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2019-02-15-JavaScript设计模式——单例模式.html
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<p>学习曾探的 《JavaScript设计模式与开发实践》并做记录。</p>
<p><a href="http://www.ituring.com.cn/book/1632/">书籍的购买链接</a></p>
<p>设计模式的定义是：在面向对象软件设计过程中针对特定问题的简洁而优雅的解决方案。</p>
<p>通俗一点，设计模式是在某种场合下对某个问题的一种解决方案。是给面向对象软件开发中的一些好的设计取个名字。</p>
<h1 id="JavaScript设计模式——单例模式"><a href="#JavaScript设计模式——单例模式" class="headerlink" title="JavaScript设计模式——单例模式"></a>JavaScript设计模式——单例模式</h1><p>单例模式的定义是：保证一个类仅有一个实例，并提供一个访问它的全局访问点</p>
<p>单例模式是一种常用的模式，有一些对象我们往往只需要一个，比如线程池、全程缓存、浏览器中的 window 对象等等。在 JavaScript 开发中，单例模式用途同样广泛。试想一下，当我们单机登录按钮的时候，页面中只会出现一个登录浮窗，而这个登录浮窗是唯一的，无论单击多少次登录按钮，这个浮窗都只会被创建一次，那么这个登录浮窗就适合用单例模式来创建。</p>
<h2 id="实现单例模式"><a href="#实现单例模式" class="headerlink" title="实现单例模式"></a>实现单例模式</h2><p>要实现一个标准的单例模式并不复杂，无非使用一个变量来标识当前是否已经为某个类创建过对象，如果是，则在下一次获取该类的实例的时候，直接返回之前创建的对象。代码如下:</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> Singleton = <span class="function"><span class="keyword">function</span> (<span class="params">name</span>) </span>{</span><br><span class="line"> <span class="keyword">this</span>.name = name;</span><br><span class="line"> <span class="keyword">this</span>.instance = <span class="literal">null</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">Singleton.prototype.getName = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="keyword">this</span>.name);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">Singleton.getInstance = <span class="function"><span class="keyword">function</span> (<span class="params">name</span>) </span>{</span><br><span class="line"> <span class="keyword">if</span> (!<span class="keyword">this</span>.instance) {</span><br><span class="line"> <span class="keyword">this</span>.instance = <span class="keyword">new</span> Singleton(name);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span>.instance;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> a = Singleton.getInstance(<span class="string">'sven1'</span>);</span><br><span class="line"><span class="keyword">var</span> b = Singleton.getInstance(<span class="string">'sven2'</span>);</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(a === b); <span class="comment">// true</span></span><br></pre></td></tr></table></figure>
<p>或者</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> Singleton = <span class="function"><span class="keyword">function</span> (<span class="params">name</span>) </span>{</span><br><span class="line"> <span class="keyword">this</span>.name = name;</span><br><span class="line">}</span><br><span class="line">Singleton.prototype.getName = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="keyword">this</span>.name);</span><br><span class="line">}</span><br><span class="line">Singleton.getInstance = (<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> instance = <span class="literal">null</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span> (<span class="params">name</span>) </span>{</span><br><span class="line"> <span class="keyword">if</span> (!instance) {</span><br><span class="line"> instance = <span class="keyword">new</span> Singleton(name);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> instance;</span><br><span class="line"> }</span><br><span class="line">})()</span><br></pre></td></tr></table></figure>
<p>我们通过 Singleton.getInstance 来获取 Singleton 类的唯一对象，这种方式相对简单，但有一个问题，就是增加了这个类的 “不透明性”。Singleton 类的使用者必须知道这是一个单例类，跟以往通过 new XX 的方式来获取对象不同，这里用到是 Singleton.getInstance 来获取对象</p>
