'Add a callback -- a function that takes no parameters -- that will
return a list of filenames to watch for changes.'

def add_file_callback(self, cls, callback):

    if (self is None):
        for instance in cls.instances:
            instance.add_file_callback(callback)
        cls.global_file_callbacks.append(callback)
    else:
        self.file_callbacks.append(callback)
