Alexei S. Alexei S. - 2 years ago 119
Swift Question

UITableView freezes after deleting an entry

My app's MasterView includes a table view which is filled with 10 cells containing some core data objects like a "title" (previously downloaded from the internet, see next paragraph). Thanks to the template "Master-Detail Application" I can make use of the built-in NSFetchedResultsController and its methods which take care of the core data handling (e.g. deleting rows). When running the app, the cells get configured with the existent core data objects. At this point I can delete those entries like one would expect.

In addition, my app also has a refresh button. When tapped, it loads some new data from the internet and converts it into objects that are saved to core data. The cells configure themselves again the same way they did before and the UITableView gets a refresh. Again I can delete an entry but now, as soon as I've deleted one which then disappears normally, the table freezes (not the whole app) in the sense that its cells are not tappable anymore/don't react to gestures. I can refresh and that work (10 entries again after having deleted one before) but the cells keep being untappable. I have to stop the app to return to normal functionality.

When restarting, the one entry deleted before is - like expected - gone, 9 entries are left. Again everything works fine at this point, I can delete multiple ones without a problem. But as soon as a manual refresh took place, we're at the same point as before when deleting an entry now.

A common pattern when missing something specific? I'm happy to post some code but I have no idea right now which parts would help...

Rob Rob
Answer Source

If the app is really freezing, common sources of this sort of problem are deadlocks. If you're using dispatch_sync anywhere, that's a common source of problems (notably using dispatch_sync from the main thread to something that eventually does another dispatch_sync back to the main thread). If you're using NSLock or @synchronized, and call them recursively, you can also deadlock that way, too.

Another source of problem is an infinite loop (such as an error in a while clause that is never satisfied). If you have such a loop in the main thread, that will freeze the UI. (Anything that blocks the main thread will freeze the UI.)

You might be able to diagnose where it's freezing by running the app through the debugger in Xcode, and when the app freezes, press the debugger's "pause" button. While it will often pause in the middle of some fairly cryptic assembler code, you can often look at the stack traces to figure out where the app was when it froze.

If it's simply that you're not seeing your UI updated as a result of your actions, try making sure that the reloadData method is called on the main queue, if it's not, e.g.:

dispatch_async(dispatch_get_main_queue()) {
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