Getting to Know the Ruby Standard Library – Delegator

This article has been republished on Monkey and Crow.

Today we will examine ruby’s implementation of the Proxy pattern, with the Delegator class. We have already seen and example of it in use with WeakRef. A Delegator can be used when you want to intercept calls to some object without concerning the caller. For example we can use a Delegator to hide the latency from http calls or other slow code:

  require 'delegate'

  class Future < SimpleDelegator
    def initialize(&block)
      @_thread = Thread.start(&block)
    end

    def __getobj__
      __setobj__(@_thread.value) if @_thread.alive?

      super
    end
  end

The Future will invoke whatever is passed to it without blocking the rest of your code until you try to access the result. You could use it to issue several http requests and then process them later:

  require 'net/http'
  # These will each execute immediately
  google = Future.new{ Net::HTTP.get_response(URI('http://www.google.com')).body  }
  yahoo  = Future.new{ Net::HTTP.get_response(URI('http://www.yahoo.com')).body  }

  # These will block until their requests have loaded
  puts google
  puts yahoo

In this example, google and yahoo will both spawn threads, however when we try to print them, the Future instance will block until the thread is done, and then pass on method calls to the result of our http call. You can grab the code from github and give it try yourself. Lets take a look at how Delegator works. Open up the source, and follow along, if you have Qwandry installed qw delegate will do the trick.

The file delegate.rb defines two classes, Delegator, an abstract class, and SimpleDelegator which implements the missing methods in Delegator. Let’s look at the first few lines of Delegator:

  class Delegator
    [:to_s,:inspect,:=~,:!~,:===].each do |m|
      undef_method m
    end
    ...

You will notice that a block of code is being executed as part of the ruby class definition. This is entirely valid, and will be executed in the scope of the Delegator class. undef_method is called to remove the default implementations of some common methods defined by Object. We will see why in a little bit. Next up is the initializer:

  def initialize(obj)
    __setobj__(obj)
  end

The method __setobj__ might look strange, but it is just a normal method with an obscure name. When a Delegator is instantiated, the object it is delegating to is stored away with __setobj__. Next look at how Delegator implements method_missing, and you’ll see what all the prep work was for:

  def method_missing(m, *args, &block)
    ...
      target = self.__getobj__
      unless target.respond_to?(m)
        super(m, *args, &block)
      else
        target.__send__(m, *args, &block)
      end
    ...

method_missing is defined on Object, and will be called any time that you try to call method that is not defined. This is the key to how Delegator works, any methods not defined on Delegator are handled by this method. Before we dive into this, we should be aware of what the arguments to method_missing are. The m is the missing method’s name as a symbol. The args are zero or more arguments that would have been passed to that method. The &block is the block that the method was called with, or nil if no block was given.

The first thing method_missing does here is call __getobj__. We’ve already seen __setobj__, it sets the object that Delegator wraps, so we can reason that __getobj__ gets it. Once the wrapped object has been obtained, it checks to see if that object implements the method we want to call. If not, then we call Object#method_missing, which is going to raise an exception. If the wrapped object does implement our method, then it passes it on. The methods that were undefined earlier are guaranteed to be passed on to the wrapped object, and the odd looking __getobj__ and __setobj__ are unlikely to collide with any other object’s methods. Ruby’s flexibility really shines in this example, in just a few lines of code, we get a very useful class that can be used to implement advanced behavior.

Now let’s figure out why there are two classes defined here. If you look down at Delegator#__setobj__ and Delegator#__getobj__ we’ll see something interesting:

  def __getobj__
    raise NotImplementedError, "need to define `__getobj__'"
  end
  ...
  def __setobj__(obj)
    raise NotImplementedError, "need to define `__setobj__'"
  end

Neither of these methods are implemented, effectively making Delegator an abstract class. For connivence, SimpleDelegator implements them in a reasonable manner. There are a few other special methods defined on Delegator as well:

  def ==(obj)
    return true if obj.equal?(self)
    self.__getobj__ == obj
  end

First Delegator checks for equality against itself, then it checks it against the wrapped object. This way == will return true if you pass in either the wrapped object, or the delegate itself. In this same manner you can intercept calls to specific methods, or override Delegator#method_missing to intercept all calls.

We have learned about a powerful design pattern that is easily implemented in ruby. We also saw a very good use for ruby’s method_missing. How have you used Delegator or found similar proxy patterns in ruby?

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3 Comments

Filed under ruby, stdlib

3 responses to “Getting to Know the Ruby Standard Library – Delegator

  1. Pingback: The Strange Ruby Splat | End of Line

  2. require 'delegate'

    BTW :-)