Programming Tutorials

SimpleDelegator in Ruby

By: James Edward Gray II and Gavin Sinclair  

The delegate.rb 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

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