Click on UIAlertView crashes app if view is dismissed

Question

A UIAlertView is displayed if an error occurs. But in the meantime the view on which the UIAlertView were called has been dismissed (and therefore released). If the user clicks on OK the app crashes because a message to a released instance is sent. This will cause your app crashing:

UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"test" message:@"test" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
[alertView show];
[alertView release];
alertView = nil;
[self.navigationController popViewControllerAnimated:YES];

I thought the UIAlertView is an independent unit. But it seems it isn't. Is there a way how I could avoid the app crashing (except not dismissing the view)?

Answer 1

Make sure you are implementing the UIAlertViewDelegate protocol. If you don't care about when the alert is dismissed just init with delegate as nil.

UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"test" message:@"test" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

Answer 2

The delegate is called when the UIAlertView is dismissed, so in your case:

delegate:self

Delegates are not retained, like an object added to an array, or a subview would be. So in your case, when you call:

[self.navigationController popViewControllerAnimated:YES];

self is most likely being released, and when the the user dismisses the alert, self is called, but has been dealloc'd so it no longer exists.

An easy way to check this is to put a logger statement, like NSLog(@"I'm gone"); in self's dealloc method, if it's ran, then you know your self isn't around anymore, and any messages sent to it will cause a crash.

Answer 3

If the UIAlertView object is to be usable from anywhere in the app, not just on the current view, then retain it inside something that is available from anywhere in the app, either some persistant root view controller under the entire possible view stack, or the app delegate.

Added:

This top level object can also retain the alert view's delegate until after it's done being needed (after alert view dismissal).