Why would delaying a thread response fix view corruption?

Question

6 times out of 10 my very simple iPhone app is getting a corrupted display on launch or crashes randomly. But it behaves fine in the simulator. The display corruption looks like mis-colored fonts, out of place font text, wrong background colors, etc.

I've found a strange work-around.. when my thread delays by 2 seconds before calling the "done" notification, everything works swimmingly. The thread reads a web page and the "done" notification loads up a PickerView with strings. So what gives? Can I not safely initiate a threaded task from viewDidLoad?

- (void) loadWebPage:(NSString *)urlAddition {
      NSAutoreleasePool *subPool = [[NSAutoreleasePool alloc] init];
      NSString *pageSource;
      NSError *err;
      NSString *urlString = [NSString stringWithFormat:@"http://server/%@",
                             urlAddition];
      pageSource = [NSString stringWithContentsOfURL:[NSURL URLWithString: urlString] 
                                            encoding:NSUTF8StringEncoding 
                                               error:&err];

      [NSThread sleepForTimeInterval:2.0]; // THIS STOPS THE DISPLAY CORRUPTION
      [[NSNotificationCenter defaultCenter] postNotificationName:@"webDoneNotification" 
                                                          object:nil]; 
      [subPool drain];
}

- (void) webDoneNotification: (NSNotification *)pNotification { 
      [mediaArray release];
      mediaArray = [[NSArray arrayWithObjects:
                     [NSString stringWithString:@"new pickerview text"], 
                     nil] retain];

      [mediaPickerView reloadAllComponents];

      [mediaPickerView selectRow:0 
                     inComponent:0 
                        animated:NO];
}


- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
   mediaArray = [[NSArray arrayWithObjects:
                 [NSString stringWithString:@"init pickerview text"], 
                 nil] retain];

   if (self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]) {
        // Custom initialization
    }

    return self;
}

- (void)viewDidLoad {
   [super viewDidLoad];

   myWebThread = [[WebThread alloc] initWithDelegate:self];
   [[NSNotificationCenter defaultCenter] addObserver:self 
                                            selector:@selector(webDoneNotification:) 
                                                name:@"webDoneNotification" 
                                              object:nil]; 
   [myWebThread performSelectorInBackground:@selector(loadWebPage:) withObject:@""];
}

Thanks!

Update: Even a delay of 0.1 seconds is enough to completely fix the problem.

Answer 1

You're updating the view from a background thread. This is what's causing your problems; UIKit views are not thread-safe. When you need to alter the view, do it in the main thread.