I have some questions regarding navigation bars and their usage in iOS apps, both from the point of view of UX and Apple´s submission and human interface guidelines:
1) Since the iOS Human Interface Guidelines state that
A navigation bar enables navigation through an information hierarchy and, optionally, management of screen contents.
You can use a navigation bar to enable navigation among different views, or provide controls that manage the items in a view.
should I assume that using a navigation bar in a view with no information hierarchy is not allowed, or just that it is not necessary? (For example, you have a tab bar and one of the tabs displays a player view or another single view, and to navigate to other view you have to tap another tab item).
2) It is also said in the Apple's document that
The navigation bar should contain no more than a view’s current title, the back button, and one control that manages the view’s contents
2.a) I've found Apple's apps "breaking" the rule of the back button at the left side (Calendar, iTunes), so... can I consider that, when they say "should" in their guidelines, it is just a recommendation but if I do things another way they won't reject my app?
2.b) I may need a button to be always visible in order to let users trigger its associated action at any moment, but I already have a tab bar at the bottom so I have to place such button within the nav bar (since I can't add a tool bar... or can I?). Is it required that such action always has "visibility" in the current view? (As, for instance, the button in the Twitter app that displays a "New Tweet" modal view whenever you want). For example, I'd want to set a button in the navigation bar for updating some data, and I'd want to have such button in all views in the app. But could it be that the data uploaded is not beign displayed in the current view but in another, or in another tab.