PropertyChangedEventHandler

PropertyChange refers to a signaling mechanism that informs interested objects when a property value on another object changes. It enables data binding, UI refresh, and synchronization across components without tight coupling. In object-oriented environments, a property change event typically includes the source object, the name of the changed property, and the old and new values.

In Java, the JavaBeans specification defines a PropertyChangeEvent and a PropertyChangeListener interface. Classes that support bound properties usually maintain a PropertyChangeSupport helper and fire a property change notification whenever a property's setter updates its value. Listener objects register with addPropertyChangeListener and are notified via propertyChange.

In the .NET framework, a closely related pattern is defined by the INotifyPropertyChanged interface, which exposes the PropertyChanged event. Data-binding frameworks such as WPF and Xamarin.Forms subscribe to this event to update views when a model or view-model property changes. Implementations typically raise the event after updating the property value, often including the property name.

Other ecosystems offer similar concepts under different names, but the core idea remains: decoupled observers receive notifications of changes and can react accordingly. Design considerations include deciding whether to expose old and new values, handling notification on the correct thread, and preventing memory leaks by unregistering listeners.

Property change mechanisms support patterns such as Model-View-ViewModel and reactive programming. They are contrasted with polling or manual refresh approaches, offering responsive, event-driven state updates.