instance method `delegate_missing_to`

Ruby on Rails 5.2.8.1

Since v5.2.8.1

Available in: v5.2.8.1 v6.0.6 v6.1.7.10 v7.0.10 v7.1.6 v7.2.3 v8.0.4 v8.1.2

Compare with:

delegate_missing_to(target)

When building decorators, a common pattern may emerge:

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

  def person
    @event.detail.person || @event.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
    @event.detail.person || @event.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 NoMethodError.

Parameters

target req

Source

# File activesupport/lib/active_support/core_ext/module/delegation.rb, line 258
  def delegate_missing_to(target)
    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.

        #{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?
              raise DelegationError, "\#{method} delegated to #{target}, but #{target} is nil"
            else
              raise
            end
          end
        end
      end
    RUBY
  end

Defined in activesupport/lib/active_support/core_ext/module/delegation.rb line 258 · View on GitHub · Improve this page · Find usages on GitHub

Defined in Module

Signature

Parameters