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<strong class="sidebar-title">开始使用</strong><a href="intro-to-liquid.html" class="sidebar-link">Liquid 简介</a><a href="setup.html" class="sidebar-link">安装</a><a href="options.html" class="sidebar-link">选项</a><a href="render-file.html" class="sidebar-link">文件渲染</a><a href="partials-and-layouts.html" class="sidebar-link">引用/继承</a><a href="use-in-expressjs.html" class="sidebar-link">Express.js 中使用</a><strong class="sidebar-title">高级主题</strong><a href="caching.html" class="sidebar-link">缓存</a><a href="escaping.html" class="sidebar-link">转义</a><a href="register-filters-tags.html" class="sidebar-link">注册标签/过滤器</a><a href="access-scope-in-filters.html" class="sidebar-link">过滤器里访问上下文</a><a href="parse-parameters.html" class="sidebar-link">参数解析</a><a href="render-tag-content.html" class="sidebar-link">渲染标签内容</a><a href="drops.html" class="sidebar-link current">Liquid Drop</a><a href="sync-and-async.html" class="sidebar-link">同步和异步</a><a href="whitespace-control.html" class="sidebar-link">换行和缩进</a><a href="plugins.html" class="sidebar-link">插件</a><a href="operators.html" class="sidebar-link">运算符</a><a href="truthy-and-falsy.html" class="sidebar-link">真和假</a><a href="dos.html" class="sidebar-link">DoS</a><strong class="sidebar-title">其他</strong><a href="migrate-to-9.html" class="sidebar-link">迁移到 LiquidJS 9</a><a href="changelog.html" class="sidebar-link">更新日志</a><a href="differences.html" class="sidebar-link">与 Shopify/liquid 的不同</a><a href="contribution-guidelines.html" class="sidebar-link">贡献指南</a>
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<h1 class="article-title" itemprop="name">Liquid Drop</h1>
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<a target="_blank" rel="noopener external nofollow noreferrer" href="https://github.com/harttle/liquidjs/edit/master/docs/source/zh-cn/tutorials/drops.md" class="article-edit-link" title="改进这篇文档"><i class="icon-pencil"></i></a>
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<p>LiquidJS 还提供了一种类似于 <a target="_blank" rel="noopener external nofollow noreferrer" href="https://github.com/Shopify/liquid/wiki/Introduction-to-Drops">Shopify Drop</a> 的机制，用于为模板作者提供在自定义解析变量值的功能。</p>
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<blockquote class="note info"><strong class="note-title">JavaScript 中的 Drop</strong><p><em>Drop</em> 接口在 LiquidJS 中实现方式与内置过滤器和其他模板功能不同。由于 LiquidJS 在 JavaScript 中运行，自定义 Drop 在 JavaScript 中一定需要重新实现。JavaScript 类与 Ruby 类之间没有兼容性可言。</p>
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</blockquote>
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<h2 id="基本用法" class="article-heading"><a href="#基本用法" class="headerlink" title="基本用法"></a>基本用法<a class="article-anchor" href="#基本用法" aria-hidden="true"></a></h2><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> { <span class="title class_">Liquid</span>, <span class="title class_">Drop</span> } <span class="keyword">from</span> <span class="string">'liquidjs'</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">SettingsDrop</span> <span class="keyword">extends</span> <span class="title class_ inherited__">Drop</span> {</span><br><span class="line"> <span class="title function_">constructor</span>(<span class="params"></span>) {</span><br><span class="line"> <span class="variable language_">super</span>()</span><br><span class="line"> <span class="variable language_">this</span>.<span class="property">foo</span> = <span class="string">'FOO'</span></span><br><span class="line"> }</span><br><span class="line"> <span class="title function_">bar</span>(<span class="params"></span>) {</span><br><span class="line"> <span class="keyword">return</span> <span class="string">'BAR'</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> engine = <span class="keyword">new</span> <span class="title class_">Liquid</span>()</span><br><span class="line"><span class="keyword">const</span> template = <span class="string">`foo: {{settings.foo}}, bar: {{settings.bar}}`</span></span><br><span class="line"><span class="keyword">const</span> context = { <span class="attr">settings</span>: <span class="keyword">new</span> <span class="title class_">SettingsDrop</span>() }</span><br><span class="line"><span class="comment">// 输出: "foo: FOO, bar: BAR"</span></span><br><span class="line">engine.<span class="title function_">parseAndRender</span>(template, context).<span class="title function_">then</span>(<span class="function"><span class="params">html</span> =></span> <span class="variable language_">console</span>.<span class="title function_">log</span>(html))</span><br></pre></td></tr></table></figure>
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<p><a target="_blank" rel="noopener external nofollow noreferrer" href="https://runkit.com/embed/2is7di4mc7kk">Runkit 链接</a></p>
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<p>如上所示，除了从上下文作用域中读取属性外，还可以调用方法。您只需创建一个继承自 <code>Drop</code> 的自定义类。</p>
