Question

Is there any clean way to initialize instance variables in a Module intended to be used as Mixin? For example, I have the following:

module Example

  def on(...)   
    @handlers ||= {} 
    # do something with @handlers
  end

  def all(...)
    @all_handlers ||= []
    # do something with @all_handlers
  end

  def unhandled(...)
    @unhandled ||= []
    # do something with unhandled
  end

  def do_something(..)
    @handlers     ||= {}
    @unhandled    ||= []
    @all_handlers ||= []

    # potentially do something with any of the 3 above
  end

end

Notice that I have to check again and again if each @member has been properly initialized in each function — this is mildly irritating. I would much rather write:

module Example

  def initialize
    @handlers     = {}
    @unhandled    = []
    @all_handlers = []
  end

  # or
  @handlers  = {}
  @unhandled = []
  # ...
end

And not have to repeatedly make sure things are initialized correctly. However, from what I can tell this is not possible. Is there any way around this, besides adding a initialize_me method to Example and calling initialize_me from the extended Class? I did see this example, but there’s no way I’m monkey-patching things into Class just to accomplish this.

Answer 1

module Example
  def self.included(base)
    base.instance_variable_set :@example_ivar, :foo
  end
end

Edit: Note that this is setting a class instance variable. Instance variables on the instance can’t be created when the module is mixed into the class, since those instances haven’t been created yet. You can, though, create an initialize method in the mixin, e.g.:

module Example
  def self.included(base)
    base.class_exec do
      def initialize
        @example_ivar = :foo
      end
    end
  end
end

There may be a way to do this while calling the including class’s initialize method (anybody?). Not sure. But here’s an alternative:

class Foo
  include Example

  def initialize
    @foo = :bar
    after_initialize
  end
end

module Example
  def after_initialize
    @example_ivar = :foo
  end
end