objective c - Showing an alert in an iPhone top-level exception handler

Question

I'm trying to display a UIAlertView in a top-level iPhone exception handler. The handler function looks like this:

void applicationExceptionHandler(NSException *ex) {
  UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Error"
                                                      message:[ex reason]
                                                      delegate:nil
                                             cancelButtonTitle:@"OK"
                                             otherButtonTitles:nil];
  [alertView show];
}

I've seen similar code elsewhere (for instance, NSSetUncaughtExceptionHandler not catch all errors on iPhone).

If I single-step in the debugger, I can see that the exception handler is called, and I can see the current screen dim as if it's going to display the alert in front of it, but nothing appears. Outside of the debugger, the app just quits immediately and goes back to the system home screen.

It does work if I trap an error in applicationDidFinishLaunching and display an alert there before returning. I assume that the alert view never gets a chance to display in the exception handler because the app is terminating (as opposed to sitting there doing nothing if I just bail out of applicationDidFinishLaunching). Is there a way to make this work?

Answer 1

Thanks heaps to benzado, here's what I think is a great generic top-level exception handler. I'm a beginner so hopefully it's done properly, but it does work :)

In my ...appDelegate.m:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{        
    [window makeKeyAndVisible];

    NSSetUncaughtExceptionHandler(&exceptionHandler);

    return YES;
}

BOOL exceptionAlertDismissed = FALSE;
void exceptionHandler(NSException *exception)
{
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"App Committed Suicide"
        message:@"Oh dear, that wasn't supposed to happen. You will have to restart the application... sorry!"
        delegate:[[UIApplication sharedApplication] delegate] cancelButtonTitle:nil otherButtonTitles:@"That's ok!", @"Erm, bye...", nil];
    [alert show];
    [alert release];

    while (exceptionAlertDismissed == FALSE)
    {
        [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.1]];
    }
}

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    exceptionAlertDismissed = TRUE;
}

And in my ...appDelegate.h:

@interface ...appDelegate : NSObject <UIApplicationDelegate, UIAlertViewDelegate>
...
void exceptionHandler(NSException *exception);