Behavior class pattern (VII): Observer (Observer)

Defined

Defines a one-to-many dependency between objects so that when each object changes state, all objects that depend on it are notified and updated automatically.

Uml

Advantages

1. It is loosely coupled between the observer and the observed, and can be individually changed independently of each other.
2. Subject when sending a broadcast notification, you do not need to specify a specific observer,observer to decide whether to subscribe to subject notifications.
3. Comply with most grasp principles and common design principles, high cohesion and low coupling.

Disadvantages

1. Loose coupling can cause code relationships to be less obvious and sometimes difficult to understand.
2. If an object is subscribed by a large number of observers, there may be an efficiency issue when broadcasting notifications. (After all, just a simple traversal)

Application Scenarios

1. Updates to an object's state require other objects to be updated synchronously, and the number of other objects is dynamically variable.
2. Objects only need to notify their own updates to other objects without needing to know the details of other objects.

Example

Implement an example of a simple observer and a observed person.

Java

1 Importjava.util.ArrayList;2 Importjava.util.List;3 4  Public classMain5 {6      Public Static voidMain (string[] args)7     {8Subject Sub =NewConcreteSubject ();9Sub.addobserver (NewConcreteObserver1 ());//Add Observer 1TenSub.addobserver (NewConcreteObserver2 ());//Add Observer 2 One sub.dosomething (); A     } -  -     /** the * Observe the interface -      */ -      Public InterfaceObserver -     { +         voidupdate (); -     } +  A     /** at * The Observer base class -      */ -      Public Static Abstract classSubject -     { -         PrivateList<observer> Obs =NewArraylist<>(); -  in          Public voidaddobserver (Observer obs) -         { to              This. Obs.add (OBS); +         } -  the          Public voiddelobserver (Observer obs) *         { $              This. Obs.remove (OBS);Panax Notoginseng         } -  the         protected voidNotifyobserver () +         { A              for(Observer o:obs) the             { + o.update (); -             } $         } $  -          Public Abstract voiddosomething (); -     } the  -     /**Wuyi * Viewer Object 1 the      */ -      Public Static classConcreteObserver1ImplementsObserver Wu     { -          Public voidUpdate () About         { $SYSTEM.OUT.PRINTLN ("Observer 1 receives the message and processes it. "); -         } -     } -  A     /** + * Viewer Object 2 the      */ -      Public Static classConcreteObserver2ImplementsObserver $     { the          Public voidUpdate () the         { theSYSTEM.OUT.PRINTLN ("Observer 2 receives the message and processes it. "); the         } -     } in  the     /** the * The specific person being observed About      */ the      Public Static classConcreteSubjectextendsSubject the     { the          Public voiddosomething () +         { -System.out.println ("The observed event is reversed"); the              This. Notifyobserver ();Bayi         } the     } the}

View Code

Behavior class pattern (VII): Observer (Observer)