如何使用Xcode检测对象的双重释放？

Question

I've got a MFMailComposeViewController object that I'm releasing in my Email button's callback, simply because I created it, and I think my doing so is intermittently but not always crashing my app.

How can I use Xcode's instrumentation program to detect such a situation?

Thanks.

Answer 1

You can set the NSZombieEnabled environment variable to YES ( Product > Edit Scheme… , the select Run (Product Name) , click the Arguments tab and edit the list of environment variables). With NSZombie , objects will not be deallocated but turned into zombies. Messaging them will log an error to the console instead of crashing with EXC_BAD_ACCESS . That way you can find out if it's really the MFMailComposeViewController that's causing trouble.

But retaining and releasing the view controller might not even be necessary. If you present the MFMailComposeViewController immediately after creating it and don't use it anymore after it's been dismissed, there's no need to retain it:

- (IBAction)composeMessage:(id)sender {
    MFMailComposeViewController *mailComposeViewController = [[[MFMailComposeViewController alloc] init] autorelease];
    mailComposeViewController.mailComposeDelegate = self;
    [self presentModalViewController:mailComposeViewController animated:YES];
}

- (void)mailComposeController:(MFMailComposeViewController *)mailComposeViewController didFinishWithResult:(MFMailComposeResult)result error:(NSError*)error {
    // Present error to the user if failed
    [self dismissModalViewControllerAnimated:YES];
}