File: delegate.rb

# delegate.rb

Path: | delegate.rb
Last Update: | Sat Oct 15 23:44:27 AUS Eastern Standard Time 2005

# 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

### Methods

DelegateClass

### Public Instance methods

DelegateClass(superclass)

The primary interface to this library. Use to setup delegation when defining your class.

      class MyClass < DelegateClass( ClassToDelegateTo )    # Step 1
        def initiaize
          super(obj_of_ClassToDelegateTo)                   # Step 2
        end
      end

[Source]

    # File delegate.rb, line 257
    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)  # :nodoc:
          unless @_dc_obj.respond_to?(m)
            super(m, *args)
          end
          @_dc_obj.__send__(m, *args)
        end
        def respond_to?(m)  # :nodoc:
          return true if super
          return @_dc_obj.respond_to?(m)
        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:
          super
          __setobj__(__getobj__.clone)
        end
        def dup  # :nodoc:
          super
          __setobj__(__getobj__.dup)
        end
      }
      for method in methods
        begin
          klass.module_eval "def \#{method}(*args, &block)\nbegin\n@_dc_obj.__send__(:\#{method}, *args, &block)\nrescue\n$@[0,2] = nil\nraise\nend\nend\n"
        rescue SyntaxError
          raise NameError, "invalid identifier %s" % method, caller(3)
        end
      end
      return klass
    end

[Validate]