How to simulate Java-like annotations in Ruby?

Question

How to simulate Java-like annotations in ruby?

(We\'ll I have the answer, generalizing http://bens.me.uk/2009/java-style-annotations-in-ruby)

Answer 1

This is the intended usage:

First you annotate a class.

class A

  extend Annotations

  extend MyAnnotations

  create_annotation("_foobar")

  _hello({:color=>'red', :ancho=>23})
  _goodbye({:color=>'green', :alto=>-123})
  _foobar({:color=>'blew'})
  def m1
  end

  def m2
  end

  _foobar({:color=>'cyan'})
  def m3
  end
end

Then you would like do inspect A's annoations like this:

anots = A.annotations
puts anots.keys

puts anots[:m1][:_hello][:color]
puts anots[:m3][:_foobar][:color]

puts anots[:m1].key?(:_goodbye)

puts "---------------"

anots.each do |met| # each annotated method
  puts "-- annotated method --"
  puts met[0] # method name
  met[1].each do |a| # each annotation for the method
    puts "-> " + a[0].to_s # annotation name
    a[1].each do |par| # each pair: key-value
      puts " key=" +   par[0].to_s + " value=" + par[1].to_s
    end
  end
end

Well. To do that, you will need this module

module Annotations

  @@annotation_list = {}
  @@pending = {}

  def method_added(met_sym)
    #puts "-> adding " + met_sym.to_s + " to class + self.to_s
    if @@pending.size > 0
      #puts met_sym.to_s + " is annotated "
      @@annotation_list[met_sym] = @@pending
      #puts @@annotation_list
    else
      #puts met_sym.to_s + " is not annotated "
    end
    @@pending = {}
  end

  def annotate_method(a,b)
    @@pending[a] = b
  end

  def create_annotation(anot_sym)
    code = "def  #{anot_sym.to_s}(val)
      annotate_method( :#{anot_sym} ,val)
      end"
    instance_eval code
  end

  def annotations
    return @@annotation_list
  end

end

and you can define a set of annotations in a module of yours:

module MyAnnotations

  def _goodbye(val)
    annotate_method(:_goodbye, val)
  end

  def _hello(val)
    annotate_method(:_hello, val)
  end
end

or define them right into the class you are annotating:

create_annotation("_foobar")