Module: Forwardable

Defined in:
lib/forwardable.rb

Overview

The Forwardable module provides delegation of specified methods to a designated object, using the methods #def_delegator and #def_delegators.

For example, say you have a class RecordCollection which contains an array @records. You could provide the lookup method #record_number(), which simply calls #[] on the @records array, like this:

require 'forwardable'

class RecordCollection
  attr_accessor :records
  extend Forwardable
  def_delegator :@records, :[], :record_number
end

We can use the lookup method like so:

r = RecordCollection.new
r.records = [4,5,6]
r.record_number(0)  # => 4

Further, if you wish to provide the methods #size, #<<, and #map, all of which delegate to @records, this is how you can do it:

class RecordCollection # re-open RecordCollection class
  def_delegators :@records, :size, :<<, :map
end

r = RecordCollection.new
r.records = [1,2,3]
r.record_number(0)   # => 1
r.size               # => 3
r << 4               # => [1, 2, 3, 4]
r.map { |x| x * 2 }  # => [2, 4, 6, 8]

You can even extend regular objects with Forwardable.

my_hash = Hash.new
my_hash.extend Forwardable              # prepare object for delegation
my_hash.def_delegator "STDOUT", "puts"  # add delegation for STDOUT.puts()
my_hash.puts "Howdy!"

Another example

We want to rely on what has come before obviously, but with delegation we can take just the methods we need and even rename them as appropriate. In many cases this is preferable to inheritance, which gives us the entire old interface, even if much of it isn't needed.

class Queue
  extend Forwardable

  def initialize
    @q = [ ]    # prepare delegate object
  end

  # setup preferred interface, enq() and deq()...
  def_delegator :@q, :push, :enq
  def_delegator :@q, :shift, :deq

  # support some general Array methods that fit Queues well
  def_delegators :@q, :clear, :first, :push, :shift, :size
end

q = Queue.new
q.enq 1, 2, 3, 4, 5
q.push 6

q.shift    # => 1
while q.size > 0
  puts q.deq
end

q.enq "Ruby", "Perl", "Python"
puts q.first
q.clear
puts q.first

This should output:

2
3
4
5
6
Ruby
nil

Notes

Be advised, RDoc will not detect delegated methods.

forwardable.rb provides single-method delegation via the def_delegator and def_delegators methods. For full-class delegation via DelegateClass, see delegate.rb.

Constant Summary

FORWARDABLE_VERSION =

Version of forwardable.rb

"1.1.0"
FILE_REGEXP =
%r"#{Regexp.quote(__FILE__)}"

Class Attribute Summary (collapse)

Instance Method Summary (collapse)

Class Attribute Details

+ (Object) debug

If true, __FILE__ will remain in the backtrace in the event an Exception is raised.



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# File 'lib/forwardable.rb', line 121

def debug
  @debug
end

Instance Method Details

- (Object) def_instance_delegator(accessor, method, ali = method) Also known as: def_delegator

Define method as delegator instance method with an optional alias name ali. Method calls to ali will be delegated to accessor.method.

class MyQueue
  extend Forwardable
  attr_reader :queue
  def initialize
    @queue = []
  end

  def_delegator :@queue, :push, :mypush
end

q = MyQueue.new
q.mypush 42
q.queue    #=> [42]
q.push 23  #=> NoMethodError


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# File 'lib/forwardable.rb', line 179

def def_instance_delegator(accessor, method, ali = method)
  line_no = __LINE__; str = %{
    def #{ali}(*args, &block)
      begin
        #{accessor}.__send__(:#{method}, *args, &block)
      rescue Exception
        $@.delete_if{|s| Forwardable::FILE_REGEXP =~ s} unless Forwardable::debug
        ::Kernel::raise
      end
    end
  }
  # If it's not a class or module, it's an instance
  begin
    module_eval(str, __FILE__, line_no)
  rescue
    instance_eval(str, __FILE__, line_no)
  end

end

- (Object) def_instance_delegators(accessor, *methods) Also known as: def_delegators

Shortcut for defining multiple delegator methods, but with no provision for using a different name. The following two code samples have the same effect:

def_delegators :@records, :size, :<<, :map

def_delegator :@records, :size
def_delegator :@records, :<<
def_delegator :@records, :map


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# File 'lib/forwardable.rb', line 152

def def_instance_delegators(accessor, *methods)
  methods.delete("__send__")
  methods.delete("__id__")
  for method in methods
    def_instance_delegator(accessor, method)
  end
end

- (Object) instance_delegate(hash) Also known as: delegate

Takes a hash as its argument. The key is a symbol or an array of symbols. These symbols correspond to method names. The value is the accessor to which the methods will be delegated.

:call-seq:

delegate method => accessor
delegate [method, method, ...] => accessor


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# File 'lib/forwardable.rb', line 132

def instance_delegate(hash)
  hash.each{ |methods, accessor|
    methods = [methods] unless methods.respond_to?(:each)
    methods.each{ |method|
      def_instance_delegator(accessor, method)
    }
  }
end