Module::ExpandConstant

expand_constant.rb

# frozen_string_literal: true

require 'concurrent/map'

# = Expand Constant Namespace
#
# To expand namespace in classic way it have to be included or extended
# to a target module or class.
#
# This module provides +expand_constant+ method to expand external module namespace
# only inside a block also (with support of nesting) without copying all constants
# to a target. It is implemented by <tt>Module#const_missing</tt>.
#
# @example
#
#   require 'module/expand_constant'
#
#   Module.prepend Module::ExpandConstant
#
#   module Foo
#     expand_constant 'Arel::Nodes' do
#       p Node # => Arel::Nodes::Node
#
#       expand_constant 'ActiveRecord' do
#         p Node      # => Arel::Nodes::Node
#         p Relation  # => ActiveRecord::Relation
#       end
#
#       p Node     # => Arel::Nodes::Node
#       p Relation # => raise NameError: Foo::Relation
#     end
#
#     p Node # => raise NameError: Foo::Node
#   end
#
module Module::ExpandConstant
  # @return [Concurrent::Map]
  def self.cache
    @cache ||= Concurrent::Map.new
  end

  # @return [Boolean]
  def self.allowed?(klass, name)
    !!@cache.get([klass.object_id, name].hash)
  end

  TEMPLATE = <<~RUBY
    def const_missing(name)
      return ::%1$s.const_get(name) if ::Module::ExpandConstant.allowed?(self, %1$p) && ::%1$s.const_defined?(name)
      return super if defined?(super)
      const_get(name)
    end
  RUBY

  # @!visibility public
  # @param mod [String] namespace module name
  # @return [void]
  def expand_constant(mod)
    mixin = Module.new { class_eval(TEMPLATE % mod) }
    input = [object_id, mod].hash
    begin
      ::Module::ExpandConstant.cache.put(input, true)
      singleton_class.prepend(mixin)
      yield
    ensure
      ::Module::ExpandConstant.cache.put(input, false)
    end
  end

  private_constant :TEMPLATE
end