It should:

• help position CupertinoTabBar and CupertinoNavigationBar to the right places
• let content be between them or behind them depending on their opacity
• automatically create pageviews and navigators per each tab if there is a CupertinoTabBar
• handle back gestures per navigator
• coordinate animations between the page transition in CupertinoPageTransition and a parallax-fade transition on the app bar (moved to Create an app bar parallax transition effect for iOS #9446)
• have an API to preserve or discard offscreen inactive tabs