Value not being set the second time on iOS 6. Threading issue? - ios

Question

I'm writing an iOS application that populates an array using the data retrieved from the server and displays it in a picker view.
Everything goes smoothly the first time the view is displayed; However, when switching to another view which uses a camera to scan stuff and switch back using a slide-out menu (built using SWRevealViewController), it fails to populate the array. When we access it on the UI thread after the background task has finished to retrieve the records, an NSRangeException is thrown with the error index 0 beyond bounds for empty array.
I'm pretty sure the background task is being run and the data is being retrieved successfully as I log every single request to the server.
I believe it might be an issue with concurrency and the background thread not updating the variable.
As far as we have tested, this issue is only present on iOS 6, and does not happen, or at least has not yet, on iOS 7.
This is the code used to retrieve and set the array:
dispatch_queue_t queue = dispatch_queue_create("com.aaa.bbb", NULL);
dispatch_async(queue, ^{
_events = [_wi getEvents:_auth_token];
dispatch_async(dispatch_get_main_queue(), ^{
// code to be executed on the main thread when background task is finished
[_mPicker reloadComponent:0];
// Set the default on first row
[self pickerView:_mPicker didSelectRow:0 inComponent:0];
[pDialog dismissWithClickedButtonIndex:0 animated:YES];
});
});
This is the prepareforSegue method in my SidebarViewController that is responsible for switching between views when an item is selected from the slide-out menu.
- (void) prepareForSegue: (UIStoryboardSegue *) segue sender: (id) sender
{
// Set the title of navigation bar by using the menu items
NSIndexPath *indexPath = [self.tableView indexPathForSelectedRow];
UINavigationController *destViewController = (UINavigationController*)segue.destinationViewController;
destViewController.title = [[_menuItems objectAtIndex:indexPath.row] capitalizedString];
if ( [segue isKindOfClass: [SWRevealViewControllerSegue class]] ) {
SWRevealViewControllerSegue *swSegue = (SWRevealViewControllerSegue*) segue;
swSegue.performBlock = ^(SWRevealViewControllerSegue* rvc_segue, UIViewController* svc, UIViewController* dvc) {
UINavigationController* navController = (UINavigationController*)self.revealViewController.frontViewController;
[navController setViewControllers: #[dvc] animated: NO ];
[self.revealViewController setFrontViewPosition: FrontViewPositionLeft animated: YES];
};
}
}
The views are linked together from the storyboard for switching.
The error occurs when I try to retrieve a specific entry from my events array in the pickerView:didSelectRow:inComponent: method :
- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component {
// Set the current id
_currentId = [NSString stringWithFormat:#"%d", row];
_currentName = [[_events objectAtIndex:row] objectForKey:#"event_name"];
-----------^
...
Here's the list of running threads and the call stack.
From my experience in Android, I think that this might have to do something with me not finishing the background task properly the first time, or somehow the code that is supposed to be run after the background task, is run alongside it the second time.
I would appreciate any suggestions that might help me with this issue!
Thanks

Answer 1

The variable _events is accessed from different threads without any synchronization mechanism.
Try this:
dispatch_queue_t queue = dispatch_queue_create("com.aaa.bbb", NULL);
dispatch_async(queue, ^{
NSArray* events = [_wi getEvents:_auth_token];
dispatch_async(dispatch_get_main_queue(), ^{
_events = [events copy];
// code to be executed on the main thread when background task is finished
[_mPicker reloadComponent:0];
// Set the default on first row
[self pickerView:_mPicker didSelectRow:0 inComponent:0];
[pDialog dismissWithClickedButtonIndex:0 animated:YES];
});
});
Also _events should be an NSArray, not NSMutableArray, to avoid such problems.