Question

I have an app based on a tab bar and data retrieved from the Internet. The main tab shows a map and one of the other tabs shows incidents around the center point of the map displayed using a UITableView. If the user moves the map and then moves to the incidents page, I need to refresh the list of incidents displayed in the table. To do this I request the incidents in viewWillAppear:animated: and when that completes (asynchronously) I call the table view’s reloadData method.

This works beautifully unless the user taps between the tabs quickly (e.g. display incidents, move to map, move map, move back to incidents, move back to map, etc.). At some point the incidents data source (an NSArray) is modified while the table view is trying to access it.

Here is a question that is similar:
UITableView Crashes if Data Source Is Updated During Scrolling

One of the solutions for that question describes a solution at a high level that is exactly what I want: Freeze the data source while the table is being updated. The thing I can’t figure out, however, is when to unfreeze the data source. The problem is I can’t find any way to be notified when the table is done being updated.

Any ideas? How do I freeze the data source while the table is being updated and then unfreeze it once the table is done being updated?

Answer 1

Although I’d really like to receive a notification when the table view is done accessing the data source, I found that my problem was due to modifying the data container from the work thread. The answer to this question led me to the solution:

Refreshing XML data and updating a UITableView

What I do now is fill a separate array in the worker thread and then perform a selector on the main thread to swap the updated data into the data container used by the table view.