dealloc on Background Thread

Question

Is it an error to call dealloc on a UIViewController from a background thread? It seems that UITextView (can?) eventually call _

Answer 1

The second post here had useful info, but their answer didn't work for me.

UIWebView in multithread ViewController

It also seems that this may have been addressed in iPhone OS 4, but not sure.

I ended up not releasing my UIWebView in the controller's dealloc when [NSThread isMainThread] was NO. Would rather leak than crash (until I get a better solution).

Answer 2

It is an error to call dealloc on anything at any time. You should only ever call release.

You should not access any UI related instances from a background thread. This includes using getter methods because they may modify things internally. However, retain and release are thread safe for any object at any time, as long as the normal rules for retain and release are followed. UI related instances include any object that is referenced by an active UIView or UIViewController.

performSelectorOnMainThread does not do anything more than retain an object until it gets to the main thread. It is safe to call on any UI related object.

Answer 3

Yes, it is an error to make a UIViewController releasing in a background thread (or queue). In UIKit, dealloc is not thread safe. This is explicitly described in Apple's TN2109 doc:

When a secondary thread retains the target object, you have to ensure that the thread releases that reference before the main thread releases its last reference to the object. If you don't do this, the last reference to the object is released by the secondary thread, which means that the object's -dealloc method runs on that secondary thread. This is problematic if the object's -dealloc method does things that are not safe to do on a secondary thread, something that's common for UIKit objects like a view controller.

Answer 4

The simple rule is that it's an error to do anything on a UI* from a background thread.