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delegate.rb

# = delegate -- Support for the Delegation Pattern
#
# Documentation by James Edward Gray II and Gavin Sinclair
#
# == Introduction
#
# This library provides three different ways to delegate method calls to an
# object. The easiest to use is SimpleDelegator. Pass an object to the
# constructor and all methods supported by the object will be delegated. This
# object can be changed later.
#
# Going a step further, the top level DelegateClass method allows you to easily
# setup delegation through class inheritance. This is considerably more
# flexible and thus probably the most common use for this library.
#
# Finally, if you need full control over the delegation scheme, you can inherit
# from the abstract class Delegator and customize as needed. (If you find
# yourself needing this control, have a look at _forwardable_, also in the
# standard library. It may suit your needs better.)
#
# == Notes
#
# Be advised, RDoc will not detect delegated methods.
#
# delegate.rb provides full-class delegation via the
# DelegateClass() method. For single-method delegation via
# def_delegator(), see forwardable.rb.
#
# == Examples
#
# === SimpleDelegator
#
# Here's a simple example that takes advantage of the fact that
# SimpleDelegator's delegation object can be changed at any time.
#
# class Stats
# def initialize
# @source = SimpleDelegator.new([])
# end
# 
# def stats( records )
# @source.__setobj__(records)
# 	
# "Elements: #{@source.size}\n" +
# " Non-Nil: #{@source.compact.size}\n" +
# " Unique: #{@source.uniq.size}\n"
# end
# end
# 
# s = Stats.new
# puts s.stats(%w{James Edward Gray II})
# puts
# puts s.stats([1, 2, 3, nil, 4, 5, 1, 2])
#
# Prints:
#
# Elements: 4
# Non-Nil: 4
# Unique: 4
# 
# Elements: 8
# Non-Nil: 7
# Unique: 6
#
# === DelegateClass()
#
# Here's a sample of use from tempfile.rb.
#
# A _Tempfile_ object is really just a _File_ object with a few special rules
# about storage location and/or when the File should be deleted. That makes for
# an almost textbook perfect example of how to use delegation.
#
# class Tempfile < DelegateClass(File)
# # constant and class member data initialization...
# 
# def initialize(basename, tmpdir=Dir::tmpdir)
# # build up file path/name in var tmpname...
# 
# @tmpfile = File.open(tmpname, File::RDWR|File::CREAT|File::EXCL, 0600)
# 
# # ...
# 
# super(@tmpfile)
# 
# # below this point, all methods of File are supported...
# end
# 
# # ...
# end
#
# === Delegator
#
# SimpleDelegator's implementation serves as a nice example here.
#
# class SimpleDelegator < Delegator
# def initialize(obj)
# super # pass obj to Delegator constructor, required
# @_sd_obj = obj # store obj for future use
# end
# 
# def __getobj__
# @_sd_obj # return object we are delegating to, required
# end
# 
# def __setobj__(obj)
# @_sd_obj = obj # change delegation object, a feature we're providing
# end
# 
# # ...
# end

#
# Delegator is an abstract class used to build delegator pattern objects from
# subclasses.  Subclasses should redefine \_\_getobj\_\_.  For a concrete
# implementation, see SimpleDelegator.
#
class Delegator
  IgnoreBacktracePat = %r"\A#{Regexp.quote(__FILE__)}:\d+:in `"

#
 # Pass in the _obj_ to delegate method calls to. All methods supported by
 # _obj_ will be delegated to.
 #
 def initialize(obj)
 preserved = ::Kernel.public_instance_methods(false)
 preserved -= ["to_s","to_a","inspect","==","=~","==="]
 for t in self.class.ancestors
 preserved |= t.public_instance_methods(false)
 preserved |= t.private_instance_methods(false)
 preserved |= t.protected_instance_methods(false)
 break if t == Delegator
 end
 preserved << "singleton_method_added"
 for method in obj.methods
 next if preserved.include? method
 begin
	eval <<-EOS, nil, __FILE__, __LINE__+1
	 def self.#{method}(*args, &block)
	 begin
	 __getobj__.__send__(:#{method}, *args, &block)
	 ensure
	 $@.delete_if{|s|IgnoreBacktracePat=~s} if $@
	 end
	 end
	EOS
 rescue SyntaxError
 raise NameError, "invalid identifier %s" % method, caller(4)
 end
 end
 end
 alias initialize_methods initialize

# Handles the magic of delegation through \_\_getobj\_\_.
  def method_missing(m, *args, &block)
    target = self.__getobj__
    unless target.respond_to?(m)
      super(m, *args, &block)
    end
    target.__send__(m, *args, &block)
  end

