From: jason@... (Jason Voegele)
Date: 2001-09-29T04:30:30+09:00
Subject: [ruby-talk:21814] Pseudo-Become operator for Ruby (was: Transparent Proxies)

With some help from this newsgroup, I've come up with a scheme for a
"become" like mechanism for Ruby.  It is not a true "become" operator
because it does not allow any arbitrary object to become any arbitrary
thing, but it does allow instances of a proxy class to become any
object you desire.  The idea is that there is a proxy object that
undefines all methods, and then uses the "method_missing" method to
trap messages, and then "becomes" the object which it was fronting
for.  This allows for such things as lazy initialization, and most
importantly for my application (a persistent object store) loading
objects on demand.

Here's an example:

class ProxyTest
    def initialize

        # create a proxy object so that the array object is only
        # created when it is first accessed.

        @lazily_loaded_array = ObjectProxy.new(self,
"@lazily_loaded_array") {
            # This "factory" block simply returns a new array,
            # but it could conceivably do anything it wants
            # to retrieve the object, such as load it from
            # an OO database.
            Array.new
        }
    end

    def test_proxy
        # once we call a method on the proxy object,
        # it should "become" an array
        puts @lazily_loaded_array.type    # -> "Array"
        @lazily_loaded_array.push(10)
        puts @lazily_loaded_array.length  # -> 1
    end
end

ProxyTest.new.test_proxy

Below is the code for the ObjectProxy class.  Please let me know if
you have any suggestions or comments.

Thanks
Jason Voegele

class ObjectProxy
    undef_method :==
    undef_method :===
    undef_method :=~
    undef_method :class
    undef_method :clone
    undef_method :display
    undef_method :dup
    undef_method :eql?
    undef_method :equal?
    undef_method :extend
    undef_method :freeze
    undef_method :frozen?
    undef_method :hash
    undef_method :id
    undef_method :inspect
    undef_method :instance_eval
    undef_method :instance_of?
    undef_method :instance_variables
    undef_method :is_a?
    undef_method :kind_of?
    undef_method :method
    undef_method :method_missing
    undef_method :methods
    undef_method :nil?
    undef_method :private_methods
    undef_method :protected_methods
    undef_method :public_methods
    undef_method :respond_to?
    undef_method :send
    undef_method :singleton_methods
    undef_method :taint
    undef_method :tainted?
    undef_method :to_a
    undef_method :to_s
    undef_method :type
    undef_method :untaint

    # The proxy object takes a block that retrieves
    # or creates the object. The container reference is
    # necessary so that we can break into it and replace
    # its reference to the proxy with a reference to the
    # actual object.
    def initialize(container, variable_name, *args, &factory)
        @container = container
        @name = variable_name
        @factory_args = args
        @factory = factory
    end

    def method_missing(method, *args)
        become = @factory.call
        @container.instance_eval("#@name = become")
        become.send(method, *args)
    end
end