valueforKey

ValueForKey is a method provided by the Objective-C runtime as part of the Key-Value Coding (KVC) system in the Foundation framework. It enables retrieving the value associated with a named key from an object at runtime, enabling dynamic access to properties without a statically coded accessor.

How valueForKey resolves keys: the runtime first looks for a getter method named after the key (for example, -name). If no such method exists, it looks for an instance variable named after the key or for other KVC-compliant accessors. If a value cannot be obtained, the default behavior is to raise NSUndefinedKeyException, unless the class overrides the behavior. For nested access, valueForKeyPath: can be used to traverse a dotted path, applying valueForKey: to each segment (for example, "address.city").

Common usage and examples: valueForKey is widely used in Cocoa bindings, Core Data, and data-driven UI models. Example: id v = [person valueForKey:@"name"]; If a key points to a to-many relationship, the result can be a collection, and valueForKeyPath can operate across nested objects (for example, [order valueForKeyPath:@"customer.name"]). This mechanism supports dynamic property access without explicit method calls and integrates with other KVC and KVO features.

Limitations and compliance: to use valueForKey effectively, a class should expose properties or ivars that conform to KVC expectations (or override valueForKey: to customize behavior). The technique relies on the object’s KVC compliance, and failure to provide a valid key can lead to runtime exceptions. In Swift, KVC usage requires bridging via Objective-C compatibility attributes and dynamic accessors.