An alphanumeric string unique to each device, used only for serving advertisements.
identifierForVendor property of the
UIDevice, the same value is returned to all vendors. This identifier may change—for example, if the user erases the device—so you should not cache it.
If the value is
nil, wait and get the value again later. This happens, for example, after the device has been restarted but before the user has unlocked the device.