From: Tom Werner <tom@...>
Date: 2006-07-15T06:48:56+09:00
Subject: Re: Preferred monkeypatching technique

dblack@wobblini.net wrote:
> Hi --
>
> On Fri, 14 Jul 2006, Tom Werner wrote:
>
>> Allow me to present a scenario:
>>
>> class Firetruck
>>   def put_out_fire(options = {})
>>      # code
>>   end
>> end
>>
>> Pretend Firetruck is in a 3rd party application (like Rails) that is 
>> happy to allow plugins to modify core code. Now, let's say I want to 
>> write some code that always adds a certain attribute to the options 
>> hash. I could do this:
>>
>> class Firetruck
>>   alias_method :__old_put_out_fire, :put_out_fire
>>   def put_out_fire(options = {})
>>      __old_put_out_fire(options.merge({:nozzle => :big}))
>>   end
>> end
>>
>> Which works just fine until someone else comes up with a plugin that 
>> wants to modify the same method (doing something similar to me) and 
>> just so happens to also use :__old_put_out_fire as THEIR alias. Now 
>> we've got my plugin's method as the alias calling itself, which leads 
>> to, you know, badness.
>>
>> So I'm wondering if there's a better way. Perhaps some way to turn 
>> Firetruck into an ancestor of itself, so to speak, so that my plugin 
>> would create a new Firetruck class, pushing the old Firetruck 
>> backward in the chain and allowing me to call super instead and 
>> preventing alias_method explosions. Or would that just end up causing 
>> more havoc?
>
> I don't know whether it qualifies as "monkeypatching" (I always
> thought that meant doing something sloppy and ill-advised, which I
> hope I'm not :-) but see if this helps:
>
>   class FireTruck
>     def put_out_fire(options = {})
>       p options
>     end
>   end
>
>   FireTruck.class_eval do
>     m = instance_method(:put_out_fire)
>     define_method(:put_out_fire) do |options|
>       m.bind(self).call(options.merge({ :nozzle => "big" }))
>     end
>   end
>
>   FireTruck.new.put_out_fire(1 => 2)
>
>   # Now someone else uses the alias:
>   class FireTruck
>     alias old_put_out_fire put_out_fire
>     def put_out_fire(options = {})
>       puts "New version!"
>       old_put_out_fire(options)
>     end
>   end
>
>   FireTruck.new.put_out_fire(1 => 2)
>
> Output:
>
> {1=>2, :nozzle=>"big"}
> New version!
> {1=>2, :nozzle=>"big"}
>
>
> David
>
This is a bit unrelated, but is there a way to give the block parameter 
of define_method a default (because otherwise you now have to send 
put_out_fire at least an empty hash)?

define_method(:put_out_fire) do |options|
  m.bind(self).call(options.merge({ :nozzle => "big" }))
end

-- 
Tom Werner
Helmets to Hardhats
Software Developer
tom@helmetstohardhats.org
www.helmetstohardhats.org