Would a UIViewController currently in UINavigationContoller's viewController stack be unloaded?

Also would anybody tell me what's the difference between viewDidUn开发者_如何学编程load and dealloc?

viewDidUnload : called during low-memory conditions when the view controller needs to release its view and any objects associated with that view to free up memory. More

dealloc: Deallocates the memory occupied by the receiver, An object’s dealloc method is invoked indirectly through the release NSObject protocol method. More

Yes, it could be unloaded if a memory warning occurs.

As for the difference between viewDidUnload and dealloc - the former is called when your view is unloaded, typically because of a low memory situation. The latter is called when your object's retain count reaches zero (i.e. it is completely released from memory)

UIViewControllers are never unloaded. The UIViews they own can be.

So, if you question is if a UIView can be unloaded although its controller has been pushed on to the navigation controller, the answer is yes. Only the currently displayed UIView will not be unloaded (if you don't prevent the unloading mechanism from working).

Furthermore, viewDidUnload is a message that is sent to a UIViewController when the view it manages has been unloaded, which usually is not the same as deallocating the view. In fact, when a view is actually deallocated, it is unloaded for sure, but viewDidUnload is not sent.

A UIViewController that has been pushed onto a nav controller stack, and hasn't yet been popped, will not be dealloc'd. However, its view property may be unloaded -- in particular, if a low memory condition occurs AND the view for that view controller isn't currently visible (something is covering it, like a modal dialog, or another VC pushed on top of it), the view may be unloaded by the system.

viewDidUnload is a bad name for what the method does. It is called when a low memory condition caused the view to be unloaded -- i.e. it is not the 'opposite' method to viewDidLoad, which I think you might reasonably expect.

More info:

When should I release objects in -(void)viewDidUnload rather than in -dealloc?

Also important is that viewDidLoad will be called again after viewDidUnload. If you perform setup in viewDidLoad, you should deal with it in such a way that a second call to it will not cause memory leaks.

Either tear everything down in viewDidUnload (polite, if they are memory intensive), or check for their existence when you set them up and don't do it twice. (And, of course, totally tear them down before or during the dealloc method.)

You are not guaranteed that viewDidUnload will be invoked before dealloc is.