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<blockquote class="note tip"><strong class="note-title">异步方法</strong><p>LiquidJS 完全支持异步，您可以在 Drop 的方法中安全地返回 Promise，或将 Drop 的方法定义为 <code>async</code>。</p>
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</blockquote>
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<h2 id="liquidMethodMissing" class="article-heading"><a href="#liquidMethodMissing" class="headerlink" title="liquidMethodMissing"></a>liquidMethodMissing<a class="article-anchor" href="#liquidMethodMissing" aria-hidden="true"></a></h2><p>如果属性名不能静态地确定的情况下，可以利用 <code>liquidMethodMissing</code> 来动态解析变量的值。</p>
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<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">import</span> { <span class="title class_">Liquid</span>, <span class="title class_">Drop</span> } <span class="keyword">from</span> <span class="string">'liquidjs'</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">SettingsDrop</span> <span class="keyword">extends</span> <span class="title class_ inherited__">Drop</span> {</span><br><span class="line"> <span class="title function_">liquidMethodMissing</span>(<span class="params">key</span>) {</span><br><span class="line"> <span class="keyword">return</span> key.<span class="title function_">toUpperCase</span>()</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> engine = <span class="keyword">new</span> <span class="title class_">Liquid</span>()</span><br><span class="line"><span class="comment">// 输出: "COO"</span></span><br><span class="line">engine.<span class="title function_">parseAndRender</span>(<span class="string">"{{settings.coo}}"</span>, { <span class="attr">settings</span>: <span class="keyword">new</span> <span class="title class_">SettingsDrop</span>() })</span><br><span class="line"> .<span class="title function_">then</span>(<span class="function"><span class="params">html</span> =></span> <span class="variable language_">console</span>.<span class="title function_">log</span>(html))</span><br></pre></td></tr></table></figure>
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<p><code>liquidMethodMissing</code> 支持 Promise，这意味着您可以在其中进行异步调用。一个更有用的例子是通过使用 Drop 动态地从数据库获取值。通过使用 Drop，您可以避免将每个属性都硬编码到上下文中。例如：</p>
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<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">import</span> { <span class="title class_">Liquid</span>, <span class="title class_">Drop</span> } <span class="keyword">from</span> <span class="string">'liquidjs'</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">DBDrop</span> <span class="keyword">extends</span> <span class="title class_ inherited__">Drop</span> {</span><br><span class="line"> <span class="keyword">async</span> <span class="title function_">liquidMethodMissing</span>(<span class="params">key</span>) {</span><br><span class="line"> <span class="keyword">const</span> record = <span class="keyword">await</span> db.<span class="title function_">getRecordByKey</span>(key)</span><br><span class="line"> <span class="keyword">return</span> record.<span class="property">value</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> engine = <span class="keyword">new</span> <span class="title class_">Liquid</span>()</span><br><span class="line"><span class="keyword">const</span> context = { <span class="attr">db</span>: <span class="keyword">new</span> <span class="title class_">DBDrop</span>() }</span><br><span class="line">engine.<span class="title function_">parseAndRender</span>(<span class="string">"{{db.coo}}"</span>, context).<span class="title function_">then</span>(<span class="function"><span class="params">html</span> =></span> <span class="variable language_">console</span>.<span class="title function_">log</span>(html))</span><br></pre></td></tr></table></figure>
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<h2 id="valueOf-NaN" class="article-heading"><a href="#valueOf-NaN" class="headerlink" title="valueOf"></a>valueOf<a class="article-anchor" href="#valueOf-NaN" aria-hidden="true"></a></h2><p>Drop 可以实现一个 <code>valueOf()</code> 方法，用于在输出中替换自身。例如：</p>
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<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">import</span> { <span class="title class_">Liquid</span>, <span class="title class_">Drop</span> } <span class="keyword">from</span> <span class="string">'liquidjs'</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">ColorDrop</span> <span class="keyword">extends</span> <span class="title class_ inherited__">Drop</span> {</span><br><span class="line"> <span class="title function_">valueOf</span>(<span class="params"></span>) {</span><br><span class="line"> <span class="keyword">return</span> <span class="string">'red'</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> engine = <span class="keyword">new</span> <span class="title class_">Liquid</span>()</span><br><span class="line"><span class="keyword">const</span> context = { <span class="attr">color</span>: <span class="keyword">new</span> <span class="title class_">ColorDrop</span>() }</span><br><span class="line"><span class="comment">// 输出: "red"</span></span><br><span class="line">engine.<span class="title function_">parseAndRender</span>(<span class="string">"{{color}}"</span>, context).<span class="title function_">then</span>(<span class="function"><span class="params">html</span> =></span> <span class="variable language_">console</span>.<span class="title function_">log</span>(html))</span><br></pre></td></tr></table></figure>
