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forwardable.rb
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# = forwardable - Support for the Delegation Pattern # # $Release Version: 1.1$ # $Revision: 16857 $ # $Date: 2008-06-06 17:05:24 +0900 (Fri, 06 Jun 2008) $ # by Keiju ISHITSUKA(keiju@ishitsuka.com) # # Documentation by James Edward Gray II and Gavin Sinclair # # == Introduction # # This library allows you delegate method calls to an object, on a method by # method basis. You can use Forwardable to setup this delegation at the class # level, or SingleForwardable to handle it at the object level. # # == Notes # # Be advised, RDoc will not detect delegated methods. # # <b>forwardable.rb provides single-method delegation via the # def_delegator() and def_delegators() methods. For full-class # delegation via DelegateClass(), see delegate.rb.</b> # # == Examples # # === Forwardable # # Forwardable makes building a new class based on existing work, with a proper # interface, almost trivial. 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 # # <i>Prints:</i> # # 2 # 3 # 4 # 5 # 6 # Ruby # nil # # === SingleForwardable # # printer = String.new # printer.extend SingleForwardable # prepare object for delegation # printer.def_delegator "STDOUT", "puts" # add delegation for STDOUT.puts() # printer.puts "Howdy!" # # <i>Prints:</i> # # Howdy! # # 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 <tt>@records</tt>. You could provide the lookup method # #record_number(), which simply calls #[] on the <tt>@records</tt> # array, like this: # # class RecordCollection # extend Forwardable # def_delegator :@records, :[], :record_number # end # # 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 # # extend Forwardable, but we did that above # def_delegators :@records, :size, :<<, :map # end # # Also see the example at forwardable.rb. # module Forwardable @debug = nil class<<self # force Forwardable to show up in stack backtraces of delegated methods attr_accessor :debug end # # 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 # # See the examples at Forwardable and forwardable.rb. # def def_instance_delegators(accessor, *methods) for method in methods def_instance_delegator(accessor, method) end end # # Defines a method _method_ which delegates to _obj_ (i.e. it calls # the method of the same name in _obj_). If _new_name_ is # provided, it is used as the name for the delegate method. # # See the examples at Forwardable and forwardable.rb. # def def_instance_delegator(accessor, method, ali = method) accessor = accessor.id2name if accessor.kind_of?(Integer) method = method.id2name if method.kind_of?(Integer) ali = ali.id2name if ali.kind_of?(Integer) module_eval(<<-EOS, "(__FORWARDABLE__)", 1) def #{ali}(*args, &block) begin #{accessor}.__send__(:#{method}, *args, &block) rescue Exception $@.delete_if{|s| /^\\(__FORWARDABLE__\\):/ =~ s} unless Forwardable::debug Kernel::raise end end EOS end alias def_delegators def_instance_delegators alias def_delegator def_instance_delegator end # # The SingleForwardable module provides delegation of specified # methods to a designated object, using the methods #def_delegator # and #def_delegators. This module is similar to Forwardable, but it works on # objects themselves, instead of their defining classes. # # Also see the example at forwardable.rb. # module SingleForwardable # # Shortcut for defining multiple delegator methods, but with no # provision for using a different name. The following two code # samples have the same effect: # # single_forwardable.def_delegators :@records, :size, :<<, :map # # single_forwardable.def_delegator :@records, :size # single_forwardable.def_delegator :@records, :<< # single_forwardable.def_delegator :@records, :map # # See the example at forwardable.rb. # def def_singleton_delegators(accessor, *methods) for method in methods def_singleton_delegator(accessor, method) end end # # Defines a method _method_ which delegates to _obj_ (i.e. it calls # the method of the same name in _obj_). If _new_name_ is # provided, it is used as the name for the delegate method. # # See the example at forwardable.rb. # def def_singleton_delegator(accessor, method, ali = method) accessor = accessor.id2name if accessor.kind_of?(Integer) method = method.id2name if method.kind_of?(Integer) ali = ali.id2name if ali.kind_of?(Integer) instance_eval(<<-EOS, "(__FORWARDABLE__)", 1) def #{ali}(*args, &block) begin #{accessor}.__send__(:#{method}, *args,&block) rescue Exception $@.delete_if{|s| /^\\(__FORWARDABLE__\\):/ =~ s} unless Forwardable::debug Kernel::raise end end EOS end alias def_delegators def_singleton_delegators alias def_delegator def_singleton_delegator end