Method: Module#delegate_missing_to

Defined in:

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

#delegate_missing_to(target, allow_nil: nil) ⇒ Object

When building decorators, a common pattern may emerge:

class Partition
  def initialize(event)
    @event = event
  end

  def person
    detail.person || creator
  end

  private
    def respond_to_missing?(name, include_private = false)
      @event.respond_to?(name, include_private)
    end

    def method_missing(method, *args, &block)
      @event.send(method, *args, &block)
    end
end

With Module#delegate_missing_to, the above is condensed to:

class Partition
  delegate_missing_to :@event

  def initialize(event)
    @event = event
  end

  def person
    detail.person || creator
  end
end

The target can be anything callable within the object, e.g. instance variables, methods, constants, etc.

The delegated method must be public on the target, otherwise it will raise DelegationError. If you wish to instead return nil, use the :allow_nil option.

The marshal_dump and _dump methods are exempt from delegation due to possible interference when calling Marshal.dump(object), should the delegation target method of object add or remove instance variables.

# File 'activesupport/lib/active_support/core_ext/module/delegation.rb', line 318

def delegate_missing_to(target, allow_nil: nil)
  target = target.to_s
  target = "self.#{target}" if DELEGATION_RESERVED_METHOD_NAMES.include?(target)

  module_eval <<-RUBY, __FILE__, __LINE__ + 1
    def respond_to_missing?(name, include_private = false)
      # It may look like an oversight, but we deliberately do not pass
      # +include_private+, because they do not get delegated.

      return false if name == :marshal_dump || name == :_dump
      #{target}.respond_to?(name) || super
    end

    def method_missing(method, *args, &block)
      if #{target}.respond_to?(method)
        #{target}.public_send(method, *args, &block)
      else
        begin
          super
        rescue NoMethodError
          if #{target}.nil?
            if #{allow_nil == true}
              nil
            else
              raise DelegationError, "\#{method} delegated to #{target}, but #{target} is nil"
            end
          else
            raise
          end
        end
      end
    end
    ruby2_keywords(:method_missing)
  RUBY
end