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;
        }
    }