Question

I have a simple application which was working fine until I added some code which launches a new thread at some point and then tries to show an alert from that thread. Now, the app crashes whenever the code for showing the alert is hit.

UIAlertView * addAlert = [[UIAlertView alloc] initWithTitle:@"New alert"
message:@"Example alert" 
delegate:nil 
cancelButtonTitle:@"Cancel", otherButtonTitles:@"OK", nil];
[addAlert show];
[addAlert release];

My question is: is it possible to display UI elements such as alerts from multiple threads on iOS?

Answer 1

You definitely don’t want to be displaying an alert (or anything UI-related) from any thread other than the main thread. I’d suggest putting your alert code in a function and call one of the performSelectorOnMainThread calls.

- (void) showAlert
{
    UIAlertView * addAlert = [[UIAlertView alloc] initWithTitle:@"New alert"
        message:@"Example alert" 
        delegate:nil 
        cancelButtonTitle:@"Cancel", otherButtonTitles:@"OK", nil];
    [addAlert show];
    [addAlert release];
}

// ... somewhere in the worker thread ...
[self performSelectorOnMainThread:@selector(showAlert) withObject:nil waitUntilDone:NO];