Question

I would like to write a method in ruby that takes a class with certain methods and modifies its behavior by adding methods or changing how existing methods work. I would like to do this in a way that doesn’t modify the base class so basically I want a function that takes a class and returns a new modified class without harming the initial class. I’m pretty sure this is possible but I’m not sure where to start.

Answer 1

You have a couple options:

• you can use x = Class.new(Parent) { def meth; puts "hello"; super; puts "bye"; end } to dynamically define a class and override methods (& define new ones)
• you can use a Delegator

So for instance, if you wanted to dynamically create classes that logged certain method calls:

class Class
  def logging_subclass(*methods)
    Class.new(self) do
      methods.each do |method|
        define_method(method) do |*args,&blk|
          puts "calling #{method}"
          ret = super(*args,&blk)
          puts "#{method} returned #{ret.inspect}"
          ret
        end
      end
    end
  end
end

class One
  def foo
    "I'm foo!"
  end
end

# this prints nothing
One.new.foo #=> returns :foo

# this prints:
#   > calling foo
#   > foo returned "I'm foo!"
One.logging_subclass(:foo).new.foo #=> returns :foo

Note that you need ruby 1.9 to support capturing do |&blk| (capturing blocks in block arguments).