KVO

KVO (Key‑Value Observing) is a mechanism provided by Apple’s Cocoa and Cocoa Touch frameworks that enables objects to be notified when a property of another object changes. It is built upon the key‑value coding (KVC) infrastructure, allowing observers to register for specific key paths and receive callbacks without requiring the observed object to implement explicit delegate methods or notifications.

To use KVO, an object adds itself as an observer of another object’s property by calling addObserver:forKeyPath:options:context:. The observed object must be KVC‑compliant, meaning its properties are accessible through standard getter and setter methods or are dynamically provided via the runtime. When the property’s value changes—typically through a KVC‑compatible setter—the runtime automatically generates willChangeValueForKey:/didChangeValueForKey: calls and dispatches an observation notification. The observer receives these events in its observeValueForKeyPath:ofObject:change:context: method, where it can inspect the change dictionary for old and new values.

KVO is widely used for synchronizing UI elements with underlying data models, implementing bindings, and decoupling components in Model‑View‑Controller architectures. However, developers must manage observer lifecycles carefully; failing to remove observers before deallocation can cause runtime exceptions. Since iOS 11 and macOS 10.13, Apple introduced block‑based KVO APIs that simplify registration and automatic deregistration, improving safety and readability.