# 
  # Checks for a method provided by this the delegate object by fowarding the 
  # call through \_\_getobj\_\_.
  # 
  def respond_to?(m, include_private = false)
    return true if super
    return self.__getobj__.respond_to?(m, include_private)
  end

#
  # This method must be overridden by subclasses and should return the object
  # method calls are being delegated to.
  #
  def __getobj__
    raise NotImplementedError, "need to define `__getobj__'"
  end

# Serialization support for the object returned by \_\_getobj\_\_.
  def marshal_dump
    __getobj__
  end
  # Reinitializes delegation from a serialized object.
  def marshal_load(obj)
    initialize_methods(obj)
    __setobj__(obj)
  end
end

#
# A concrete implementation of Delegator, this class provides the means to
# delegate all supported method calls to the object passed into the constructor
# and even to change the object being delegated to at a later time with
# \_\_setobj\_\_ .
#
class SimpleDelegator<Delegator

# Pass in the _obj_ you would like to delegate method calls to.
  def initialize(obj)
    super
    @_sd_obj = obj
  end

# Returns the current object method calls are being delegated to.
  def __getobj__
    @_sd_obj
  end

#
  # Changes the delegate object to _obj_.
  #
  # It's important to note that this does *not* cause SimpleDelegator's methods
  # to change.  Because of this, you probably only want to change delegation
  # to objects of the same type as the original delegate.
  #
  # Here's an example of changing the delegation object.
  #
  #   names = SimpleDelegator.new(%w{James Edward Gray II})
  #   puts names[1]    # => Edward
  #   names.__setobj__(%w{Gavin Sinclair})
  #   puts names[1]    # => Sinclair
  #
  def __setobj__(obj)
    raise ArgumentError, "cannot delegate to self" if self.equal?(obj)
    @_sd_obj = obj
  end

# Clone support for the object returned by \_\_getobj\_\_.
  def clone
    new = super
    new.__setobj__(__getobj__.clone)
    new
  end
  # Duplication support for the object returned by \_\_getobj\_\_.
  def dup
    new = super
    new.__setobj__(__getobj__.clone)
    new
  end
end

# :stopdoc:
# backward compatibility ^_^;;;
Delegater = Delegator
SimpleDelegater = SimpleDelegator
# :startdoc:

#
# The primary interface to this library. Use to setup delegation when defining
# your class.
#
# class MyClass < DelegateClass( ClassToDelegateTo ) # Step 1
# def initialize
# super(obj_of_ClassToDelegateTo) # Step 2
# end
# end
#
def DelegateClass(superclass)
 klass = Class.new
 methods = superclass.public_instance_methods(true)
 methods -= ::Kernel.public_instance_methods(false)
 methods |= ["to_s","to_a","inspect","==","=~","==="]
 klass.module_eval {
 def initialize(obj) # :nodoc:
 @_dc_obj = obj
 end
 def method_missing(m, *args, &block) # :nodoc:
 unless @_dc_obj.respond_to?(m)
 super(m, *args, &block)
 end
 @_dc_obj.__send__(m, *args, &block)
 end
 def respond_to?(m, include_private = false) # :nodoc:
 return true if super
 return @_dc_obj.respond_to?(m, include_private)
 end
 def __getobj__ # :nodoc:
 @_dc_obj
 end
 def __setobj__(obj) # :nodoc:
 raise ArgumentError, "cannot delegate to self" if self.equal?(obj)
 @_dc_obj = obj
 end
 def clone # :nodoc:
 new = super
 new.__setobj__(__getobj__.clone)
 new
 end
 def dup # :nodoc:
 new = super
 new.__setobj__(__getobj__.clone)
 new
 end
 }
 for method in methods
 begin
 klass.module_eval <<-EOS, __FILE__, __LINE__+1
 def #{method}(*args, &block)
	 begin
	 @_dc_obj.__send__(:#{method}, *args, &block)
	 ensure
	 $@.delete_if{|s| ::Delegator::IgnoreBacktracePat =~ s} if $@
	 end
	end
 EOS
 rescue SyntaxError
 raise NameError, "invalid identifier %s" % method, caller(3)
 end
 end
 return klass
end

# :enddoc:

if __FILE__ == $0
 class ExtArray<DelegateClass(Array)
 def initialize()
 super([])
 end
 end

ary = ExtArray.new
  p ary.class
  ary.push 25
  p ary

foo = Object.new
  def foo.test
    25
  end
  def foo.error
    raise 'this is OK'
  end
  foo2 = SimpleDelegator.new(foo)
  p foo.test == foo2.test	# => true
  foo2.error			# raise error!
end

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