单例模式分析和思想

singleton(单例)模式被熟知的原因是因为它限制了类的实例化次数只能一次，单例模式，在该实例不存在的勤快下，可以通过一个方法创建一个类来实现创建类的新实例；如果实例已经存在，则会简单返回该对象的引用。单例模式不同于静态类（或对象），因为我们可以推迟它们的初始化，这通常是因为它需要一些信息，而这些信息在初始化期间可能无法获取，对于没有察觉到之前的引用代码，它们不会提供方便检索方法，这是因为它既不是对象，也不是由一个single返回的类，而是一个结构，在js中，singleton充当共享资源命名空间，从全局命名空间中隔离出代码实现，从而函数提供单一访问点。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; background: transparent; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> mySingleton = (<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//实例保持了singleton的一个引用</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> instacnce;
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">init</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//私有方法和变量</span>
        funciton privateMethod(){
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//...</span>
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> privateVariable = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"private"</span>;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> privateRandomNumber = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Math</span>.random();
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/*这里定义单例代码*/</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//公有方法和变量</span>
            publicMethod: <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
                console.log(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'hello world'</span>);
            },
            publicProperty: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'test'</span>,
            getRandomNumber:<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> privateRandomNumber;
        }
        };
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获取singleton的实例，如果存在就返回，不存在创建新的实例</span>
        getInstance: <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!instance) {
                instance= init();
            }
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> instance;
        }
    };
})();

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/*调用公有的方法来获取实例:*/</span>
mySingleton .getInstance().publicMethod();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right: 1px solid rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li></ul>

singleton模式适用性：

1. 当类只能有一个实例而且客户可以从一个众所周知的访问点访问它时；

2. 当唯一的实例应该是通过子类化可扩展的，并且客户应该无需要更改代码就能使用一个扩展的实例时。

   类的静态实例和singleton之间的区别：当singleton可以作为一个静态的实例实现时，它可以延迟构建，直到需要使用静态实例时，无需使用资源和内存。 
   如果我们有一个可以直接被初始化的静态对象，需要确保执行代码顺序总是相同，当我们有大量的源文件时，它并不能伸缩。

在实践中，当在系统中确实需要一个对象来协调其他对象时，singleton模式是很有用的。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; background: transparent; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> SingletonTester = (<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//参数：传递给单例的一个参数集合</span>
    <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">Singleton</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(options)</span> {</span>

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置options变量为接收的参数或者为空（如果没有提供的话）</span>
        options= options|| {};
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置name参数</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.name = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'SingletonTester'</span>;
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置pointX的值</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.pointX = options.pointX || <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//从接收的参数里获取，或者设置为默认值</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置pointY的值</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.pointY = options.pointY || <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>;

    }

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//实例容器</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> instance;

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//静态变量和方法模拟</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> _static = {
        name: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'SingletonTester'</span>,

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获取实例的方法</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//返回Singleton的实例</span>
        getInstance: <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(options)</span> {</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (instance === <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">undefined</span>) {
                instance = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Singleton(options);
            }
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> instance;
        }
    };
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> _static;
})();

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> singletonTest = SingletonTester.getInstance({ pointX: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> });
console.log(singletonTest.pointX); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 输出 5 </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right: 1px solid rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li></ul>

singleton的存在往往表明系统中的模块要么是系统紧密耦合，要么是其逻辑过于分散在代码库中多个部分。由于一系列的问题：从隐藏的依赖到创建多个实例的难度、底层依赖的难度等等，singleton测试会更麻烦