Understanding Design Patterns (Viewer)

Observer pattern to ensure consistency between the subject and several observers.

Overview

sometimes called publish-subscribe subscribe> mode, model-view view> mode, source-listener listener> mode, or slave mode

This pattern is often used to implement an event-handling system

Specific environment

• When one object changes and some other objects change
• Relatively independent between actively changing objects and passively changing objects for reuse and maintenance

A class of questions

A system that consists of an object and several collaboration objects, while avoiding the coupling of these objects to each other

Solution Solutions

• Abstract theme Roles
• Specific theme Roles
• Abstract Observer role
• Specific observer roles

Abstract theme Roles

generally implemented by interfaces or abstract classes

Register a set of references that have several abstract observers

At the same time, the ability to delete the abstract observer, and change itself, notify the observer to make corresponding changes

Specific theme Roles

Abstract theme roles are generally implemented by specific subclasses with their own state

Abstract Observer role

generally implemented by interfaces or abstract classes

Define the behavior specification for all specific observers

and define methods, receive notifications and update their status

Specific observer roles

The abstract observer role is generally implemented by a specific subclass, with its own state, which changes with the subject state.

Push models and pull models

In the observer pattern, it is also divided into:

• Push model
  The subject object pushes the details of the topic to the observer, and the information that is pushed is usually all or part of the data of the subject object, regardless of whether or not the observer needs them.
• Pull model
  Subject object When notifying the observer, only a small amount of information is passed
  If the observer needs more specific information, the Observer takes the initiative to get to the subject object, which is equivalent to the observer pulling data from the subject object
  In the general implementation of this model, the subject object itself is passed to the observer through the Update method
  So that when the observer needs to get the data, it can use this reference to get the

Comparison of two implementation methods

• The push model is the assumption that the subject object knows the data that the observer needs;
  While the pull model is the subject object does not know what the observer specifically needs data, there is no way, simply pass itself to the observer, let the observer himself to the value of the need

• Pushing a model may make it difficult to reuse the observer object because the Observer's Update method is a parameter that is defined as needed, and may not be able to take into account the unused usage. This means that new update methods may be available when new situations arise, or simply re-implement the observer;
  The pull model does not cause this situation, because the parameter of the Update method under the pull model is the subject object itself, which is basically the largest collection of data that can be passed by the subject object, and can be adapted to the needs of various situations basically.

Observer interfaces and Observable classes in Java

observer Interface: Viewer

observable class: The subject, that is, the person being observed

by inheriting or implementing them, you can quickly implement the observer pattern, and thread-safe

In addition, these two types use a pull model

Reference

The observer pattern of Java and patterns, Java_my_life

Understanding Design Patterns (Viewer)