Challenge:

Concurrency, competing user/signal interactions, responsiveness.

Criteria:

• A frame containing a gauge for an elapsed time.
• A label which shows the elapsed time as a numerical value.
• A slider by which the duration of the timer can be adjusted while the timer is running.
• A reset button.
• Adjusting the slider must immediately reflect on the duration & gauge (not only when the slider is released).
• When the elapsed time ≥ duration, the timer stops (and the gauge will be full). If, thereafter, the duration is increased such that the duration > elapsed time, the timer starts to tick until elapsed time ≥ duration again.
• Clicking the reset button will reset elapsed time to zero.

Timer deals with concurrency in the sense that a timer process that updates the elapsed time runs concurrently to the user’s interactions with the GUI application. This also means that the solution to competing user and signal interactions is tested. The fact that slider adjustments must be reflected immediately moreover tests the responsiveness of the solution. A good solution will make it clear that the signal is a timer tick and, as always, has not much scaffolding.

Timer is directly inspired by the timer example in the paper Crossing State Lines: Adapting Object-Oriented Frameworks to Functional Reactive Languages.