Function callback.add()

Description

Turn this callback into a wrapper for other callbacks.

Your callback function will be called after all of the callback functions you've added are called, and you'll get multiple arguments passed to your callback function (one per callback).

Return Value

As a shortcut, if you don't specify a callback, this will create one for you and then return it. Otherwise it will return the callback object again, so you can use this for method chaining.