Why would delaying a thread response fix view corruption? - iphone

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.