Most user interfaces are implemented using objects and event handlers associated with objects. At a given time, a subset of the objects is able to accept certain events. Since an event handler processes user inputs and generates new information for the user, it forces the user interface into a state, which can be the same as or different from the original one. Therefore, event handlers with the corresponding triggering user inputs and events correspond to the transition arcs of the STD. A user delay is incurred as soon as the event is generated. Therefore, the user interface knows when user delays occur. If we assume an event handler transitions the user interface into a unique state, an STD is naturally defined by the event handlers. Such an STD is actually the most detailed STD that can be extracted for the object/event handler based implementation. It provides the complete system-user interaction description. Note that an event handler in the implementation may correspond to more than one transition arc since the same event handler can be activated from different states. Therefore, all the arcs entering a state correspond to the same event handler.
If we only care about some specific user delays during system-user
interaction or it is hard to infer the complete STD, we can
identify event handlers which lead to important states only.
Instead of a complete STD, we will then have a set of its
snapshots. Since the user delay of a state is always associated
with the corresponding event handler, such a set of snapshots is
still useful for knowing when and how long these user delays will
be. Moreover, the complete STD can be compacted by clustering
similar states into a single state, based on content or profiling.
For content-based clustering, let denote the set of
objects for state which are able to accept events, and
denote the set of events that object
is able to accept. We define the content set for state