I have an iOS screen that is usually used for data entry but is occasionally used to later view the data that's been entered.
For data entry, it seems like the iOS convention (e.g. stock email app) is to use a button (e.g. Cancel) in the upper-left and a Submit button in the upper-right.
But for data viewing (e.g. stock camera roll app), the convention is for the upper-left button to be a navigation button that looks like a backwards-pointing arrow.
For a screen which is sometimes viewing and sometimes editing, should I use a navigation button in the upper-left, a Cancel button in the upper-left, or should I change the button from navigation arrow button to a Cancel button after the user makes any changes on the screen?