From: Massimiliano Mirra - bard <mmirra@...>
Date: 2003-08-27T05:48:48+09:00
Subject: Re: Aspect oriented Everything?

Mark Wilson <mwilson13@cox.net> writes:

> I have yet to read a description of what aspect oriented development is 
> and how one would use it, that I could understand (i.e., with a code 
> example that explains what is being done). Could someone provide such a 
> description?

Disclaimer: I've never used AspectJ nor AspectR, nor do actually know
much more about AOP than what has been said in this thread in in
earlier ones in this group.  The following was whipped together in
about ten minutes after seeing Dan's post; if you find stupidity atoms
in it, they probably all belong to me.


module Wrappable
  def Wrappable.append_features(c)
    c.public_instance_methods.each do |method|
      c.class_eval "
        alias_method :unwrapped_#{method}, :#{method}
        remove_method :#{method}
        def #{method}(*args)
          call_prewrap(:#{method}, *args)
          result = unwrapped_#{method}(*args)
          call_postwrap(:#{method}, result)
          result
        end
"
    end
  end
end

module Aspectable
  def add_prewrap(method_id, &block)
    @prewraps ||= {}
    @prewraps[method_id] ||= []
    @prewraps[method_id] << block
  end

  def add_postwrap(method_id, &block)
    @postwraps ||= {}
    @postwraps[method_id] ||= []
    @postwraps[method_id] << block
  end

  def call_prewrap(method_id, *args)
    @prewraps ||= {}
    @prewraps[method_id].each { |wrap| wrap.call(method_id, *args) }
  end

  def call_postwrap(method_id, *args)
    @postwraps ||= {}
    @postwraps[method_id].each { |wrap| wrap.call(method_id, *args) }
  end
end


class Test
  def greet(someone)
    puts "Hello, #{someone}!"
  end
end

t = Test.new
t.greet('world')

# >> Hello, world!


class Test
  include Wrappable
  include Aspectable
end
log_enter = proc { |method, *args| STDERR.puts "Now entering '#{method}' with args #{args.inspect}" }
log_exit  = proc { |method, result| STDERR.puts "Now exiting '#{method}' with result #{result.inspect}" }


t = Test.new
t.add_prewrap(:greet, &log_enter)
t.add_postwrap(:greet, &log_exit)
t.greet('matz')

# >> Now entering 'greet' with args ["matz"]
# >> Hello, matz!
# >> Now exiting 'greet' with result nil


Massimiliano


p.s.: Yes, the modules are tightly coupled and that is bad, I didn't
find a way to have it work with a single module (not that I've
searched that hard...).