Class: Module

Inherits:
Object
  • Object
show all
Defined in:
lib/active_support/core_ext/module/delegation.rb

Overview

This file is part of Ruby on Rails (http://rubyonrails.org/) (original location: https://github.com/rails/rails/raw/v4.0.0/activesupport/lib/active_support/core_ext/module/delegation.rb)

Ruby on Rails is released under the MIT License (http://www.opensource.org/licenses/MIT).

"Ruby on Rails" is a registered trademark of David Heinemeier Hansson.

Instance Method Summary collapse

Instance Method Details

#delegate(*methods) ⇒ Object

Provides a +delegate+ class method to easily expose contained objects' public methods as your own.

The macro receives one or more method names (specified as symbols or strings) and the name of the target object via the :to option (also a symbol or string).

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 target is +nil+ and does not respond to the delegated method a +NoMethodError+ is raised, as with any other value. Sometimes, however, it makes sense to be robust to that situation and that is the purpose of the :allow_nil option: If the target is not +nil+, or it is and responds to the method, everything works as usual. But if it is +nil+ and does not respond to the delegated method, +nil+ is returned.

class User < ActiveRecord::Base has_one :profile delegate :age, to: :profile end

User.new.age # raises NoMethodError: undefined method `age'

But if not having a profile yet is fine and should not be an error condition:

class User < ActiveRecord::Base has_one :profile delegate :age, to: :profile, allow_nil: true end

User.new.age # nil

Note that if the target is not +nil+ then the call is attempted regardless of the :allow_nil option, and thus an exception is still raised if said object does not respond to the method:

class Foo def initialize(bar) @bar = bar end

delegate :name, to: :@bar, allow_nil: true

end

Foo.new("Bar").name # raises NoMethodError: undefined method `name'



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# File 'lib/active_support/core_ext/module/delegation.rb', line 139

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'

    # The following generated methods call the target exactly once, storing
    # the returned value in a dummy variable.
    #
    # Reason is twofold: On one hand doing less calls is in general better.
    # On the other hand it could be that the target has side-effects,
    # whereas conceptualy, from the user point of view, the delegator should
    # be doing one call.
    if allow_nil
      module_eval(<<-EOS, file, line - 3)
        def #{method_prefix}#{method}(#{definition})        # def customer_name(*args, &block)
          _ = #{to}                                         #   _ = client
          if !_.nil? || nil.respond_to?(:#{method})         #   if !_.nil? || nil.respond_to?(:name)
            _.#{method}(#{definition})                      #     _.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 - 2)
        def #{method_prefix}#{method}(#{definition})        # def customer_name(*args, &block)
          _ = #{to}                                         #   _ = client
          _.#{method}(#{definition})                        #   _.name(*args, &block)
        rescue NoMethodError                                # rescue NoMethodError
          if _.nil?                                         #   if _.nil?
            #{exception}                                    #     # add helpful message to the exception
          else                                              #   else
            raise                                           #     raise
          end                                               #   end
        end                                                 # end
      EOS
    end
  end
end