Extending instance methods from a class object

ruby-talk
1

Hi all,

I'm currently drowning in a pool of instance_methods and define_methods
and I can't seem to get out. I know there's an elegant solution
somewhere....

I have a Class object klass. It has an instance method meth(). I'd like
to "extend" meth() so that when it's called, the output is changed---I
want to call the original meth(), tweak its result, and then return it.
I need to return a Class object that has this tweak in place; I can
either change klass and return that or I can subclass it and return the
subclass.

Something like:

def tweaked_class(klass)
  old_meth = klass.instance_method(:to_html)
  klass.define_method(:meth) do |*a|
    r = old_meth(*a)
    # ... transform r ...
    r
  end
  klass
end

But that doesn't work, and I might be on the wrong track completely.
So what's the best way to do this?

Thanks,

···

--
William <wmorgan-ruby-talk@masanjin.net>

2

William Morgan wrote:

Hi all,

I'm currently drowning in a pool of instance_methods and define_methods
and I can't seem to get out. I know there's an elegant solution
somewhere....

I have a Class object klass. It has an instance method meth(). I'd like
to "extend" meth() so that when it's called, the output is changed---I
want to call the original meth(), tweak its result, and then return it.
I need to return a Class object that has this tweak in place; I can
either change klass and return that or I can subclass it and return the
subclass.

Something like:

def tweaked_class(klass)
  old_meth = klass.instance_method(:to_html)
  klass.define_method(:meth) do |*a|
    r = old_meth(*a)
    # ... transform r ...
    r
  end
  klass
end

But that doesn't work, and I might be on the wrong track completely.
So what's the best way to do this?

Thanks,

Here are two ways. There are probably more, knowing ruby :slight_smile:

def tweak(klass, old_meth, new_meth)

   if old_meth == new_meth

     old_meth_unbound = klass.instance_method(old_meth)

     klass.class_eval do
       define_method(new_meth) do |*args|
         yield old_meth_unbound.bind(self).call(*args)
       end
     end

   else

     klass.class_eval do
       define_method(new_meth) do |*args|
         yield send(old_meth, *args)
       end
     end

   end

   klass
end

class Foo
   def whazzup; "nothing"; end
end

tweak(Foo, :whazzup, :whaddaya) do |r|
   r + " much"
end

p Foo.new.whaddaya

tweak(Foo, :whazzup, :whazzup) do |r|
   r + " else"
end

p Foo.new.whazzup

3

Excerpts from Joel VanderWerf's mail of 30 Oct 2004 (EDT):

Here are two ways. There are probably more, knowing ruby :slight_smile:

Thanks! It was the class_eval I was forgetting.

···

--
William <wmorgan-ruby-talk@masanjin.net>