Detailed explanation of the definition, classification, and usage examples of the factory pattern of JavaScript design patterns

Concept: The definition of factory pattern: Provides an interface for creating objects, which means producing corresponding products (objects) according to the instructions (parameters) of the leader (caller).

Creating an object often requires a complex process, so it is not suitable for a complex object.

Creating objects may result in a lot of duplication of code, and may not provide a sufficient level of abstraction.

The factory transfers the creation of member objects to an external object. The advantage is to eliminate the coupling between objects (that is, mutual influence)

Category:

Simple Factory Pattern: Use a class, usually a singleton, to generate instances.

Complex Factory PatternThe definition is: push the instantiation of its member objects to the subclass, and the subclass can override the parent class interface method to create When specifying your own object type.

The parent class only handles general issues during the creation process. These processes will be inherited by the subclasses. The subclasses are independent of each other. , specific Business logic will be written in subclasses.

Code implementation:

Simple factory pattern: 　

var XMLHttpFactory =function(){};　　　　　　//这是一个简单工厂模式
　　XMLHttpFactory.createXMLHttp =function(){
　　　 var XMLHttp = null;
　　　　if (window.XMLHttpRequest){
　　　　　　XMLHttp = new XMLHttpRequest()
　　　 }else if (window.ActiveXObject){
　　　　　　XMLHttp = new ActiveXObject("Microsoft.XMLHTTP")
　　　　}
　　return XMLHttp;
　　}
　　//XMLHttpFactory.createXMLHttp()这个方法根据当前环境的具体情况返回一个XHR对象。
　　var AjaxHander =function(){
　　　　var XMLHttp = XMLHttpFactory.createXMLHttp();
　　　　...
　　}

Complex Factory Pattern: Process ==》 First design an abstract class. This class cannot be instantiated and can only be used to derive subclasses. Finally, the factory method is implemented by extending the subclass

var XMLHttpFactory =function(){};　   //这是一个抽象工厂模式
XMLHttpFactory.prototype = {
　　//如果真的要调用这个方法会抛出一个错误，它不能被实例化，只能用来派生子类
　　createFactory:function(){
 　　throw new Error('This is an abstract class');
　　}
}
var XHRHandler =function(){}; //定义一个子类
// 子类继承父类原型方法
extend( XHRHandler , XMLHttpFactory );
XHRHandler.prototype =new XMLHttpFactory(); //把超类原型引用传递给子类,实现继承
XHRHandler.prototype.constructor = XHRHandler; //重置子类原型的构造器为子类自身
//重新定义createFactory 方法
XHRHandler.prototype.createFactory =function(){
　　var XMLHttp =null;
　　if (window.XMLHttpRequest){
 　　XMLHttp =new XMLHttpRequest();
　　}else if (window.ActiveXObject){
 　　XMLHttp =new ActiveXObject("Microsoft.XMLHTTP")
　　}
　　return XMLHttp;
}

Application scenarios:

The following The factory pattern is particularly useful in situations where:

(1) The construction of objects is very complex

(2) Different instances need to be created depending on the specific environment

(3) Processing a large number of objects with the same Small object of attributes

Advantages:

can implement some of the same methods, and we can write code for these same methods in the parent class , then you need to implement specific business logic, then you can override the method of the parent class in a subclass to implement your own business logic;

In other words, there are two points:

1. Weaken the coupling between objects to prevent code duplication. Instantiating a class in a method eliminates repetitive code.

2. Repetitive code can be written in the parent class. The subclass inherits all member properties and methods of the parent class, and the subclass only focuses on implementing its own business logic.

Disadvantages:

When the factory increases to a certain level, the complexity of the code increases and the readability decreases. And it does not solve the problem of object recognition, that is, how to know the type of an object.

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