information for developers on making sliders accessible
Look for the "Developers" link on each AppleVis page and you will find a page of resources with accessibility information for developers (for example, here.) one of the best general overview articles, that can be read by lay users and developers alike, is the blog article by Matt Legend Gemmell. For insight into how things get fixed on a more "techie" level, I always liked the early Stanford University iTunes U course on iOS development that included a lecture by Chris Fleizach showing how to fix an actual app being designed to make it accessible. These are all quite old examples. The iTunes U course was in Spring 2009 (and back when it was still possible to point to individual podcast episodes in iTunes). They were great to view, because everything was so new at that point that the explanations had to set a context and work for all listeners, because there were very few experts at the time. So, even though the examples were simple, you could follow along what was being done.
How can I implement for the two methods below to change adjustable controls with VoiceOver?
1. Simply swipe up/down with a one finger.
2. Double tap and hold with one finger, wait for VoiceOver sound effect, and slide your finger up/down.
How do you implement for the second behavior?
I'm not an app developer, but I found <a href="https://developer.apple.com/documentation/objectivec/nsobject/1615076-a… information on incrementing a control</a> and <a href="https://developer.apple.com/documentation/objectivec/nsobject/1615076-a… information on decrementing a control.</a> I hope this is what you were looking for. A VoiceOver user can increment or decrement a control in this way by flicking up or down.
I discourage use of the double-tap, hold and flick gesture because it is very difficult to perform correctly. If you want to increment or decrement a control by a smaller element than by flicking, consider having "increment" and "decrement" buttons that allow the user to manipulate the slider by a more specific amount.
If you want more specific information on programming accessibility for iOS, don't forget <a href="developer.apple.com/accessibility/ios">Apple's official information</a>.