Module: Templatable

Defined in:
lib/classy/templatable.rb

Overview

Templatable allows a class to set default variables for its instances.

Example

class Widget
  extend Templatable

  templatable_attr :awesomeness, :temperature
  awesomeness :total
end

Widget.awesomeness    # => :total
Widget.temperature    # => nil

doodad = Widget.new
doodad.awesomeness    # => :total
doodad.temperature    # => nil

# New defaults affect existing instances.
Widget.temperature = :cool
Widget.temperature    # => :cool
doodad.temperature    # => :cool

# Instances can override the defaults.
doodad.awesomeness = nil
doodad.temperature = :cool
doodad.awesomeness    # => nil
doodad.temperature    # => :cool

Note

The default values are stored as class variables of the same name as the associated instance variables. This may lead to some surprising results, eg, if you set a default value on a subclass.

Constant Summary collapse

@@ever_been_set =

A hash to track templatable variables that have been locally set. (Required to distinguish between variables explicitly set to nil and those left as nil by default.)

:nodoc:

Hash.new { |hash, key| hash[key] = {} }

Instance Method Summary collapse

Instance Method Details

#templatable_attr(*symbols) ⇒ Object

Defines one or more templatable attrs, which will add instance methods similar to Ruby’s standard attr_accessor method, and will also add class methods to set or get default values, as described above.



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# File 'lib/classy/templatable.rb', line 51

def templatable_attr (*symbols)
  symbols.each do |symbol|
    # define the instance setter method
    #
    define_method("#{symbol}=".to_sym) do |value|
      instance_variable_set("@#{symbol}", value)
      @@ever_been_set[self][symbol] = true
    end

    # define the instance getter method
    #
    define_method(symbol) do 
      if @@ever_been_set[self][symbol]
        return instance_variable_get("@#{symbol}")
      elsif self.class.class_variable_defined?("@@#{symbol}")
        return self.class.class_exec { class_variable_get("@@#{symbol}") }
      else
        return nil
      end
    end

    class_exec do
      # define the class setter method
      #
      # We have to use this send hack, since define_method is private.
      #
      self.class.send(:define_method, "#{symbol}=".to_sym) do |value|
        class_variable_set("@@#{symbol}", value)
      end

      # define the class getter/setter method
      #
      # We want to be able to call this like a dsl as a setter, or like a
      # class method as a getter, so we need variable arity, which
      # define_method doesn't support.  In order to use the standard `def`
      # with variable arity, eval'ing a string seems to be the only option.
      #
      # Oh man would I love for somebody to submit a patch to do this in a
      # less horrible way.
      #
      self.class.class_eval "
        def #{symbol} (value = nil)
          if value
            class_variable_set(\"@@#{symbol}\", value)
          end
          class_variable_get(\"@@#{symbol}\") if class_variable_defined?(\"@@#{symbol}\")
        end
      "
    end
  end
end