ss:`Monitor`.

Usual actions are:

``'add'``
  A device has been added (e.g. a USB device was plugged in)
``'remove'``
  A device has been removed (e.g. a USB device was unplugged)
``'change'``
  Something about the device changed (e.g. a device property)
``'online'``
  The device is online now
``'offline'``
  The device is offline now

.. warning::

   Though the actions listed above are the most common, this property
   *may* return other values, too, so be prepared to handle unknown
   actions!

.. versionadded:: 0.16
N)