Why is tableView:cellForRowAtIndexPath: being called on a background thread?

Question

In my iPhone app, I am occasionally seeing a crash caused by tableView:cellForRowAtIndexPath: being called on a background thread.

Obviously, this should not be happening. I'm not calling it, my object is a delegate to a UITableView and the foundation is calling it - the only thing I see in the stack for the thread in question is

-_WebTryThreadLock(bool)
-_dequeuReusableViewOfType
-tableView:cellForRowAtIndexPath:
-_createPreparedCellForGlobalRow:withIndexPath
-_pthread_qathread

The crash occurs in WebTryThreadLock - no surprise really, it's not threadsafe and should not be called off the main thread.

But how do I figure out WHY my tableView delegate is being called on a background thread?

I wonder - if I were to call [tableView reloadData] on a background thread, would that do it?

I always thought it would just dispatch the call on the main thread regardless. I'm not sure I am even doing that, but I might be, and I will check for that, but really, shouldn't UIKit check for that and call the delegate methods on the main thread anyway?

Answer 1

You can't call [tableView reloadData] on a secondary thread. You can't call any UIKit stuff on a secondary thread (with a few exceptions, such as UIImage). This includes all tableView methods, including straight getters and setters. It doesn't matter whether it is rendering related or not.

Answer 2

It might be interesting to sprinkle some of these around your view controller:

if (![NSThread isMainThread]) { NSLog(@"Huh?"); }

I'm pretty sure UIKit/IOS would not decide on it's own to call a table view delegate method on a background thread. Do you have any dispatch_async, detachNewThreadSelector, performSelectorInBackground?