What works in viewDidUnload should be moved to did

问题:

In new iOS 6, viewDidUnload is deprecated and we have been instructed to use didReceiveMemoryWarning instead, to manage objects in UIViewController instances and subclasses. Is it equally effective to assign nils to UIView kinds inside didReceiveMemoryWarning like the way it has been done inside viewDidUnload?

I am asking this because these two methods seems to be working differently. It seems like didReceiveMemoryWarning doesn't guarantee viewDidLoad to be called again to re-instantiate any necessary UIViews.

I suspect with iOS 6, memory management is done without requiring to manually deallocate UIView. Please help me to know what I have missed in understanding the lifecycle of UIViewController.

回答1:

My preferred method is now the following:

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    if (self.isViewLoaded && !self.view.window) {
        self.view = nil;
    }
    // Do additional cleanup if necessary
}

Note that the test self.isViewLoaded is essential, as otherwise accessing the view causes it to load - even the WWDC videos tend to miss that.

If your other references to subviews are weak references, you don't have to nil them out here, otherwise you want to set them to nil, too.

You should get rid of viewDidUnload completely, and every code there should move to appropriate places. It wasn't guaranteed to be called prior to iOS 6 before anyway.

回答2:

In the iOS reference for viewDidUnload:, it states that this is deprecated for iOS 6 because

Views are no longer purged under low-memory conditions and so this method is never called

It doesn't say anything about placing this code in didReceiveMemoryWarning:. Since views are no longer purged under low memory conditions, you never have to worry about cleaning up your views in either method.

回答3:

The answer by Eiko is not correct, and we should NOT set self.view to nil when receiving low memory warning. Doing so is useless and may be harmful.

iOS 6 will automatically freeing bitmaps of views which is not currently displayed, See http://thejoeconwayblog.wordpress.com/2012/10/04/view-controller-lifecycle-in-ios-6/ for details.