<h2 id="透明的单例模式"><a href="#透明的单例模式" class="headerlink" title="透明的单例模式"></a>透明的单例模式</h2><p>实现一个 “透明” 的单例类，用户从这个类中创建对象的时候，可以像使用其他任何普通类一样。在下面的例子中，我们将使用 CreateDiv 单例类，它的作用是负责在页面中创建唯一的 div 节点，代码如下：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> CreateDiv = (<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> instance;</span><br><span class="line"> <span class="comment">//CreateDiv 的构造函数实际上负责了两件事情，第一是创建对象和执行初始化 init 方法，第二是保证只有一个对象。</span></span><br><span class="line"> <span class="keyword">var</span> CreateDiv = <span class="function"><span class="keyword">function</span> (<span class="params">html</span>) </span>{</span><br><span class="line"> <span class="keyword">if</span> (instance) {</span><br><span class="line"> <span class="keyword">return</span> instance;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">this</span>.html = html;</span><br><span class="line"> <span class="keyword">this</span>.init();</span><br><span class="line"> <span class="keyword">return</span> instance = <span class="keyword">this</span>;</span><br><span class="line"> }</span><br><span class="line"> CreateDiv.prototype.init = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> div = <span class="built_in">document</span>.createElement(<span class="string">'DIV'</span>);</span><br><span class="line"> div.innerHTML = <span class="keyword">this</span>.html;</span><br><span class="line"> <span class="built_in">document</span>.body.appendChild(div);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> CreateDiv;</span><br><span class="line">})();</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> c = <span class="keyword">new</span> CreateDiv(<span class="string">'sven1'</span>);</span><br><span class="line"><span class="keyword">var</span> d = <span class="keyword">new</span> CreateDiv(<span class="string">'sven2'</span>);</span><br><span class="line"><span class="built_in">console</span>.log(c === d);</span><br></pre></td></tr></table></figure>
<p>虽然现在是完成了一个透明的单例类的编写，但是它有一些缺点。</p>
<p>为了把 instance 封装起来，使用自执行匿名函数和闭包，并且让这个匿名函数返回真正的 Singleton 构造方法，增加了一些程序的复杂度，阅读起来也不是很舒服。</p>
<h2 id="用代理实现单例模式"><a href="#用代理实现单例模式" class="headerlink" title="用代理实现单例模式"></a>用代理实现单例模式</h2><p>在上面的代码中进行改造，首先在 CreateDiv 构造函数中，把负责管理单例的代码移出去，让它成为一个普通的创建 div 的类：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> CreateDiv = <span class="function"><span class="keyword">function</span> (<span class="params">html</span>) </span>{</span><br><span class="line"> <span class="keyword">this</span>.html = html;</span><br><span class="line"> <span class="keyword">this</span>.init();</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">CreateDiv.prototype.init = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> div = <span class="built_in">document</span>.createElement(<span class="string">'div'</span>);</span><br><span class="line"> div.innerHTML = <span class="keyword">this</span>.html;</span><br><span class="line"> <span class="built_in">document</span>.body.appendChild(div);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>接下来映日 代理类 ProxySingletonCreateDiv:</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> ProxySingletonCreateDiv = (<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> instance;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span> (<span class="params">html</span>) </span>{</span><br><span class="line"> <span class="keyword">if</span> (!instance) {</span><br><span class="line"> instance = <span class="keyword">new</span> CreateDiv(html);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> instance;</span><br><span class="line"> }</span><br><span class="line">})();</span><br></pre></td></tr></table></figure>
<p>通过引用代理类的方法，我们同样完成了一个单例模式的编写，跟之前不同的是，现在我们把负责管理单例的逻辑移到了代理类中，这样一来，CreateDiv 就变成了一个普通的类，它跟代理类组合起来可以达到单例模式的效果。</p>
<p>这个例子是缓存代理的应用之一。</p>
<h2 id="JavaScript-中的单例模式"><a href="#JavaScript-中的单例模式" class="headerlink" title="JavaScript 中的单例模式"></a>JavaScript 中的单例模式</h2><p><strong>单例的核心是确保只有一个实例，并提供全局访问。</strong></p>
<p>单例对象是从 “类” 中创建而来的，在以类为中心的语言中，这是很自然的做法。比如在 Java 中，需要某个对象，就必须先定义一个，对象总是从类中创建的。</p>
<p>但 JavaScript 其实一门 无类的语言，正是因为这样，生搬单例模式的概念并无意义。</p>
<p>全局变量不是单例模式，但在 JavaScript 开发中，我们经常会把全局变量当做单例来使用。</p>
<p>例如：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = {};</span><br></pre></td></tr></table></figure>
