I have a UINavigationController into which I push several views. Inside viewDidLoad for one of these views I want to set the self.navigationI
If your end goal is to simply replace the image used for the back button, you can use a new method on UIBarButtonItem available in iOS 5.0:
setBackButtonBackgroundImage:forState:barMetrics:
Apple Docs: http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIBarButtonItem_Class/Reference/Reference.html
Here's a simple example that sets a custom background image for all back buttons in your app:
UIImage *toolbarBackButtonBackgroundPortrait = [[UIImage imageNamed:@"toolbarBackButtonPortrait"] resizableImageWithCapInsets:UIEdgeInsetsMake(0, 17, 0, 6)];
UIImage *toolbarBackButtonBackgroundLandscape = [[UIImage imageNamed:@"toolbarBackButtonLandscape"] resizableImageWithCapInsets:UIEdgeInsetsMake(0, 17, 0, 6)];
[[UIBarButtonItem appearance] setBackButtonBackgroundImage:toolbarBackButtonBackgroundPortrait forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
[[UIBarButtonItem appearance] setBackButtonBackgroundImage:toolbarBackButtonBackgroundLandscape forState:UIControlStateNormal barMetrics:UIBarMetricsLandscapePhone];