def METHOD_NAME(self, size):
    """
    The application has informed us that it has processed a certain number
    of bytes. This may cause us to want to emit a window update frame. If
    we do want to emit a window update frame, this method will return the
    number of bytes that we should increment the window by.
    :param size: The number of flow controlled bytes that the application
        has processed.
    :type size: ``int``
    :returns: The number of bytes to increment the flow control window by,
        or ``None``.
    :rtype: ``int`` or ``None``
    """
    self._bytes_processed += size
    return self._maybe_update_window()