async preloading cache with code rewriting

aync_preloader.rb

# Ad-hoc code (no library)
#
module GemUtils

  def self.gem_names  # version 1 : 1st call
    GEM_NAMES_PRELOADER.join.value  # **1 : wait and return this if the cache is not ready
  end

private

  GEM_NAMES_PRELOADER = Thread.new do
    @@_cached_gem_names = `gem list --no-version` # version 1 : slow

    def GemUtils.gem_names     # version 2 : fast
      @@_cached_gem_names      # **2
    end                         #

    @@_cached_gem_names  # for **1
  end
end

############
# Usage example :
############

3.times do
  start = Time.now
  puts GemUtils.gem_names.size.to_s + " " + (Time.now - start).to_s
end

# 204 3.858853
# 204 6.0e-06
# 204 4.0e-06

with_library.rb

# extracted from code above : cattr_reader_preloaded
#
module Kernel

  # Usage :   
  #   cattr_reader_preloaded :gem_names do
  #     `gem list --no-version`
  #   end
  #  
  def cattr_reader_preloaded(name, &block)
    raise "initialization block is missing" unless block_given?
    cache     = "@_cached_#{name}"   .to_sym
    preloader = "@_#{name}_preloader".to_sym
 
    self.class.send :define_method, name, -> {
     instance_variable_get(preloader).join.value
    }
 
    instance_variable_set(preloader, Thread.new do
        instance_variable_set cache, block.call
        self.class.send :define_method, name, -> {
          instance_variable_get cache
        }
        instance_variable_get cache
    end)
  end
  end

end


############
# Usage example :
############

module GemUtils
  cattr_reader_preloaded :gem_names do
    `gem list --no-version`
  end
end

3.times do
  start = Time.now
  puts GemUtils.gem_names.size.to_s + " " + (Time.now - start).to_s
end

# 204 3.858853
# 204 6.0e-06
# 204 4.0e-06

module GemUtils
  cattr_reader_preloaded :slow   do sleep 1 ; "AAAA" end
  cattr_reader_preloaded :slow_2 do sleep 1 ; "BBBB" end
end

3.times { start = Time.now; puts GemUtils.slow   + " " + (Time.now - start).to_s }
3.times { start = Time.now; puts GemUtils.slow_2 + " " + (Time.now - start).to_s }

# AAAA 1.001
# AAAA 5.0e-06
# AAAA 4.0e-06
# BBBB 5.0e-06
# BBBB 4.0e-06
# BBBB 4.0e-06