js简单工厂

我以计算器为例写一个简单工厂模式，只完成加减乘除4个计算功能，考虑到其他功能方便日后扩展，遵循开放-封闭原则。

简单工厂类图：

先看一下C#的简单工厂是如何实现的：

定义抽象类Operation，加减乘除的实现由子类派生，最后由OperationFactory决定实例化哪一个子类。

namespace OperationLibrary
{
    public abstract class Operation
    {
        private int num_A = 0;
        public int Num_A
        {
            get { return num_A; }
            set { num_A = value; }
        }

        private int num_B = 0;
        public int Num_B
        {
            get { return num_B; }
            set { num_B = value; }
        }

        public abstract double getResult();
    }

    public class OperationAdd : Operation
    {
        public override double getResult()
        {
            return Num_A + Num_B;
        }
    }

    public class OperationSub : Operation
    {
        public override double getResult()
        {
            return Num_A - Num_B;
        }
    }

    public class OperationMul : Operation
    {
        public override double getResult()
        {
            return Num_A * Num_B;
        }
    }

    public class OperationDiv : Operation
    {
        public override double getResult()
        {
            if (Num_B == 0)
                throw new Exception("除数不能为0");
            return Num_A / Num_B;
        }
    }

    public class OperationFactory
    {
        public static Operation createOperate(string operate)
        {
            Operation oper = null;
            switch (operate)
            {
                case "+":
                    oper = new OperationAdd();
                    break;
                case "-":
                    oper = new OperationSub();
                    break;
                case "*":
                    oper = new OperationMul();
                    break;
                case "/":
                    oper = new OperationDiv();
                    break;
            }
            return oper;
        }
    }
}

客户端调用：

Operation oper = OperationFactory.createOperate("/");
oper.Num_A = 6;
oper.Num_B = 2;
oper.getResult(); //3

js模拟C#的简单工厂：

var Operation = function(){
        this.num_A = this.num_B = 0;
    };

    var OperationAdd = function(){};
    OperationAdd.prototype = new Operation();
    OperationAdd.prototype.getResult = function(){
        return this.num_A + this.num_B;
    };

    var OperationSub = function(){};
    OperationSub.prototype = new Operation();
    OperationSub.prototype.getResult = function(){
        return this.num_A - this.num_B;
    };

    var OperationMul = function(){};
    OperationMul.prototype = new Operation();
    OperationMul.prototype.getResult = function(){
        return this.num_A * this.num_B;
    };

    var OperationDiv = function(){};
    OperationDiv.prototype = new Operation();
    OperationDiv.prototype.getResult = function(){
        if(this.num_B == 0)
            throw new Error('除数不能为0');
        return this.num_A / this.num_B;
    };

    var OperateFactory = function(){};
    OperateFactory.createOperate = function(operate){
        var oper;
        switch(operate){
            case "+":
                oper = new OperationAdd();
            break;
            case "-":
                oper = new OperationSub();
            break;
            case "*":
                oper = new OperationMul();
            break;
            case "/":
                oper = new OperationDiv();
            break;
        }
        return oper;
    };

        //调用：
    var oper = OperateFactory.createOperate('+');
    oper.num_A = 1;
    oper.num_B = 2;
    alert(oper.getResult()); //3

js完全照搬C#的模式显然太复杂，根据js语言的特性实现一个：

var Operation = function(){
        this.num_A = this.num_B = 0;
    };
    Operation.prototype.OperationAdd = function(){
        return this.num_A + this.num_B;
    };
    Operation.prototype.OperationSub = function(){
        return this.num_A - this.num_B;
    };
    Operation.prototype.OperationMul = function(){
        return this.num_A * this.num_B;
    };
    Operation.prototype.OperationDiv = function(){
        if(this.num_B == 0)
            throw new Error('除数不能为0');
        return this.num_A / this.num_B;
    };
   
    //策略
    var operateStrategy = {
        '+':'OperationAdd',
        '-':'OperationSub',
        '*':'OperationMul',
        '/':'OperationDiv'
    };

    //工厂
    var OperateFactory = (function(){

        var oper;

        return function(operate){

            //只实例化一次
            oper = oper || new Operation();

            return {
                setNumber:function(a,b){
                    oper.num_A = a;
                    oper.num_B = b;
                },
                getResult:function(){
                    var fName = operateStrategy[operate],
                        fun = oper[fName] || function(){};
                    return fun.call(oper);
                }
            };
        };    
    })();


    //调用：
    var oper = OperateFactory('+');
    oper.setNumber(1,3);
    alert(oper.getResult());

    oper = OperateFactory('*');
    oper.setNumber(2,3);
    alert(oper.getResult());

总结：如果日后需要扩展计算器的功能，我们需要维护2个地方，一个是为Operation的原型添加所需要的新功能，另一个是维护策略对象：operateStrategy， 基本上不需要修改程序的内部方法，而是为它增加扩展，基本符合开放-封闭原则。我认为只要对面向对象编程思想有所了解，都能写出简单工厂模式，使用场景无需考虑因为在编码过程中这是一件很自然的事情，他简单到都不能算是一个模式。