A one-shot sensor is triggered once after it is scheduled, when the delay queue is processed. Like all delay queue sensors, one-shot sensors with a non-zero priority are just added to the delay queue when scheduled; if they are scheduled again before the delay queue is processed nothing happens, and they are guaranteed to be called only once when the delay queue is processed. For example, a one-shot sensor whose delegateredraws the scene might be scheduled whenever the scene graph changes and whenever a window-system event reporting that the window changed size occurs. By using a one-shot, the scene will only be redrawn once even if a window-changed-size event occurs just after the scene graph is modified (or if several window-changed-size events occur in a row).

Calling schedule() in the delegateis a useful way of getting something to happen repeatedly as often as possible, while still handling events and timeouts.

A priority 0 one-shot sensor isn't very useful, since scheduling it is exactly the same as directly calling its delegate.

Reference