Shuriken.defineExtension (baseNS) ->
  baseNS.defineMixin 'Callbacks', (mixin) ->

    mixin.callbacks = {}

    mixin.defineCallback = (key) ->
      @["on#{key}"]    = (callback) -> @hasCallback key, callback
      @["invoke#{key}"] = (args...)  -> @invokeCallbacks key, args...
      true

    mixin.hasCallback = (name, callback) ->
      callbacks = mixin.callbacks[name]?= []
      callbacks.push callback
      true

    mixin.callbacksFor = (name) ->
      existing = mixin.callbacks[name]
      if existing? then existing else []

    mixin.invokeCallbacks = (name, args...) ->
      for callback in mixin.callbacksFor(name)
        return false if callback(args...) is false
      true