NSKeyValueCoding

NSKeyValueCoding, often abbreviated as KVC, is a fundamental design pattern and an Objective-C runtime capability provided by Apple's Cocoa and Cocoa Touch frameworks. It allows objects to access and modify properties of other objects indirectly, using string keys instead of direct method calls. This mechanism enables a dynamic and flexible way to interact with an object's state.

The core of NSKeyValueCoding relies on conventions. When you request a value for a given key, an object will first look for a method named `key` or `isKey` (for boolean values). If such an accessor method is not found, it will then look for a method named `primitiveForKey:` (where `primitive` is the capitalized key). Similarly, when setting a value for a key, it searches for `setKey:` or `setPrimitiveForKey:`. If none of these conventional methods are found, the object's `valueForUndefinedKey:` or `setValue:forUndefinedKey:` methods are invoked, which typically result in an exception.

Key benefits of NSKeyValueCoding include its role in data binding, object graph management, and enabling powerful features like Key-Value Observing (KVO). It allows for runtime introspection and manipulation of object properties, making it a cornerstone for many higher-level frameworks within Apple's ecosystem. For instance, when using `NSUserDefaults` or when performing data serialization and deserialization with `NSCoding`, NSKeyValueCoding is often operating behind the scenes. It also facilitates the use of selectors with dynamic string keys, offering a way to invoke methods indirectly.