Why my UIViewController's view is being unloaded when visible?

Question

This issue never happened to me. I have an UIViewController inside an UINavigationController. When a memory warning is received (nevermind the level), the viewDidUnload method of the visible controller is called, so the view is unloaded and I get an awesome black screen (with a navigation bar at the top).

I'm testing with an iPad 1 on iOS 4.3.3.

Any suggestions?

Answer 1

From what I understand, the viewDidUnload method is called by didRecieveMemoryWarning function in the UIViewController (the super class). Basically iOS gives you couple of warnings and expect to see your memory usage go down. If you continue to ignore these, OS will kill your app.

Sometimes, though, it is critical to keep some views up and running so the way I get around this is to simply override the didRecieveMemoryWarning method and inside it, don't do anything.

Or better yet, check if self is the current view in the self.navigationController.visibleViewController, and if so, don't pass the memory warning call down to [super didRecieveMemoryWarning].

If you are holding image caches or something, just empty those instead.

HTH

Answer 2

According to Apple memory management guidelines when a viewcontroller recieves memory warnings in critical situations it directly calls viewDidUnload so that memory can be managed by releasing the view.

Its actually ios providing chance to purge your temporary data which wil be useful while recreating the view. Since your UIViewCotroller is the root viewcontroller of the navigationcontroller you see oly navigationbar the view gets unloaded.

Answer 3

You receive viewDidUnload in low memory situation on controllers, where iOS has determined that the views are not longer needed. Remember that Apple made some improvements on the implementations on later versions of iOS, so it might be worse seeing what happens under 5.x. Second you should review your view controller hierarchy.