they are interacting with the service through the devices, not with a bunch of
separate devices. There are two key components here. Data and content must
be synchronized, and cross-device interactions must be clearly signposted. In
my experience, some of the biggest usability challenges in IoT are continuity
issues.
Data and content synchronization
It sounds obvious that different device UIs should each give the same
information on system state.
Kindle Whispersync is a great example of synchronization. You can switch
between reading on different devices - even swapping between the book and
the audiobook - and your place in the book is always up to date (see figure
9.27).