Question

my app crashed when nsstring getting contents of URL.

website = [NSString stringWithFormat:@"hidden."]; 
contents = [NSString stringWithContentsOfURL:[NSURL URLWithString:website] encoding:NSUTF8StringEncoding error:nil];

the crash log is:

void _WebThreadLockFromAnyThread(bool), 0x865cc40: Obtaining the web lock from a thread other than the main thread or the web thread. UIKit should not be called from a secondary thread.
2012-07-10 11:15:17.654 MyApplication[1505:19407] OK // some output
2012-07-10 11:15:17.660 MyApplication[1505:19407] bool _WebTryThreadLock(bool), 0x865cc40: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
1   WebThreadLock
2   -[UITextRangeImpl isEmpty]
3   -[UITextRange(UITextSelectionAdditions) _isCaret]
4   -[UITextSelectionView setCaretBlinks:]
5   -[UIKeyboardImpl setCaretBlinks:]
6   -[UIKeyboardImpl setDelegate:force:]
7   -[UIKeyboardImpl setDelegate:]
8   -[UIPeripheralHost(UIKitInternal) _reloadInputViewsForResponder:]
9   -[UIResponder(UIResponderInputViewAdditions) reloadInputViews]
10  -[UIResponder(Internal) _windowBecameKey]
11  -[UITextField _windowBecameKey]
12  -[UIWindow makeKeyWindow]
13  -[UIWindow makeKeyAndVisible]
14  -[MyApplicationAppDelegate checkLogin]
15  -[NSThread main]
16  __NSThread__main__
17  _pthread_start
18  thread_start

the code is called with [self performSelectorInBackground:@selector(checkLogin) withObject:nil]; from IBAction. what i am doing wrong?

(Xcode 4.3, Mac OS X Lion, iPhone Simulator 5.1)

Answer 1

According to that error message, you’re trying to update a UIKit object from a background thread, and the UI can only be updated from the main thread.

Instead, use self performSelectorOnMainThread to call back to the main thread to update your UI with whatever information you received when your web call returns.

From your main thread:

[self performSelectorInBackground:@selector(checkLogin) withObject:nil];

In the background:

- (void)checkLogin {
    // Do web stuff
    ...

    [self performSelectorOnMainThread:@selector(updateUI) 
                           withObject:nil 
                        waitUntilDone:NO];
}

- (void)updateUI {
    [statusLabel setText:@"Done!"];
}