Question

Alright this is a really weird one so I am going to layout what is happening and then give some code after. For my example I am going to use a static amount of views, 2.

The Basics
I have a UIPageControl with X many Subviews added. On each subviews viewDidLoad is an NSXMLParse to grab an XML feed. Once the feed is obtained, it’s parsed and the table is reloaded using the parsed array. There is also a Settings button on each view. When the Settings button is pressed, UIModalTransitionStyleCoverVertical:Animated:YES is run and a UINavigationController slides up into view with full animation. Dismiss also shows animation sliding out back to the previous view. If you are in Settings, you can PushViews two levels deep (Slide In Animation).

The Problem
A random amount of the time, when the app is built and run (Not Resumed) when you tap the Settings button, the Animation does not occur. Everything is functional except for all Core Animations are removed. DismissModal simply swaps back to the previous screen. PushView in the NavigationController no longer has any animation, the next view simply appears.

If you quit the app (Kill Process) and relaunch it, it may work fine for a period of time but at some point when you tap the Settings button, it will lose all animations.

The Details
I started with Apples PageControl Application for the groundwork. It creates a dynamic amount of views based on user settings.

- (void)awakeFromNib
{   
    kNumberOfPages = 2;

    // view controllers are created lazily
    // in the meantime, load the array with placeholders which will be replaced on demand
    NSMutableArray *controllers = [[NSMutableArray alloc] init];
    for (int i = 0; i < kNumberOfPages; i++)
    {
        [controllers addObject:[NSNull null]];
    }

    self.viewControllers = controllers;

    // a page is the width of the scroll view
    scrollView.pagingEnabled = YES;
    scrollView.contentSize = CGSizeMake(scrollView.frame.size.width * kNumberOfPages, scrollView.frame.size.height);
    scrollView.showsHorizontalScrollIndicator = NO;
    scrollView.showsVerticalScrollIndicator = NO;
    scrollView.scrollsToTop = NO;
    scrollView.delegate = self;

    pageControl.numberOfPages = kNumberOfPages;
    pageControl.currentPage = 0;

    // pages are created on demand
    // load the visible page
    // load the page on either side to avoid flashes when the user starts scrolling
    [self loadScrollViewWithPage:0];
    [self loadScrollViewWithPage:1];
}


- (void)loadScrollViewWithPage:(int)page
{
    if (page < 0)
        return;
    if (page >= kNumberOfPages)
        return;

    // replace the placeholder if necessary
    SecondViewController *controller = [viewControllers objectAtIndex:page];
    if ((NSNull *)controller == [NSNull null])
    {
        controller = [[SecondViewController alloc] initWithPageNumber:page];
        [viewControllers replaceObjectAtIndex:page withObject:controller];
        [controller release];
    }

    // add the controller's view to the scroll view
    if (controller.view.superview == nil)
    {
        CGRect frame = scrollView.frame;
        frame.origin.x = frame.size.width * page;
        frame.origin.y = 0;
        controller.view.frame = frame;
        [scrollView addSubview:controller.view];
    }

}

As each view is generated, it runs an NSXMLParse in its viewDidLoad. Everything works fine up to this point. Both views are generated and you can swipe between them.

If you push the Settings Button

- (IBAction)settingsButtonPressed:(id)sender;
{
    SettingsViewController *settingsViewController = [[SettingsViewController alloc] initWithNibName:@"SettingsViewController" bundle:nil];

    UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:settingsViewController];  

    navigationController.modalTransitionStyle = UIModalTransitionStyleCoverVertical;

    [self presentModalViewController:navigationController animated:YES];

    [settingsViewController release];
    [navigationController release];
}

At this point, SettingsViewController appears into view. However, sometimes it slides up with its proper animation. Other times it will simply appear and all further core animations are broken until the process is restarted.

I went through and checked all of NSXMLParse and have narrowed down the problem to one line. On each of my subviews, is a tableView, after the XML Parsing is done, I created an array with the results and ran [self.tableview reloadData]. If I comment out that line, the table obviously only loads blank but it doesn’t have any issues with Animations.

- (void)parserDidEndDocument:(NSXMLParser *)parser
{   
    NSMutableArray *tableData = ARRAY_GENERATED_HERE;
    [self.tableView reloadData];
}

My Testing
I will note from my tests, everything is fine if kNumberOfPages is set to 1 instead of 2. Only 1 view gets generated, the Animation glitch never occurs. Add a second view in, usually within opening Settings five times, it will glitch.

Still haven’t come to a solution but it has to do with [tableView reloadData]. Any insight would be great.

Daniel pointed out something that makes sense.

My XML is fetched in the viewDidLoad using:

[NSThread detachNewThreadSelector:@selector(parseXMLFileAtURL:) toTarget:self withObject:path];

- (void)parseXMLFileAtURL:(NSString *)URL
{   
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; 

    stories = [[NSMutableArray alloc] init];

    //you must then convert the path to a proper NSURL or it won't work
    NSURL *xmlURL = [NSURL URLWithString:URL];

    // here, for some reason you have to use NSClassFromString when trying to alloc NSXMLParser, otherwise you will get an object not found error
    // this may be necessary only for the toolchain
    rssParser = [[NSXMLParser alloc] initWithContentsOfURL:xmlURL];

    // Set self as the delegate of the parser so that it will receive the parser delegate methods callbacks.
    [rssParser setDelegate:self];

    // Depending on the XML document you're parsing, you may want to enable these features of NSXMLParser.
    [rssParser setShouldProcessNamespaces:NO];
    [rssParser setShouldReportNamespacePrefixes:NO];
    [rssParser setShouldResolveExternalEntities:NO];

    [rssParser parse];

    [pool release];
}

Answer 1

From your comment, you said you are running the parser in a background thread…UIKit is not thread safe and i suspect that is whats causing your problems…try making the reloadData call on the main thread, you can use NSObjects performSelectorInMainThread to do this…

[self performSelectorOnMainThread:@selector(operationComplete) withObject:nil waitUntilDone:false];