Patterns Are Not Scary: Method Missing Proxy

Method missing proxy? Ooooh! Sounds scary, right? I got news for you Walter Cronkite, it’s not. Lets start with the definition of proxy, according to Wikipedia.

Definition

A proxy, in its most general form, is a class functioning as an interface to something else.

An interface to something else. That sounds easy enough. You might be thinking that you have never used a proxy, but if you are reading this blog, you are wrong. Chances are you have used Rails, and if you have used Rails, chances are you have used has_many or some other ActiveRecord association, all of which are implemented using proxies under the hood.

Creating Your Own

Now that we have definition out of the way and have confirmed your use of proxies, let’s make one! Yay! The people rejoice! The basic idea of a proxy is a class that is an interface to something else. Lets call something else subject from now on. In order to get started, we’ll make a new proxy that has a subject.

class Proxy
  def initialize(subject)
    @subject = subject
  end
end

proxied_array = Proxy.new([1,2,3])
puts proxied_array.size
# NoMethodError: undefined method ‘size’

FAIL! Our proxy has a subject (the array), but does not proxy anything yet. In order to proxy up the girl, lets throw in some method missing magic.

class Proxy
  def initialize(subject)
    @subject = subject
  end
  
  private
    def method_missing(method, *args, &block)
      @subject.send(method, *args, &block)
    end
end

proxied_array = Proxy.new([1,2,3])
puts proxied_array.size # 3

Method missing takes 3 arguments: the method called, the arguments passed to the method and a block if one is given. With just that tiny method missing addition, we can now do fun things like this:

proxied_array = Proxy.new([1,2,3])
puts proxied_array.size # 3
puts proxied_array[0] # 1
puts proxied_array[1] # 2
puts proxied_array[2] # 3
puts proxied_array.select { |a| a > 1 }.inspect # [2, 3]
proxied_array << 4
puts proxied_array.size # 4
puts proxied_array[3] # 4

Just like that our proxied array behaves just like the original array. Well, almost like the original array.

puts proxied_array.class # Proxy

BlankSlate and BasicObject

Hmm, that is not quite what you would expect. We told the proxy to send everything to the subject, so it should output Array, not Proxy as the class, right? The problem is that any new class automatically has some methods included with it. In order for our Proxy class to be a true proxy, we need to remove those methods as well. In the Ruby 1.8 series, this is often done by defining a BlankSlate object which removes those methods and then have our Proxy inherit from BlankSlate.

class BlankSlate #:nodoc:
  instance_methods.each { |m| undef_method m unless m =~ /^__|instance_eval|object_id/ }
end

class Proxy < BlankSlate
  def initialize(subject)
    @subject = subject
  end
  
  private
    def method_missing(method, *args, &block)
      @subject.send(method, *args, &block)
    end
end

proxied_array = Proxy.new([1,2,3])
puts proxied_array.class # Array

Yay! Now we in fact get Array as one would expect. The great news is that Ruby 1.9 comes with a class like this already named BasicObject. The easy way to make this work with Ruby 1.8 and Ruby 1.9 is to just define BasicObject if it does not exist and then inherit from BasicObject, instead of dealing with BlankSlate.

class BasicObject #:nodoc:
  instance_methods.each { |m| undef_method m unless m =~ /^__|instance_eval/ }
end unless defined?(BasicObject)

class Proxy < BasicObject
  def initialize(subject)
    @subject = subject
  end
  
  private
    def method_missing(method, *args, &block)
      @subject.send(method, *args, &block)
    end
end

proxied_array = Proxy.new([1,2,3])
puts proxied_array.class # Array

Just like that our proxy is a full fledged proxy and it works with Ruby 1.8 and 1.9.

Example: MongoMapper Pagination

So other than ActiveRecord where else can you check out some proxies in the wild? In MongoMapper, pagination uses a method missing proxy. When someone uses paginate, instead of find, I wanted the result that was returned to also function much like WillPaginate::Collection does, but I didn’t want to inherit from Array.

You can view the pagination proxy on github. The paginate method that uses it looks like this:

def paginate(options)        
  per_page      = options.delete(:per_page)
  page          = options.delete(:page)
  total_entries = count(options[:conditions] || {})
  
  collection = Pagination::PaginationProxy.new(total_entries, page, per_page)
  
  options[:limit] = collection.limit
  options[:offset]  = collection.offset
  
  collection.subject = find_every(options)
  collection
end

Just like that, paginate returns results just like find, but also includes methods for total_pages, previous and next pages, total_entries and the like.

Example: HTTParty Response

In HTTParty, at first I just returned a ruby hash that was the parsed xml or json. Then, people started begging for response codes and headers, so I went with a Response proxy that looks like this:

module HTTParty
  class Response < BlankSlate #:nodoc:
    attr_accessor :body, :code, :message, :headers
    attr_reader :delegate

    def initialize(delegate, body, code, message, headers={})
      @delegate = delegate
      @body = body
      @code = code.to_i
      @message = message
      @headers = headers
    end

    def method_missing(name, *args, &block)
      @delegate.send(name, *args, &block)
    end
  end
end

Now I just pass the parsed response, the codes, headers, and such to Response.new and the people who want that information get it and those who don’t have no API change to wrestle with.

Conclusion

Hope this little primer on the Proxy pattern, specifically using Ruby’s method missing, is helpful. I also hope that because of this you check out some of the other great patterns that are out there. I know I avoided them for far too long. When applied correctly, they really lead to elegant solutions.