javascript模式——Flyweight

Flyweight是一种共享数据内存的模式。

Flyweight模式是将一些公有属性从个人中剔除，放在共享的对象当中。

下面以一个项目实例，通过不断的改进，以显示Flyweight模式的优点。

现在我们想做一个电脑生产程序，用以生产电脑，电脑的一般有生产商，机型，CPU类型，内存，和型号。

我们的最初代码如下：

// 电脑类
var Computer = function (make, model, processor, memory, tag) {
    this.make = make;
    this.model = model;
    this.processor = processor;
    this.memory = memory;
    this.tag = tag;
}

// 测试生产两台不同的电脑
var computer1 = new Computer("Dell", "Studio XPS", "Intel", "4G", "Y755P");
var computer2 = new Computer("Dell", "Studio XPS", "Intel", "8G", "Y755P");

console.log(computer1);
console.log(computer2);

对代码简单的分析，我们可以发现，两台不同的电脑的前三个属性，品牌，机型和CPU都是一样的。而我们却每次都为其重新定义赋值。

试想一下，如果这样有一百上千台的电脑，那对内存来说就是一笔不小的开销。我们可不可以将这些相同的属性共有化呢？

// 电脑类
var Computer = function (make, model, processor, memory, tag) {

    // 引用共享单元中的数据
    if(make + model + processor === "DellStudio XPSIntel"){
        this.flyweight = flyweight1;
    }
    this.memory = memory;
    this.tag = tag;
}

// 共享单元
var flyweight = {
    make: 'Dell',
    model: "Studio XPS",
    processor: "Intel"
}

// 测试
var computer1 = new Computer("Dell", "Studio XPS", "Intel", "4G", "Y755P");
var computer2 = new Computer("Dell", "Studio XPS", "Intel", "8G", "Y755P");

console.log(computer1);
console.log(computer2);

在这里我们将共有几乎固定的属性从类中剔除，建立了一个很生硬的对象flyweight以存放他们用来共享。就这样，虽然新生产每台电脑都有5个属性，但其中前三个属性都是共享flyweight对象中的数据。节省了内存开支。尤其这样前三个属性相同的电脑比较多时，这已经就是Flyweight的精髓了，只是我们的模式还很初步。

继续我们的思路，如果我们需要再多一个共享单元呢，难道重新创建一个对象？不是的。此时我们用一个类创建各种享元，且用一个对象作为列表来存放来会更好一些。

// 享元列表
var flyweightList = {};

var Computer = function (make, model, processor, memory, tag) {

    if(flyweightList[make+model+processor]){

        // 享元列表中存在，直接引用
        this.flyweight = flyweightList[make+model+processor];
    }else {

        // 不存在，创建新的享元对象并存入列表中
        this.flyweight = new Flyweight(make, model, processor);
        flyweightList[make+model+processor] = this.flyweight;
    }

    this.memory = memory;
    this.tag = tag;
}

// 享元类
function Flyweight (make, model, processor) {

    this.make = make;
    this.model = model;
    this.processor = processor;

};

var computer1 = new Computer("Dell", "Studio XPS", "Intel", "4G", "Y755P");
var computer2 = new Computer("Dell", "Studio XPS", "Intel", "8G", "Y755P");

var computer3 = new Computer("HP", "Envy", "Intel", "2G", "801632312");

console.log(computer1);
console.log(computer2);
console.log(computer3);

这样其实已经很不错了，只是享元列表作为一个全局变量似乎不太好，在比较高级的享元模式运用，我们还需要一些其他的方法，如获取享元中的数量等。进一步我们可以用模块模式将享元包裹一下。

function Flyweight (make, model, processor) {

    this.make = make;
    this.model = model;
    this.processor = processor;

};

var FlyweightFactory = (function(){

    // 私有
    var flyweightList = {};

    return {

        // 新增享元
        addFlyweight: function (make, model, processor){
            if(!flyweightList[make+model+processor]){
                flyweightList[make+model+processor] = new Flyweight(make, model, processor);
            }
            return flyweightList[make+model+processor];
        },

        // 计数
        count: function(){
            var count = 0;
            for (var f in flyweightList) count++;
            return count;
        }
    }
})()

var Computer = function (make, model, processor, memory, tag) {

    this.flyweight = FlyweightFactory.addFlyweight(make, model, processor)
    this.memory = memory;
    this.tag = tag;

}

var computer1 = new Computer("Dell", "Studio XPS", "Intel", "4G", "Y755P");
var computer2 = new Computer("Dell", "Studio XPS", "Intel", "8G", "Y755P");
var computer3 = new Computer("HP", "Envy", "Intel", "2G", "801632312");

console.log(computer1);
console.log(computer2);
console.log(computer3);
console.log(FlyweightFactory.count())

享元是一种非常常用的设计模式，对性能的优化有很大的作用。