Terminating app due to uncaught exception 'NSUnknownKeyException' : iOS app crash

Question

I'm a beginner programmer working on iPhone apps. I was wondering if anybody could help me with a bug in my app. When programming on XCode I found that my app suddenly crashed on launch after adding a little more code. The output panel showed the following error:

* Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key play.' * First throw call stack: (0x1c91012 0x10cee7e 0x1d19fb1 0xb7ae41 0xafc5f8 0xafc0e7 0xb26b58 0x230019 0x10e2663 0x1c8c45a 0x22eb1c 0xf37e7 0xf3dc8 0xf3ff8 0xf4232 0x433d5 0x4376f 0x43905 0x8eceab6 0x4c917 0x1096c 0x1194b 0x22cb5 0x23beb 0x15698 0x1becdf9 0x1becad0 0x1c06bf5 0x1c06962 0x1c37bb6 0x1c36f44 0x1c36e1b 0x1117a 0x12ffc 0x222d 0x2155) libc++abi.dylib: terminate called throwing an exception (lldb)

The code that shows up when the app terminates is as follows

#import <UIKit/UIKit.h>
#import "CIAAppDelegate.h"

int main(int argc, char *argv[])
{
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([CIAAppDelegate class]));
    }
}

My ViewController.h shows:

#import <UIKit/UIKit.h>

@interface CIAViewController : UIViewController {
    IBOutlet UILabel *cruzia;
    IBOutlet UILabel *textarea;
    IBOutlet UIButton *playbtn;
    IBOutlet UIButton *tutorialbtn;
    IBOutlet UIButton *optionsbtn;
    IBOutlet UIButton *trainingbtn;
    IBOutlet UIButton *back;
}

-(IBAction)press;
-(IBAction)press2;
-(IBAction)press3;
-(IBAction)press4;
-(IBAction)press5;

Also my ViewController.m shows

#import "CIAViewController.h"

@interface CIAViewController ()
@end

@implementation CIAViewController

-(IBAction)press {
    cruzia.hidden = 0;
    textarea.hidden = 0;
    playbtn.hidden = 1;
    tutorialbtn.hidden = 1;
    optionsbtn.hidden = 1;
    trainingbtn.hidden = 1;
    back.hidden = 0;
    cruzia.text = @"Play";
    textarea.text = @"Hello! You are playing the game of Cruzia!";
}

-(IBAction)press2 {
    cruzia.hidden = 0;
    textarea.hidden = 0;
    playbtn.hidden = 1;
    tutorialbtn.hidden = 1;
    optionsbtn.hidden = 1;
    trainingbtn.hidden = 1;
    back.hidden = 0;
    cruzia.text = @"Tutorial";
    textarea.text = @"Welcome! You are watching the Cruzia tutorial!";
}

-(IBAction)press3 {
    cruzia.hidden = 0;
    textarea.hidden = 0;
    playbtn.hidden = 1;
    tutorialbtn.hidden = 1;
    optionsbtn.hidden = 1;
    trainingbtn.hidden = 1;
    back.hidden = 0;
    cruzia.text = @"Options";
    textarea.text = @"You are now in the Options screen. You can edit the settings of the game!";
}

-(IBAction)press4 {
    cruzia.hidden = 0;
    textarea.hidden = 0;
    playbtn.hidden = 1;
    tutorialbtn.hidden = 1;
    optionsbtn.hidden = 1;
    trainingbtn.hidden = 1;
    back.hidden = 0;
    cruzia.text = @"Training";
    textarea.text = @"This is the training area. You can improve your Cruzia skills here!";
}

-(IBAction)press5 {
    cruzia.hidden = 0;
    textarea.hidden = 0;
    playbtn.hidden = 0;
    tutorialbtn.hidden = 0;
    optionsbtn.hidden = 0;
    trainingbtn.hidden = 0;
    back.hidden = 1;
    cruzia.text = @"Cruzia";
}


- (void)viewDidLoad
{
    textarea.hidden = 1;
    playbtn.hidden = 0;
    tutorialbtn.hidden = 0;
    optionsbtn.hidden = 0;
    trainingbtn.hidden = 0;
    back.hidden = 1;
    cruzia.text = @"Cruzia";
    [super viewDidLoad];
}

@end

I am sorry to waste your time but I have had this error for a long time and would like it if you had a solution.

Answer 1

What The Problem Is:

At runtime your view controller is going to get deserialized from its XIB/Storyboard. During this process, it's going to read all the objects that are in your IB files and start instantiating them with an NSCoder. As it is instantiating those objects in your view, it's going to read the IBOutlet and IBAction connections in your storyboard and start assigning the hydrated objects to those properties that were connected at design time so that you have programmatic access to them at run time. But here's the kicker: If you have a connection that points to a property that doesn't exist, then this exception will be thrown because it was told a property was there that really isn't.

What's happening is that your Storyboard/XIB is expecting your class to have a property called play. As your interface shows, you do not have a property called that. I suspect though, that you used to, but renamed it playbtn.

This is what is meant by

this class is not key value coding-compliant for the key play

It means it tried to do something like this under the covers:

[self setValue:button forKey:@"play"]

For additional enlightenment about Key/Value Coding and serialization, grab a comfy chair and read Key Value Coding Programming Guide and Archives and Serializations Programming Guide

How To Fix It:

In Interface Builder, right click on your view controller and see the list of connections:

See how there is a connection for the property play and it has a little yellow triangle next to it? This is Interface Builder trying to warn you that you have a connection set there, but the header isn't showing that there is an IBOutlet of that name.

All you need to do is click that little x to break the connection to the missing property.

How To Prevent This In The Future

Anytime you change a property in your code, you'll need to make sure that you break any associated connection in Interface Builder.