Re: Using any trick to omit () in constant lookup?

What you are asking for is possible, simply by overriding the new 
method:

class GTKButton; end
class OldButton; end

class Button
  def self.new(*args,&blk)
    if $use_gtk
      GTKButton.new(*args,&blk)
    else
      OldButton.new(*args,&blk)
    end
  end
end

$use_gtk = true
puts Button.new.class

This is unusual, and it may surprise users of your library to call 
Button.new and actually receive an object of a different class. But it's 
not unreasonable - I did this in a ruby-ip library once, so that 
IP.new(...) would return either an IP::V4 object or an IP::V6 object 
depending on the argument format.

Gunther's solution is the more general and flexible one. It allows your 
Button wrapper to be composed of one or more other objects, which can be 
changed dynamically as the object runs. A method call x(a,b,c) on your 
wrapper object can then be mapped to whatever calls are required on the 
underlying objects, e.g. @obj1.y(a) and @obj2.z(c,b)

I suspect you will want to do both: return different classes for each 
type of Button, and within those classes, map the method calls to an 
underlying object or objects. This will allow your Button facade objects 
to implement the same API calls and translate them to what is actually 
required.

This is of course only part of the story. If you want your UI toolkit to 
be interchangeable then it will have to expose the same programming 
model and data structures, and unifying that across the various 
underlying libraries may be more difficult than you expect.

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