Crash in UITableView sending message to deallocated UIViewController

Question

I have been fighting with the following stack trace for the whole day. I'm unable to reproduce the issue but I know it happens for a large number of users (Crashlytics).

Crashed: com.apple.main-thread
EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x0000000000000010

Thread : Crashed: com.apple.main-thread
0  libobjc.A.dylib                0x0000000194187bd0 objc_msgSend + 16
1  UIKit                          0x000000018860639c -[UITableView _createPreparedCellForGlobalRow:withIndexPath:willDisplay:] + 544
2  UIKit                          0x00000001885fafc4 -[UITableView _updateVisibleCellsNow:isRecursive:] + 2360
3  UIKit                          0x00000001883f0c60 -[UITableView layoutSubviews] + 172
4  UIKit                          0x000000018830d874 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 572
5  QuartzCore                     0x0000000187c65d58 -[CALayer layoutSublayers] + 168
6  QuartzCore                     0x0000000187c60944 CA::Layer::layout_if_needed(CA::Transaction*) + 320
7  QuartzCore                     0x0000000187c607e8 CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 32
8  QuartzCore                     0x0000000187c5ffe8 CA::Context::commit_transaction(CA::Transaction*) + 276
9  QuartzCore                     0x0000000187c5fd6c CA::Transaction::commit() + 436
10 UIKit                          0x0000000188304848 _afterCACommitHandler + 156
11 CoreFoundation                 0x0000000183b46388 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 32
12 CoreFoundation                 0x0000000183b43314 __CFRunLoopDoObservers + 360
13 CoreFoundation                 0x0000000183b436f4 __CFRunLoopRun + 836

...

This happens in a UIViewController who owns a UITableView. The view controller is also the data source and the delegate for the table view.

Looking at the stack trace, I was guessing that the view controller would be deallocated while some animation or slow operation was going on. After trying everything, I can't find a configuration where this would happen.

Would you have any suggestion on why a VC could be deallocated while one of its view is still animating?

Answer 1

I noticed that this crash coincides with switching from UITableViewController to UIViewController + UITableView.

Presumably, the UITableViewController was nillifying the table view's delegate and datasource when it was deallocated. It wasn't the case with the new code.

To solve the issue, I set them to nil in - (void)dealloc:

- (void)dealloc {
    _tableView.dataSource = nil;
    _tableView.delegate = nil;
}

The reason why they need to be set to nil is that both the UITableView's dataSource and delegate are unsafe_unretained references.