<p>当用这种方式创建对象 a 时，对象 a 确实是独一无二的。如果 a 变量被声明在全局作用域下，则我们可以在代码中的任何位置使用这个变量，全局变量提供给全局访问是理所当然的。这样就满足了单例模式的两个条件。</p>
<p>但是全局变量存在很多问题，它很容易造成命名空间污染。</p>
<p>我们有必要尽量减少全局变量的使用，即使需要，也要把它的污染降到最低。以下几种方式可以相对降低全局变量带来的命名污染。</p>
<h3 id="1-使用命名空间"><a href="#1-使用命名空间" class="headerlink" title="1.使用命名空间"></a>1.使用命名空间</h3><p>适当地使用命名空间，并不会杜绝全局变量，但是可以减少全局变量的数量</p>
<p>最简单的方法依然是使用对象字面量的方式：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> namespace1 = {</span><br><span class="line"> a:<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="number">1</span>)</span><br><span class="line"> },</span><br><span class="line"> b:<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="number">2</span>)</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 把 a 和 b 都定义为 namespace1 的属性，可以减少变量和全局作用域打交道的机会。另外，还可以动态地创建命名空间。</span></span><br><span class="line"><span class="keyword">var</span> MyApp = {};</span><br><span class="line">MyApp.namespace = <span class="function"><span class="keyword">function</span>(<span class="params">name</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> parts = name.split(<span class="string">'.'</span>);</span><br><span class="line"> <span class="keyword">var</span> current = MyApp;</span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">var</span> i <span class="keyword">in</span> parts){</span><br><span class="line"> <span class="keyword">if</span>(!current[parts[i]]){</span><br><span class="line"> current[parts[i]] = [];</span><br><span class="line"> }</span><br><span class="line"> current = current[parts[i]]</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line">MyApp.namespace(<span class="string">'event'</span>);</span><br><span class="line">MyApp.namespace(<span class="string">'dom.style'</span>);</span><br><span class="line"><span class="built_in">console</span>.log(MyApp);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 上面代码等价于 </span></span><br><span class="line"><span class="keyword">var</span> MyApp = {</span><br><span class="line"> event:{},</span><br><span class="line"> dom:{</span><br><span class="line"> style:{}</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="2-使用闭包封装私有变量"><a href="#2-使用闭包封装私有变量" class="headerlink" title="2.使用闭包封装私有变量"></a>2.使用闭包封装私有变量</h3><p>这种方法把一些变量封闭在闭包的内部，只暴露一些接口跟外界通信：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> user = (<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> __name = <span class="string">'sven'</span>,</span><br><span class="line"> __age = <span class="number">29</span>;</span><br><span class="line"> <span class="keyword">return</span> {</span><br><span class="line"> getUserInfo:<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">return</span> __name + <span class="string">'_'</span> + __age;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">})();</span><br></pre></td></tr></table></figure>
<p>这里用下划线来约定私有变量，它们被封装在闭包产生的作用域中，外部是访问不到这两个变量的，这就避免了对全局的命令污染。（现在没有这样来定义私有变量，一般是使用 Symbol）</p>
<h2 id="惰性单例"><a href="#惰性单例" class="headerlink" title="惰性单例"></a>惰性单例</h2><p>惰性单例指的是需要的时候才创建对象实例。惰性单例是单例模式的重点，这种技术是实际开发中非常有用的，有用的程度可能超出了我们的想象。instance 实例对象总是在我们调用 Singleton.getInstance 的时候才被创建，而不是在页面加载好的时候就被创建。代码如下：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">Singleton.getInstance = (<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> instance = <span class="literal">null</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params">name</span>)</span>{</span><br><span class="line"> <span class="keyword">if</span>(!instance){</span><br><span class="line"> instance = <span class="keyword">new</span> Singelton(name);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> instance;</span><br><span class="line"> }</span><br><span class="line">})();</span><br></pre></td></tr></table></figure>
<p>不过这是基于“类” 的单例模式，基于 “类” 的单例模式在 JavaScript 中并不适用。下面用 WebQQ 的登录浮窗为例子，介绍与全局变量结合实现惰性的实例。</p>
