FingerFiddle

FingerFiddle is one of the best music instrument applications I've ever purchased on any platform, especially considering its low cost in comparison to its high quality.

It's the only physically-modeled bowed instrument software that has a natural-feeling performance GUI, and that's largely due to being developed for iOS and touch screen interaction (the larger the screen, the better, and I recommend it on an iPad Pro 12.9"). Playing FingerFiddle is leaps and bounds beyond any other bowed instrument software on any platform that I've tried. It even simulates pressure by the rarely utilized touch-surface-area feature of iOS touch screen devices (the harder you press on the screen, the larger the contact area is, as your finger squishes out onto the glass).

The developer has always been responsive to my inquiries.

It's a shame so few people seem to know about or have interest in it.