javascript设计模式-工厂模式（简单工厂）

接口在工厂模式中起着很重要的作用，如果不对对象进行某种类型检查的以确保其实现了必要的方法，工厂模式的好处也就所剩无几了，举个简单的例子。

Interface.js

 // Constructor.

 var Interface = function(name, methods) {
     if(arguments.length != 2) {
         throw new Error("Interface constructor called with " + arguments.length
           + "arguments, but expected exactly 2.");
     }

     this.name = name;
     this.methods = [];
     for(var i = 0, len = methods.length; i < len; i++) {
         if(typeof methods[i] !== 'string') {
             throw new Error("Interface constructor expects method names to be "
               + "passed in as a string.");
         }
         this.methods.push(methods[i]);
     }
 };    

 // Static class method.

 Interface.ensureImplements = function(object) {
     if(arguments.length < 2) {
         throw new Error("Function Interface.ensureImplements called with " +
           arguments.length  + "arguments, but expected at least 2.");
     }

     for(var i = 1, len = arguments.length; i < len; i++) {
         var interface = arguments[i];
         if(interface.constructor !== Interface) {
             throw new Error("Function Interface.ensureImplements expects arguments "
               + "two and above to be instances of Interface.");
         }

         for(var j = 0, methodsLen = interface.methods.length; j < methodsLen; j++) {
             var method = interface.methods[j];
             if(!object[method] || typeof object[method] !== 'function') {
                 throw new Error("Function Interface.ensureImplements: object "
                   + "does not implement the " + interface.name
                   + " interface. Method " + method + " was not found.");
             }
         }
     }
 };

声明一个接口Bicycle，定义该接口应该有哪些方法

 var Bicycle = new Interface("Bicycle",["assemble","wash","ride","repair"]);

构造一个简单的工厂BicycleShop，根据传递的模型，卖出不同类型的自行车。

 var BicycleShop  = function(){};

     BicycleShop.prototype = {
         sellBicycle:function(model){
             var bicycle;
             switch(model){
                 case "The Speedster":
                     bicycle = new Speedster();
                     break;
                 case "The Lowrider":
                     bicycle = new Lowrider();
                     break;
                 case "The Comfort Cruiser":
                 default:
                     bicycle = new ComfortCruiser();
             }
             Interface.ensureImplements(bicycle,Bicycle);
             bicycle.assemble();
             bicycle.wash();
             return bicycle;
         }
     }

建立一个Speedster类型的自行车，且该类提供了必要的方法

 var Speedster = function(){
     };
     Speedster.prototype = {
         assemble:function(){
             console.log("assemble method is exected");
         },
         wash:function(){
             console.log("wash method is exected");
         },
         ride:function(){
             console.log("ride method is exected");
         },
         repair:function(){
             console.log("repair method is exected");
         }
     }

创建一个工厂，传入模型参数

 var californiaCruisers = new BicycleShop();
 californiaCruisers.sellBicycle("The Speedster");

结果：

然而，如果需要加入一款新车型，那么必须为此修改BicycleShop代码，哪怕这个类的实际功能并没有发生变化。因此，更好的解决办法是把SellBicycle中的方法中创建实例的这部分交给一个简单的工厂。

 var BicycleFactory = {
         createBicycle:function(model){
             var bicycle;
             switch(model){
                 case "The Speedster":
                     bicycle = new Speedster();
                     break;
                 case "The Lowrider":
                     bicycle = new Lowrider();
                     break;
                 case "The Comfort Cruiser":
                 default:
                     bicycle = new ComfortCruiser();
             }
             Interface.ensureImplements(bicycle,Bicycle);
             return bicycle;
         }
     }

     var BicycleShop  = function(){};

     BicycleShop.prototype = {
         sellBicycle:function(model){
             var bicycle = BicycleFactory.createBicycle(model);
             bicycle.assemble();
             bicycle.wash();
             return bicycle;
         }
     }