<p> 假设我们是 WebQQ 的开发人员（网址是web.qq.com），当点击左边导航里 QQ 头像时，会弹出一个登录浮窗很明显这个浮窗在页面里总是唯一的，不可能出现同时存在两个登录窗口的情况。</p>
<p>第一种解决方案就是在页面加载完成的时候创建好这个 div 浮窗，这个浮窗一开始肯定是隐藏状态，当用户点击登录按钮的时候就显示：</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag"><<span class="name">html</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">button</span> <span class="attr">id</span>=<span class="string">"loginBtn"</span>></span>登录<span class="tag"></<span class="name">button</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">script</span>></span><span class="undefined"></span></span><br><span class="line"><span class="javascript"> <span class="keyword">var</span> loginLayer = (<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span></span><br><span class="line"><span class="javascript"> <span class="keyword">var</span> div = <span class="built_in">document</span>.createElement(<span class="string">'div'</span>);</span></span><br><span class="line"><span class="javascript"> div.innerHTML = <span class="string">'我是登录浮窗'</span>；</span></span><br><span class="line"><span class="javascript"> div.style.display = <span class="string">'none'</span>;</span></span><br><span class="line"><span class="javascript"> <span class="built_in">document</span>.body.appendChild(div);</span></span><br><span class="line"><span class="javascript"> <span class="keyword">return</span> div;</span></span><br><span class="line"><span class="undefined"> })();</span></span><br><span class="line"><span class="javascript"> <span class="built_in">document</span>.getElementById(<span class="string">'loginBtn'</span>).onClick=<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span></span><br><span class="line"><span class="javascript"> loginLayer.style.display = <span class="string">'block'</span>;</span></span><br><span class="line"><span class="undefined"> };</span></span><br><span class="line"><span class="undefined"> </span><span class="tag"></<span class="name">script</span>></span></span><br><span class="line"><span class="tag"></<span class="name">html</span>></span></span><br></pre></td></tr></table></figure>
<p>这种方式有一个问题就是，也许我们进入 WebQQ 只是玩玩游戏或者看看天气，根本不需要进行登录操作，因为登录浮窗总是一开始就被创建好，那么很有可能将白白浪费一些 DOM 节点。</p>
<p>改变一下代码，使用户点击登录按钮的时候才开始创建浮窗</p>
<figure class="highlight html"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag"><<span class="name">html</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">button</span> <span class="attr">id</span>=<span class="string">"loginBtn"</span>></span>登录<span class="tag"></<span class="name">button</span>></span></span><br><span class="line"> <span class="tag"></<span class="name">body</span>></span></span><br><span class="line"> <span class="tag"><<span class="name">script</span>></span><span class="undefined"></span></span><br><span class="line"><span class="javascript"> <span class="keyword">var</span> createLoginLayer = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span></span><br><span class="line"><span class="javascript"> <span class="keyword">var</span> div = <span class="built_in">document</span>.createElement(<span class="string">'div'</span>);</span></span><br><span class="line"><span class="javascript"> div.innerHTML = <span class="string">'我是登录浮窗'</span>；</span></span><br><span class="line"><span class="javascript"> div.style.display = <span class="string">'none'</span>;</span></span><br><span class="line"><span class="javascript"> <span class="built_in">document</span>.body.appendChild(div);</span></span><br><span class="line"><span class="javascript"> <span class="keyword">return</span> div;</span></span><br><span class="line"><span class="undefined"> };</span></span><br><span class="line"><span class="javascript"> <span class="built_in">document</span>.getElementById(<span class="string">'loginBtn'</span>).onClick=<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span></span><br><span class="line"><span class="javascript"> <span class="keyword">var</span> loginLayer = createLoginLayer();</span></span><br><span class="line"><span class="javascript"> loginLayer.style.display = <span class="string">'block'</span>;</span></span><br><span class="line"><span class="undefined"> };</span></span><br><span class="line"><span class="undefined"> </span><span class="tag"></<span class="name">script</span>></span></span><br><span class="line"><span class="tag"></<span class="name">html</span>></span></span><br></pre></td></tr></table></figure>
