CODEWITHSUNDEEP
ruby-on-railsrubyinheritance
Frank Koehl
Frank Koehl

Reputation: 3186

Define method calls for class declarations, a la Rails

I'm building a domain-specific hierarchical inheritance structure similar to ActiveRecord, i.e. I have a Base class, and then n children classes declared under Base.

I'm currently declaring specifics in the children as constants, and then calling those from the shared methods in the Base class, but it's getting unwieldy. I would like to implement the Rails style of declaring class-specific, instantiated values via method calls.

If you're familiar Rails, I'm essentially trying to duplicate the generic structure of has_many/belongs_to from ActiveRecord, or before_action from ActionPack.

A simplistic, contrived example of what I'm trying to accomplish...

class Widget < Base

  important_value :foo, :bar

end

widget = Widget.new
widget.foo
# => :bar

If someone can explain to me what to put in class Base to achieve the above, I'll be well on my way.

Upvotes: 0

Views: 57

Answers (2)

Frank Koehl
Frank Koehl

Reputation: 3186

This doesn't precisely match the conditions of my original question, but it was extremely helpful in moving me along.

class Base

  # defining variables happens at the class level
  # the first line allows us to set a sane default (skip for a nil value)
  # the second line makes the method a getter as well as a setter (this is required)
  def self.title(val = nil)
    @title ||= 'DEFAULT TITLE'
    return @title if val.nil?
    @title = val.upcase
  end

  # the instance-level reader
  # if you want to change the values from the child, use attr_accessor
  # in either case, the visibility of @title in the child is unchanged (we assume its not accessed directly)
  attr_reader :title

  # set the instance variable when the class is instantiated
  def initialize
    instance_variable_set("@title", self.class.title)
  end

end

class Foo < Base
  title "foo title"
end

class Bar < Base
  # if left commented out, the value will be set to 'DEFAULT TITLE'
  # title "BAR TITLE" 
end

f = Foo.new
f.title
# => "FOO TITLE"

b = Bar.new
b.title
# => "DEFAULT TITLE"

Upvotes: 0

rmosolgo
rmosolgo

Reputation: 1874

class Base 
  class << self
    def important_value(key, value)
      # define method named `key` which returns `value`
      define_method(key) { value } 
    end 
  end 
end 

class Widget < Base 
  important_value :foo, :bar 
end 

Widget.new.foo # => :bar

Or, if the number of "important value" methods are small and known beforehand: 

class Base 
  def foo 
    self.class.foo 
  end 

  class << self
    attr_reader :foo 

    def important_value(key, value)
      self.instance_variable_set(:"@#{key}", value)
    end 
  end 
end 

class Widget < Base 
  important_value :foo, :bar 
end 

Widget.new.foo # => :bar

Upvotes: 1

Related Questions