # returns nil
  98   #
  99   def delegate(*methods)
 100     options = methods.pop
 101     unless options.is_a?(Hash) && to = options[:to]
 102       raise ArgumentError, "Delegation needs a target. Supply an options hash with a :to key as the last argument (e.g. delegate :hello, :to => :greeter)."
 103     end
 104
 105     if options[:prefix] == true && options[:to].to_s =~ /^[^a-z_]/
 106       raise ArgumentError, "Can only automatically set the delegation prefix when delegating to a method."
 107     end
 108
 109     prefix = options[:prefix] && "#{options[:prefix] == true ? to : options[:prefix]}_"
 110
 111     allow_nil = options[:allow_nil] && "#{to} && "
 112
 113     methods.each do |method|
 114       module_eval(<<-EOS, "(__DELEGATION__)", 1)
 115         def #{prefix}#{method}(*args, &block)                           # def customer_name(*args, &block)
 116           #{allow_nil}#{to}.__send__(#{method.inspect}, *args, &block)  #   client && client.__send__(:name, *args, &block)
 117         end                                                             # end
 118       EOS
 119     end
 120   end
 121 end