<p>虽然现在达到了惰性的目的，但失去了单例的效果，当我们点击登录按钮的时候，都会创建一个新的登录浮窗 div ，虽然我们可以在点击浮窗上的关闭时把这个浮窗从页面中删除，但这样频繁地创建和删除节点明显是不合理的，也是不必要的。</p>
<p>可以用一个变量来判断是否已经创建过登录浮窗</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> createLoginLayer = (<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> div;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">if</span>(!div){</span><br><span class="line"> div = <span class="built_in">document</span>.createElement(<span class="string">'div'</span>);</span><br><span class="line"> div.innerHTML = <span class="string">'我是登录浮窗'</span>；</span><br><span class="line"> div.style.display = <span class="string">'none'</span>;</span><br><span class="line"> <span class="built_in">document</span>.body.appendChild(div); </span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> div; </span><br><span class="line"> }</span><br><span class="line">})();</span><br><span class="line"><span class="built_in">document</span>.getElementById(<span class="string">'loginBtn'</span>).onClick=<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> loginLayer = createLoginLayer();</span><br><span class="line"> loginLayer.style.display = <span class="string">'block'</span>;</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<h2 id="通用的惰性单例"><a href="#通用的惰性单例" class="headerlink" title="通用的惰性单例"></a>通用的惰性单例</h2><p>上面虽然完成了一个可用的惰性单例，但是它还有如下的一些问题。</p>
<ul>
<li>代码仍然是违反单一职责原则的，创建对象和管理单例的逻辑都放在 createLoginLayer 对象内部。</li>
<li>如果我们下次需要创建一个页面中唯一的 iframe 的时候，或者 script 标签，用来跨域请求数据，就必须如法炮制，把 createLoginLayer 函数几乎抄多一遍</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> createIframe = (<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> iframe;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">if</span>(!iframe){</span><br><span class="line"> iframe = <span class="built_in">document</span>.createElement(<span class="string">'iframe'</span>);</span><br><span class="line"> iframe.style.display = <span class="string">'none'</span>;</span><br><span class="line"> <span class="built_in">document</span>.body.appendChild(iframe); </span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> iframe; </span><br><span class="line"> }</span><br><span class="line">})();</span><br></pre></td></tr></table></figure>
<p>可以看出，实际上管理单例的逻辑其实是完全可以抽象出来的，这个逻辑始终是一样的：用一个变量来标志是否创建了对象，如果是，则在下次直接返回这个已经创建好的对象。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> obj;</span><br><span class="line"><span class="keyword">if</span>(!obj){</span><br><span class="line"> obj = xxx;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>我们把如何管理单例的逻辑从原来的代码中抽离出来，这些逻辑被封装在 getSingle 函数内部，创建对象的方法 fn 被当做参数动态传入 getSingle 函数中：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> getSingle = <span class="function"><span class="keyword">function</span> (<span class="params">fn</span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> result;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> result || (result = fn.apply(<span class="keyword">this</span>, <span class="built_in">arguments</span>))</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>接下来用于创建登录浮窗的方法用 fn 的形式传入到这个 getSingle。我们可以传入任何我们想创建的东西。之后让 getSingle 返回一个新的函数，并用一个变量 result 来保存 fn 的计算结果。result 变量因为身在闭包中，它永远不会被销毁。在将来的请求中，如果 result 已经被赋值了，那么它将返回这个值。