This one is equally good for neuropsychologists and their clients. Unfortunately, as it runs on Android smartphones, so I'll have to search for an iPhone equivalent.
StayOn Task app beeps at you at unpredictable intervals and you have to touch either the 'on task' or 'off task' button to get it quiet again. As we all know, monitoring a behaviour improves it, so it should result in more on-task behaviour.
As far as I am aware this app does not record data - it is just meant to remind you to keep on task. The frequency of alarms can be adjusted according to whether you chose 'on task' or 'off task' last time it checked.