inheritable_attributes.rb

require 'active_support/core_ext/object/duplicable'

# Retain for backward compatibility.  Methods are now included in Class.
module ClassInheritableAttributes # :nodoc:
end

# Allows attributes to be shared within an inheritance hierarchy, but where each descendant gets a copy of
# their parents' attributes, instead of just a pointer to the same. This means that the child can add elements
# to, for example, an array without those additions being shared with either their parent, siblings, or
# children, which is unlike the regular class-level attributes that are shared across the entire hierarchy.
class Class # :nodoc:
  def class_inheritable_reader(*syms)
    syms.each do |sym|
      next if sym.is_a?(Hash)
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def self.#{sym}                         # def self.after_add
          read_inheritable_attribute(:#{sym})   #   read_inheritable_attribute(:after_add)
        end                                     # end

        def #{sym}                              # def after_add
          self.class.#{sym}                     #   self.class.after_add
        end                                     # end
      EOS
    end
  end

  def class_inheritable_writer(*syms)
    options = syms.extract_options!
    syms.each do |sym|
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def self.#{sym}=(obj)                          # def self.color=(obj)
          write_inheritable_attribute(:#{sym}, obj)    #   write_inheritable_attribute(:color, obj)
        end                                            # end
                                                       #
        #{"                                            #
        def #{sym}=(obj)                               # def color=(obj)
          self.class.#{sym} = obj                      #   self.class.color = obj
        end                                            # end
        " unless options[:instance_writer] == false }  # # the writer above is generated unless options[:instance_writer] == false
      EOS
    end
  end

  def class_inheritable_array_writer(*syms)
    options = syms.extract_options!
    syms.each do |sym|
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def self.#{sym}=(obj)                          # def self.levels=(obj)
          write_inheritable_array(:#{sym}, obj)        #   write_inheritable_array(:levels, obj)
        end                                            # end
                                                       #
        #{"                                            #
        def #{sym}=(obj)                               # def levels=(obj)
          self.class.#{sym} = obj                      #   self.class.levels = obj
        end                                            # end
        " unless options[:instance_writer] == false }  # # the writer above is generated unless options[:instance_writer] == false
      EOS
    end
  end

  def class_inheritable_hash_writer(*syms)
    options = syms.extract_options!
    syms.each do |sym|
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def self.#{sym}=(obj)                          # def self.nicknames=(obj)
          write_inheritable_hash(:#{sym}, obj)         #   write_inheritable_hash(:nicknames, obj)
        end                                            # end
                                                       #
        #{"                                            #
        def #{sym}=(obj)                               # def nicknames=(obj)
          self.class.#{sym} = obj                      #   self.class.nicknames = obj
        end                                            # end
        " unless options[:instance_writer] == false }  # # the writer above is generated unless options[:instance_writer] == false
      EOS
    end
  end

  def class_inheritable_accessor(*syms)
    class_inheritable_reader(*syms)
    class_inheritable_writer(*syms)
  end

  def class_inheritable_array(*syms)
    class_inheritable_reader(*syms)
    class_inheritable_array_writer(*syms)
  end

  def class_inheritable_hash(*syms)
    class_inheritable_reader(*syms)
    class_inheritable_hash_writer(*syms)
  end

  def inheritable_attributes
    @inheritable_attributes ||= EMPTY_INHERITABLE_ATTRIBUTES
  end

  def write_inheritable_attribute(key, value)
    if inheritable_attributes.equal?(EMPTY_INHERITABLE_ATTRIBUTES)
      @inheritable_attributes = {}
    end
    inheritable_attributes[key] = value
  end

  def write_inheritable_array(key, elements)
    write_inheritable_attribute(key, []) if read_inheritable_attribute(key).nil?
    write_inheritable_attribute(key, read_inheritable_attribute(key) + elements)
  end

  def write_inheritable_hash(key, hash)
    write_inheritable_attribute(key, {}) if read_inheritable_attribute(key).nil?
    write_inheritable_attribute(key, read_inheritable_attribute(key).merge(hash))
  end

  def read_inheritable_attribute(key)
    inheritable_attributes[key]
  end

  def reset_inheritable_attributes
    @inheritable_attributes = EMPTY_INHERITABLE_ATTRIBUTES
  end