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> createLoginLayer = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> div = <span class="built_in">document</span>.createElement(<span class="string">'div'</span>);</span><br><span class="line"> div.innerHTML = <span class="string">'我是登录浮窗'</span>；</span><br><span class="line"> div.style.display = <span class="string">'none'</span>;</span><br><span class="line"> <span class="built_in">document</span>.body.appendChild(div); </span><br><span class="line"> <span class="keyword">return</span> div;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> createSingleLoginLayer = getSingle(createLoginLayer);</span><br><span class="line"><span class="built_in">document</span>.getElementById(<span class="string">'loginBtn'</span>).onClick=<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> loginLayer = createSingleLoginLayer();</span><br><span class="line"> loginLayer.style.display = <span class="string">'block'</span>;</span><br><span class="line">};</span><br><span class="line"><span class="comment">// 下面再试试创建唯一的 iframe 用于动态加载第三方页面</span></span><br><span class="line"><span class="keyword">var</span> createSingleIframe = getSingle(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> iframe = <span class="built_in">document</span>.createElement(<span class="string">'iframe'</span>);</span><br><span class="line"> <span class="built_in">document</span>.body.appendChild(iframe);</span><br><span class="line"> <span class="keyword">return</span> iframe;</span><br><span class="line">});</span><br><span class="line"><span class="built_in">document</span>.getElementById(<span class="string">'loginBtn'</span>).onClick=<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> loginLayer = createSingleIframe();</span><br><span class="line"> loginLayer.src = <span class="string">'http://laibh.top'</span>;</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>上面的例子中，把创建实例对象的职责和管理单例的职责分别放置在了两个方法里，这两个方法可以独立变化而互不影响，当它们连接在一起的时候，就完成了创建唯一实例对象的功能。</p>
<p>这种单例模式的用途远不止创建对象，比如我们通常渲染页面中的一个列表之后，接下来要给这个列表绑定一个 click 事件，如果是通过 ajax 动态往列表里追加数据，在使用事件代理的前提下，click 事件实际上只需要在第一次渲染列表的时候被绑定一次，但是我们不想去判断当前是否是第一次渲染列表，如果借助JQ，我们通常选择给节点绑定 one 事件：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> bindEvent = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> $(<span class="string">'div'</span>).one(<span class="string">'click'</span>, <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'click'</span>);</span><br><span class="line"> });</span><br><span class="line">}</span><br><span class="line"><span class="keyword">var</span> render = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'开始渲染列表'</span>);</span><br><span class="line"> bindEvent();</span><br><span class="line">}</span><br><span class="line">render();</span><br><span class="line">render();</span><br><span class="line">render();</span><br></pre></td></tr></table></figure>
<p>如果使用 getSingle 函数也可以达到一样的效果，代码如下：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> bindEvent = getSingle(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">document</span>.getElementById(<span class="string">'div1'</span>).onclick = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'click'</span>);</span><br><span class="line"> };</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line">});</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> render = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="string">'开始渲染列表'</span>);</span><br><span class="line"> bindEvent();</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">render();</span><br><span class="line">render();</span><br><span class="line">render();</span><br></pre></td></tr></table></figure>
<p>可以看到 render 函数 和 bindEvent 函数分别都执行了 3 次，但是 div 实际上只被绑定了一个事件。</p>
<h2 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h2><p>单例模式是一种简单但非常实用的模式。特别是惰性单例技术，在合适的时候才创建对象，并只创建唯一的一个。更奇妙的是，创建对象和管理单例的职责被分布在两个不同的方法中，这两个方法组合起来才具有单例模式的威力。</p>
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