I implemented a custom iOS control (looking similar to a ruler), where the user can easily scroll to select one of about 300 values.
My control also has the trait UIAccessibilityTraitAdjustable and implements the methods accessibilityIncrement() and accessibilityDecrement() of the protocol UIAccessibilityAction to update its value. So far everything works as intended.
The problem is that adjusting the value via VoiceOver can be tedious, since each swipe gesture increments/decrements the value only by one. Is there a good way to offer a second mode where the updates happen, e.g. in steps of ten?
UIAccessibilityTraitAdjustable uses a fixed, developer-defined increment size. You can adjust the slider with a larger step size in -accessibilityIncrement if you think users will find it tedious to adjust the control one unit at a time. VoiceOver users desiring finer control can use the pass-through gesture (double tap and hold) to interact with the control directly, sliding to adjust it in single-percent increments. This does, however, assume users have fine motor control.
If you feel strongly that UIAccessibility should support a "fine-grain" adjustment mode, file an enhancement request with supporting use cases.