When working with Key-Value Coding (KVC) in your classes, you might encounter an error like this:
*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<YourClass 0x12345> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key yourKey'
This error occurs when you try to set a value for a key that doesn't exist in your class or when your class doesn't support KVC. In this guide, we'll walk you through resolving this issue step by step.
Table of Contents
Understanding Key-Value Coding (KVC)
Key-Value Coding is a mechanism in Cocoa and Cocoa Touch that allows you to access an object's properties indirectly using string keys. This can be useful in various scenarios, such as when working with bindings, Core Data, or user defaults.
To be KVC-compliant, a class must implement methods for getting and setting values for its properties. These methods are typically named setValue:forKey: and valueForKey:. For more information on KVC, refer to the official documentation.
Identifying the Cause of the Error
To fix the error, you first need to identify the root cause. Here are some common causes of the setValue:forUndefinedKey: error:
- Typo in the key name
- Missing property in the class
- Incorrect implementation of KVC methods
- Inherited class not implementing KVC
Once you have identified the cause, you can proceed to resolve the issue.
Resolving the Issue
Fixing a Typo in the Key Name
Double-check the key name in your code and make sure it matches the property name in your class. If there's a typo, correct it and re-run your code.
Adding the Missing Property
If the property is missing from your class, add it with the correct data type and access level. For example:
@interface MyClass : NSObject
@property (nonatomic, strong) NSString *yourKey;
@end
Implementing KVC Methods
If your class doesn't implement the required KVC methods, you can add them like this:
- (void)setValue:(id)value forKey:(NSString *)key {
if ([key isEqualToString:@"yourKey"]) {
_yourKey = value;
} else {
[super setValue:value forKey:key];
}
}
- (id)valueForKey:(NSString *)key {
if ([key isEqualToString:@"yourKey"]) {
return _yourKey;
} else {
return [super valueForKey:key];
}
}
Replace yourKey with the name of the property you want to make KVC-compliant.
Ensuring Inherited Classes Implement KVC
If your class inherits from another class that doesn't implement KVC, make sure to implement the KVC methods in the inherited class as well.
FAQ
What is Key-Value Coding (KVC)?
Key-Value Coding is a mechanism in Cocoa and Cocoa Touch that allows you to access an object's properties indirectly using string keys.
How do I make my class KVC-compliant?
To make your class KVC-compliant, implement the setValue:forKey: and valueForKey: methods for getting and setting values for your class properties.
Can Swift classes use Key-Value Coding?
Yes, Swift classes can use Key-Value Coding when they inherit from NSObject and use the @objc attribute for their properties.
Is Key-Value Coding slower than direct property access?
Yes, Key-Value Coding can be slower than direct property access because it involves additional steps like string parsing and method resolution. However, the difference is usually negligible in most cases.
Can I use Key-Value Coding with Core Data?
Yes, Key-Value Coding is commonly used with Core Data to access managed object properties.