Fatal error handler in iOS app - ios

Question

When something goes so horribly wrong that my app can't continue and needs to exit, I want to pop up an alert box to the user, and then close the app when they tap the OK button. Sounds simple enough, right?
But here's the problem: my fatal error handler gets called by a 3rd party library (I don't have their source code). I give them a pointer to my fatal error handler on initialization, and when they encounter a fatal error they simply call that routine and expect it to never return. If it returns, the 3rd party library will assume I've handled the error and it will continue on its way (possibly corrupting data because things are now in an inconsistent state). I could just exit the application at the end of my error handler (which is what they expect), but I want to be able to display a message to the user first to tell them what the problem is.
Unfortunately, if I just do:
-(void)fatalErrorHandler:(NSString *)msg
{
// Log the error and shut down all the things that need
// to be shut down before we exit
// ...
// Show an alert to the user to tell them what went wrong
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"Error" message:msg delegate:self cancelButtonTitle:#"Close" otherButtonTitles:nil];
[alert show];
}
-(void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
exit(-1);
}
fatalErrorHandler returns right after [alert show], which tells the 3rd party library that I've handled the error and it will continue on as if nothing has happened. This is no good.
I need to NOT return from fatalErrorHandler. Ever. But since I'm on the main thread, the UIAlertView won't appear until fatalErrorHandler returns. A catch-22.
Any ideas on how I can show an alert to the user without returning from my fatal error handler?

Answer 1

I don't know if this would work, but what about starting a while loop with a sleep in its body for, let's say, 1 second each cycle? The while would exit when a Bool variable would have been set to YES, maybe from the alertViewDelegate.

Answer 2

From what you wrote "fatalErrorHandler returns right after [alert show], which tells the 3rd party library that I've handled the error and it will continue on as if nothing has happened."
I guess what you actually need is to pause everything when the fatalErrorHandler method is called. To achieve this, you can stop all NSTimer, queued methods etc. before displaying alertView.
Alternatively, you can display alertView via a different thread, and then use usleep(long long time) to pause the thread where fatalErrorHandler is in.

Answer 3

Okay, wesley6j's answer gave me an idea. Here's what I came up with:
-(void)fatalErrorHandler:(NSString *)msg
{
// Pop up an alert to tell the user what went wrong. Since this error
// handler could be called from any thread, we have to make sure this happens
// on the main thread because it does UI stuff
[self performSelectorOnMainThread:#selector(showMessage:) withObject:msg waitUntilDone:YES];
// Now, if we're NOT on the main thread, we can just sleep forever and never
// return from here. The error handler will exit the app after the user
// dismisses the alert box.
if (![NSThread isMainThread])
Sleep(0x7fffffff);
else
{
// OTOH, if we ARE on the main thread, we have to get a bit creative.
// We don't ever want to return from here, because this is a fatal error
// handler and returning means the caller can continue on its way as if
// we "handled" the error, which we didn't. But since we're on the main
// thread, we can't sleep or exit because then the user will never see
// the alert box we popped up in showMessage. So we loop forever and
// keep calling the main run loop directly to let it do its processing
// and show the alert. This is what the main run loop does anyway, so
// in effect, we just become the main run loop.
for (;;)
{
[[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode beforeDate: [NSDate date]];
}
}
}
-(void)showMessage:(NSString *)msg
{
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"Error" message:msg delegate:self cancelButtonTitle:#"Close" otherButtonTitles:nil];
[alert show];
[alert release];
}
-(void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
exit(-1);
}
This works perfectly and does exactly what I need.