ios Navigation Bar Usage

Question

Is it acceptable (i.e., to App Store) to have buttons in a UINavigationBar change on-the-fly? For example, perhaps there are two buttons on the Nav Bar for + (Add) and Trash (Delete), but once the + has been touched (for adding an item) the buttons change to Cancel/Save?

A related question, assuming it is acceptable, is whether it is preferable to activate/deactivate buttons as required, or whether it would be better better to make buttons that are not relevant to the operation underway simply disappear to be replaced by those which are relevant at any given time?

Not sure I've made myself clear here, but I haven't seen anything on this in the HIG. Thx.

Answer 1

I don't think there's anything wrong with changing buttons on UINavigationBar. I even have an app published to AppStore (and many other apps too) implementing such feature and Apple's ok with that.