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delegate.rb
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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