For a long time Apple iPads and iPhones have been very difficult (often impossible) to link any sort of pointer device to that isn’t a finger.
Two very promising solutions have been announced due for release this year (2019). The AbleNet Sidekick and the CSS AMAneo BTi .
Both allow for wired mice type devices to connect. That could be track-balls, some head-trackers, foot mice and so on. The AMAneo has tremor filtering if needed, for people who wobble a bit.
Initial news via: Ian Bean Twitter feed. Update below via Dad in a Shed.
AMAneo BTi mouse adapter for iPad.