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# = 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.
#
# <b>delegate.rb provides full-class delegation via the
# DelegateClass() method. For single-method delegation via
# def_delegator(), see forwardable.rb.</b>
#
# == 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])
#
# <i>Prints:</i>
#
# Elements: 4
# Non-Nil: 4
# Unique: 4
#
# Elements: 8
# Non-Nil: 7
# Unique: 6
#
# === DelegateClass()
#
# Here's a sample of use from <i>tempfile.rb</i>.
#
# 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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