PropertyChange

PropertyChange is a notification mechanism used in software design to indicate that a named property of an object has changed. It is typically emitted by the object's mutator or setter when the new value differs from the old one. Listeners or observers register to receive these notifications, which generally include the property name, the old value, the new value, and sometimes a reference to the source object. This pattern supports decoupled communication between components, such as models and user interfaces.

In Java, the concept is formalized in the JavaBeans framework through PropertyChangeEvent, PropertyChangeListener, and the PropertyChangeSupport helper. A bean fires a property change event by calling firePropertyChange with the property name and values. Listeners implement the propertyChange method to react to updates, enabling common data-binding and UI refresh scenarios.

In the .NET ecosystem, a closely related pattern is provided by the INotifyPropertyChanged interface and the PropertyChanged event. Frameworks and libraries use OnPropertyChanged to raise the event, typically supplying only the property name, though some implementations expose additional context. This supports data binding in UI frameworks and MVVM architectures.

Other ecosystems offer similar mechanisms, such as Cocoa’s Key-Value Observing (KVO) and various signal/slot or observer patterns in dynamic languages. Use of PropertyChange signals is common for UI synchronization, model-view communication, and any scenario requiring responsive updates to state changes.

Design considerations include avoiding unnecessary notifications, thread-safety, proper listener lifecycle management to prevent leaks, and decisions about including old and new values.