Design Pattern-Observer Pattern

Document directory

• Advantages:
• Defects:

I. Concepts

The observer mode defines a one-to-many dependency between objects, so that every time an object changes state, all objects dependent on it will be notified and automatically updated (The Observer PatternDefines a one-to-define dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically .).

Ii. Composition of observer Mode

Subject(Object interface observed)

• Specifies the unified interface of concretesubject;
• Each subject can have multiple observers;

Concretesubject(Specific object to be observed)

• Maintains a list of references to all specific observers;
• When the status changes, a notification is sent to all registered observers.

Observer(Observer Interface)

• Specifies the unified interface of concreteobserver;
• Defines an update () method, which is called when the state of the object to be observed changes.

Concreteobserver(Specific observer)

• Maintain a reference to concretesubject;
• Synchronization between the specified status and concretesubject;
• Implement the observer interface to receive concretesubject notifications through the update () method.

 

Iii. application scenarios

• To update the status of an object, you must synchronously update other objects, and the number of other objects can be dynamically changed.
• The object only needs to notify other objects of its own updates without knowing the details of other objects.

 

Iv. Advantages and DisadvantagesAdvantages:

• Subject and observer are loosely coupled and can be changed independently.
• When a subject sends a broadcast notification, it does not need to specify the specific observer. The observer can decide whether to subscribe to the subject notification.
• Comply with most grasp principles and common design principles, high cohesion and low coupling.

Defects:

• Due to loose coupling, the code relationship is not obvious and may be difficult to understand.
• If a subject is subscribed to by a large number of observers, it may be efficient during broadcast notifications. (After all, it is just a simple traversal)

 

References: http://www.cnblogs.com/justinw/archive/2007/05/02/734522.html