Overridable is a pure ruby library which helps you to make your methods which are defined in classes to be able to be overrided by mixed-in modules.

Some people are overusing alias_method_chain in their codes like what wycats mentioned in his post. One of the reasons that people like using alias_method_chain is because it's impossible for modules to override methods that are defined in classes when they are included. For example: class Thing def foo 'Thing.foo' end end

module Extension
  def foo
    'Extension.foo'
  end
end

Thing.send :include, Extension
Thing.new.foo #=> Thing.foo # not Extension.foo

In order to achieve that goal, some will write the Extension module like this: module Extension def foo_with_redef 'Extension.foo' end # you can also do this by: alias_method_chain :foo, :redef, if you use ActiveSupport. alias foo_without_redef foo alias foo foo_with_redef end

Thing.send :include, Extension
Thing.new.foo #=> Extension.foo

But according to the post, this is a bad practice. And overridable is a gem that provides a neat mean to resolve this problem.

There are two ways to do this with overridable: in class or in module.

Remember Thing and Extension in our first example? Let's make some changes: require 'overridable'

Thing.class_eval {
  include Overridable
  overrides :foo
  
  include Extension
}

Thing.new.foo #=> Extension.foo

That's it! You can specify which methods can be overrided by overrides method. One more example based on the previous one: Thing.class_eval { def bar; 'Thing.bar' end def baz; 'Thing.baz' end def id; 'Thing' end

  overrides :bar, :baz
}

Extension.module_eval {
  def bar; 'Extension.bar' end
  def baz; 'Extension baz' end
  def id;  'Extension'     end
}

thing = Thing.new
thing.bar #=> 'Extension.bar'
thing.baz #=> 'Extension.baz'
thing.id  #=> 'Thing'

Of course it's not the end of our story ;) How could I call this override if we cannot use super? Go on with our example: Extension.module_eval { def bar parent = super me = 'Extension.bar' "I'm #{me} and I overrided #{parent}" end }

Thing.new.bar => I'm Extension.bar and I overrided Thing.bar

If you have many methods in your module and find that it's too annoying to use overrides, then you can mix Overridable::ModuleMixin in your module. Example ( we all like examples, don't we? ): class Thing def method_one; ... end def method_two; ... end ... def method_n; ... end end

module Extension
  include Overridable::ModuleMixin

  def method_one; ... end
  def method_two; ... end
  ...
  def method_n;   ... end
end

Thing.send :include, Extension #=> method_one, method_two, ..., method_n are all overrided.

Since version 0.3.1, you can use overrides in your module to specify which method should be overrided and which should not. Let's rewrite the Extension module above: module Extension include Overridable::ModuleMixin overrides :only => [:method_one, :method_two]

  # define methods here...
end

Thing.send :include, Extension #=> only method_one and method_two will be overrided.

You can also use overrides :except => [:method_one, :method_two] to tell the module not to override method_one and method_two in the classes which include it.

gem source -a http://gemcutter.org # you neednot do this if you have already had gemcutter in your source list
gem install overridable

• Ruby >= 1.8.7
• riot >= 0.10.0 just for test
• yard >= 0.4.0 just for generating document

$> cd $GEM_HOME/gems/overridable-x.y.z
$> rake test # requires riot

These features only will be added when they are asked for.

• :all and :except for overrides overrides :all, :except => [:whatever, :excepts]
• override with block
  override :foo do |*args| super # or not # any other stuff end
• tell me what's missing.

• Fork the project.
• Make your feature addition or bug fix.
• Add tests for it. This is important so I don't break it in a future version unintentionally.
• Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
• Send me a pull request. Bonus points for topic branches.

Copyright (c) 2009 梁智敏. See LICENSE for details.