performSegueWithIdentifier：导致应用崩溃

Question

I have a button that is pressed after user enters email information. I have an alert view that is displayed when there is no email entered but if there is I want the button to segue to another view controller.

The following code causes my app to crash. I have no idea why. Please help.

(note: I have tried "sender:self]" "sender:nil]" and "sender:sender]" and they all make my app crash.)

- (IBAction)nextButtonPushed:(id)sender {

    if ([self.emailTextField.text  isEqual: @""]) {

        emailAlertView = [[UIAlertView alloc] initWithTitle:@"Missing Email" message:@"A destination email is required to send." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
        [emailAlertView show];
        self.nextButton.enabled = NO;
    }

    else {

        eString = self.eTextField.text;
        hString = self.hField.text;
        emailAddress = self.emailTextField.text;

        [userDefaults setObject:eString forKey:@"e"];

        [userDefaults setObject:hString forKey:@"h"];

        [userDefaults setObject:emailAddress forKey:@"email"];

        [self performSegueWithIdentifier:@"next" sender:self];
    }

}

Answer 1

There are three overwhelmingly likely possibilities:

• Perhaps the storyboard really has no segue named "next" emerging from the FirstViewController scene. Be careful: spaces and capitalization and things like that matter.

• Perhaps the storyboard has a segue named "next" but it emerges from a different scene (a different view controller).

• Perhaps the FirstViewController instance represented by self in your code is not the same as the FirstViewController instance in the storyboard, ie maybe it came into existence in some other way and not by instantiation from the storyboard. You might even have two FirstViewController objects in the storyboard, and the segue comes from the other one.

Answer 2

One great way to figure out why your app is crashing on a line like this is to disable any breakpoints you may have (including exception breakpoints). This will cause the app to crash and often tell you a reason why it is crashing. Normally you'll get error's like @matt mentioned such as bad identifiers on your seques (often typing errors), multiple of the same object in an IB scene such as the following:

 Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Receiver (<YOURAPP.YOURVIEWCONTROLLER: 0x7c8951d0>) has no segue with identifier 'TheSequeIdentifierYouHaventSetYetOrTypedInWrong''

or like in my case today where I got this error:

 Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<YOURAPP.YOURVIEWCONTROLLERYOUARETRYINGTOSHOW 0x78e5a140> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key "MyKeyOnTheOffendingObject"

Once I knew that I had an object in my storyboard with a key that it should not have all I had to do was track down the Button/Image/Other Random Object with the bad Referencing Outlet or Sent Event and all was fixed

Often performSequeWithIdentifier will come up as the last line shown before a crash since that is the last line that worked before a problem was found when presenting a new View Controller - so if you are sure the problem isn't with your seque then check references on the VC you are trying to present!

Answer 3

Crashing can also occur if the view controller cannot load from the NIB. For example if there is an incorrectly named User Defined Runtime Attribute.

For example QBFlatButton changed their API today and my app started crashing. This is why we use semantic versioning :-)