Why am I crashing after MKMapView is freed if I'm no longer using it?

Question

I have a MKMapView. Sometimes after my view controller is dismissed, I'll get a EXC_BAD_ACCESS.

I turned on NSSZombies and it looks like the MKMapView's delegate — my view controller! — is being called, despite both the MKMapView and UIViewController subclass being freed. I've checked, and my memory management is correct.

What's going on?

Answer 1

This is because of the way MKMapView works. There's an operation pending, so MapKit is retaining the MKMapView and it hasn't actually been deallocated yet. That isn't itself a problem. The problem is that it's still sending messages to your delegate.

The workaround is simple: As part of your view controller's cleanup set the map view's delegate to nil, which will prevent MKMapView from sending messages to it.

This is documented in MKMapViewDelegate Protocol Reference:

Before releasing an MKMapView object for which you have set a delegate, remember to set that object’s delegate property to nil. One place you can do this is in the dealloc method where you dispose of the map view.

Edit: Give Oscar an upvote as well, just below, who provided the documentation quote here.

Given ARC, I suggest this means you should set your map view's delegate to nil in your view controller's dealloc.

Answer 2

OK, this is the confirmation of the answer. It's from the Apple doc, but it's missing from MKMapView. It's only found under the documentation for its delegate protocol:

Before releasing an MKMapView object for which you have set a delegate, remember to set that object’s delegate property to nil. One place you can do this is in the dealloc method where you dispose of the map view.

NOTE: This also applies to UIWebView.

I set the MapView's delegate pointer to nil in the delegate's dealloc method, and our crashes seem to have been eliminated.