  private
    # Prevent this constant from being created multiple times
    EMPTY_INHERITABLE_ATTRIBUTES = {}.freeze unless const_defined?(:EMPTY_INHERITABLE_ATTRIBUTES)

    def inherited_with_inheritable_attributes(child)
      inherited_without_inheritable_attributes(child) if respond_to?(:inherited_without_inheritable_attributes)

      if inheritable_attributes.equal?(EMPTY_INHERITABLE_ATTRIBUTES)
        new_inheritable_attributes = EMPTY_INHERITABLE_ATTRIBUTES
      else
        new_inheritable_attributes = inheritable_attributes.inject({}) do |memo, (key, value)|
          memo.update(key => value.duplicable? ? value.dup : value)
        end
      end

      child.instance_variable_set('@inheritable_attributes', new_inheritable_attributes)
    end

    alias inherited_without_inheritable_attributes inherited
    alias inherited inherited_with_inheritable_attributes
end

class Class
  # Defines class-level inheritable attribute reader. Attributes are available to subclasses,
  # each subclass has a copy of parent's attribute.
  #
  # @param *syms<Array[#to_s]> Array of attributes to define inheritable reader for.
  # @return <Array[#to_s]> Array of attributes converted into inheritable_readers.
  #
  # @api public
  #
  # @todo Do we want to block instance_reader via :instance_reader => false
  # @todo It would be preferable that we do something with a Hash passed in
  #   (error out or do the same as other methods above) instead of silently
  #   moving on). In particular, this makes the return value of this function
  #   less useful.
  def extlib_inheritable_reader(*ivars)
    instance_reader = ivars.pop[:reader] if ivars.last.is_a?(Hash)

    ivars.each do |ivar|
      self.class_eval <<-RUBY, __FILE__, __LINE__ + 1
        def self.#{ivar}
          return @#{ivar} if self.object_id == #{self.object_id} || defined?(@#{ivar})
          ivar = superclass.#{ivar}
          return nil if ivar.nil? && !#{self}.instance_variable_defined?("@#{ivar}")
          @#{ivar} = ivar && !ivar.is_a?(Module) && !ivar.is_a?(Numeric) && !ivar.is_a?(TrueClass) && !ivar.is_a?(FalseClass) ? ivar.dup : ivar
        end
      RUBY
      unless instance_reader == false
        self.class_eval <<-RUBY, __FILE__, __LINE__ + 1
          def #{ivar}
            self.class.#{ivar}
          end
        RUBY
      end
    end
  end

  # Defines class-level inheritable attribute writer. Attributes are available to subclasses,
  # each subclass has a copy of parent's attribute.
  #
  # @param *syms<Array[*#to_s, Hash{:instance_writer => Boolean}]> Array of attributes to
  #   define inheritable writer for.
  # @option syms :instance_writer<Boolean> if true, instance-level inheritable attribute writer is defined.
  # @return <Array[#to_s]> An Array of the attributes that were made into inheritable writers.
  #
  # @api public
  #
  # @todo We need a style for class_eval <<-HEREDOC. I'd like to make it
  #   class_eval(<<-RUBY, __FILE__, __LINE__), but we should codify it somewhere.
  def extlib_inheritable_writer(*ivars)
    instance_writer = ivars.pop[:writer] if ivars.last.is_a?(Hash)
    ivars.each do |ivar|
      self.class_eval <<-RUBY, __FILE__, __LINE__ + 1
        def self.#{ivar}=(obj)
          @#{ivar} = obj
        end
      RUBY
      unless instance_writer == false
        self.class_eval <<-RUBY, __FILE__, __LINE__ + 1
          def #{ivar}=(obj) self.class.#{ivar} = obj end
        RUBY
      end

      self.send("#{ivar}=", yield) if block_given?
    end
  end

  # Defines class-level inheritable attribute accessor. Attributes are available to subclasses,
  # each subclass has a copy of parent's attribute.
  #
  # @param *syms<Array[*#to_s, Hash{:instance_writer => Boolean}]> Array of attributes to
  #   define inheritable accessor for.
  # @option syms :instance_writer<Boolean> if true, instance-level inheritable attribute writer is defined.
  # @return <Array[#to_s]> An Array of attributes turned into inheritable accessors.
  #
  # @api public
  def extlib_inheritable_accessor(*syms, &block)
    extlib_inheritable_reader(*syms)
    extlib_inheritable_writer(*syms, &block)
  end
end