blankslate.rb

#!/usr/bin/env ruby
#---
# Excerpted from "Metaprogramming Ruby 2",
# published by The Pragmatic Bookshelf.
# Copyrights apply to this code. It may not be used to create training material, 
# courses, books, articles, and the like. Contact us if you are in doubt.
# We make no guarantees that this code is fit for any purpose. 
# Visit http://www.pragmaticprogrammer.com/titles/ppmetr2 for more book information.
#---
#--
# Copyright 2004, 2006 by Jim Weirich (jim@weirichhouse.org).
# All rights reserved.

# Permission is granted for use, copying, modification, distribution,
# and distribution of modified versions of this work as long as the
# above copyright notice is included.
#++

class String
  if instance_methods.first.is_a?(Symbol)
    def _blankslate_as_name
      to_sym
    end
  else
    def _blankslate_as_name
      self
    end
  end
end

class Symbol
  if instance_methods.first.is_a?(Symbol)
    def _blankslate_as_name
      self
    end
  else
    def _blankslate_as_name
      to_s
    end
  end
end

######################################################################
# BlankSlate provides an abstract base class with no predefined
# methods (except for <tt>\_\_send__</tt> and <tt>\_\_id__</tt>).
# BlankSlate is useful as a base class when writing classes that
# depend upon <tt>method_missing</tt> (e.g. dynamic proxies).
#
# [Note from Paolo: I use this code as an example in Chapter 2, but I don't talk about
# the "class << self" idiom until chapter 5. So I took the liberty of moving the definition
# of hide() outside the "class << self" construct, and define it with the good old
# "def self.method_name" idiom.]
class BlankSlate
  # Hide the method named +name+ in the BlankSlate class.  Don't
  # hide +instance_eval+ or any method beginning with "__".
  def self.hide(name)
    warn_level = $VERBOSE
    $VERBOSE = nil
    # ...
    if instance_methods.include?(name._blankslate_as_name) &&
        name !~ /^(__|instance_eval$)/
      @hidden_methods ||= {}
      @hidden_methods[name.to_sym] = instance_method(name)
      undef_method name
    end
  ensure
    $VERBOSE = warn_level
  end

  class << self
    def find_hidden_method(name)
      @hidden_methods ||= {}
      @hidden_methods[name] || superclass.find_hidden_method(name)
    end

    # Redefine a previously hidden method so that it may be called on a blank
    # slate object.
    def reveal(name)
      hidden_method = find_hidden_method(name)
      fail "Don't know how to reveal method '#{name}'" unless hidden_method
      define_method(name, hidden_method)
    end
  end

  # ...
  
  instance_methods.each { |m| hide(m) }
end

######################################################################
# Since Ruby is very dynamic, methods added to the ancestors of
# BlankSlate <em>after BlankSlate is defined</em> will show up in the
# list of available BlankSlate methods.  We handle this by defining a
# hook in the Object and Kernel classes that will hide any method
# defined after BlankSlate has been loaded.
#
module Kernel
  class << self
    alias_method :blank_slate_method_added, :method_added

    # Detect method additions to Kernel and remove them in the
    # BlankSlate class.
    def method_added(name)
      result = blank_slate_method_added(name)
      return result if self != Kernel
      BlankSlate.hide(name)
      result
    end
  end
end

######################################################################
# Same as above, except in Object.
#
class Object
  class << self
    alias_method :blank_slate_method_added, :method_added

    # Detect method additions to Object and remove them in the
    # BlankSlate class.
    def method_added(name)
      result = blank_slate_method_added(name)
      return result if self != Object
      BlankSlate.hide(name)
      result
    end

    def find_hidden_method(name)
      nil
    end
  end
end

######################################################################
# Also, modules included into Object need to be scanned and have their
# instance methods removed from blank slate.  In theory, modules
# included into Kernel would have to be removed as well, but a
# "feature" of Ruby prevents late includes into modules from being
# exposed in the first place.
#
class Module
  alias blankslate_original_append_features append_features
  def append_features(mod)
    result = blankslate_original_append_features(mod)
    return result if mod != Object
    instance_methods.each do |name|
      BlankSlate.hide(name)
    end
    result
  end
end