Hook: React To Changes In Object's State

Requirement

An object's state must be monitored and reactions to changes of the object's state must be implemented.

Method of Adaptation

Add Feature

Level of Support

Parameter Pattern

HotSpot

Application architecture

Participants

object , eventType , NewListener

Changes

1. if object sends events to inform about state changes:
1. select appropriate eventType.
2. implementation NewListener of < eventType > Listener or subclass NewListener of < eventType > Adapter.
3. NewListener implements/overrides the appropriate notification method.
4. call object.add < eventType > Listener with parameter instance of NewListener.

Constraints

• If you need to maintain the connection over serialization and deserialization of the object graph, NewListener must also implement the util.SerializableListener tag interface. Usually, adapters already implement this interface.

Comments

• The hook describes on a generic level how to react to changes in objects that support events. This is an implementation of the Observer design pattern.

  Classes/Interfaces in the Framework that support events are:

  • data.ListenableCatalog
  • data.ListenableStock
  • data.ListenableDataBasket
  • data.Nameable
  • sale.Display
  • sale.Timer
  • users.User
  • users.UserManager
  And subclasses thereof. Some classes may provide alternative ways of informing clients about changes in their state. For these cases usually a separate hook is provided.

• Listener interfaces usually provide several methods that are called in response to specific subtypes of the eventType selected. If there are several methods in a Listener interface, as a rule, there will also be an adapter class that you can use as a base class for your listener.
• An example of listener usage is the implementation of referential integrity in the Pure Java implementation of the data management classes. Here, every Stock listens to its Catalog to be informed whenever a CatalogItem gets removed or edited. If such an event occurs, the Stock will react appropriately to maintain referential integrity.