UITabBarController's new prominentTabIdentifier property is a really nice addition! Thank you!
Will there be official support via the UITab API for using the underlying _UITabBarAuxiliaryView as a primary action as many apps, such as the Apple Design Award Finalist, Structured, does for presenting a sheet, for example?
A prominent tab provides additional emphasis to an important destination in your app. Use it to highlight a persistent, top-level destination that users may need to access frequently or monitor at a glance.
Prominent tabs should elevate destinations, not actions. Users should be able to move in and out of the destination naturally, just as they would any other tab.
If tapping the item primarily performs an action, opens a modal experience, or temporarily reveals functionality, it is generally not a good candidate for a prominent tab.