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<h2 id="toLiquid" class="article-heading"><a href="#toLiquid" class="headerlink" title="toLiquid"></a>toLiquid<a class="article-anchor" href="#toLiquid" aria-hidden="true"></a></h2><p><code>toLiquid()</code> 不是 <code>Drop</code> 的方法，但它可以用于返回一个 <code>Drop</code>。在您有一个上下文中固定结构且不能更改其值的情况下，您可以实现 <code>toLiquid()</code>，以便让 LiquidJS 使用返回的值而不是自身来渲染模板。</p>
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<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></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> { <span class="title class_">Liquid</span>, <span class="title class_">Drop</span> } <span class="keyword">from</span> <span class="string">'liquidjs'</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> context = {</span><br><span class="line"> <span class="attr">person</span>: {</span><br><span class="line"> <span class="attr">firstName</span>: <span class="string">"Jun"</span>,</span><br><span class="line"> <span class="attr">lastName</span>: <span class="string">"Yang"</span>,</span><br><span class="line"> <span class="attr">name</span>: <span class="string">"Jun Yang"</span>,</span><br><span class="line"> <span class="attr">toLiquid</span>: <span class="function">() =></span> ({</span><br><span class="line"> <span class="attr">firstName</span>: <span class="variable language_">this</span>.<span class="property">firstName</span>,</span><br><span class="line"> <span class="attr">lastName</span>: <span class="variable language_">this</span>.<span class="property">lastName</span>,</span><br><span class="line"> <span class="comment">// 使用不同的 `name`</span></span><br><span class="line"> <span class="attr">name</span>: <span class="string">"Yang, Jun"</span></span><br><span class="line"> })</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> engine = <span class="keyword">new</span> <span class="title class_">Liquid</span>()</span><br><span class="line"><span class="comment">// 输出: "Yang, Jun"</span></span><br><span class="line">engine.<span class="title function_">parseAndRender</span>(<span class="string">"{{person.name}}"</span>, context).<span class="title function_">then</span>(<span class="function"><span class="params">html</span> =></span> <span class="variable language_">console</span>.<span class="title function_">log</span>(html))</span><br></pre></td></tr></table></figure>
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<p>当然，您还可以在 <code>toLiquid()</code> 方法中返回一个 <code>PersonDrop</code> 实例，并在 <code>PersonDrop</code> 中实现此功能：</p>
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<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></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> { <span class="title class_">Liquid</span>, <span class="title class_">Drop</span> } <span class="keyword">from</span> <span class="string">'liquidjs'</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">PersonDrop</span> <span class="keyword">extends</span> <span class="title class_ inherited__">Drop</span> {</span><br><span class="line"> <span class="title function_">constructor</span>(<span class="params">person</span>) {</span><br><span class="line"> <span class="variable language_">super</span>()</span><br><span class="line"> <span class="variable language_">this</span>.<span class="property">person</span> = person</span><br><span class="line"> }</span><br><span class="line"> <span class="title function_">name</span>(<span class="params"></span>) {</span><br><span class="line"> <span class="keyword">return</span> <span class="variable language_">this</span>.<span class="property">person</span>.<span class="property">lastName</span> + <span class="string">", "</span> + <span class="variable language_">this</span>.<span class="property">person</span>.<span class="property">firstName</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> context = {</span><br><span class="line"> <span class="attr">person</span>: {</span><br><span class="line"> <span class="attr">firstName</span>: <span class="string">"Jun"</span>,</span><br><span class="line"> <span class="attr">lastName</span>: <span class="string">"Yang"</span>,</span><br><span class="line"> <span class="attr">name</span>: <span class="string">"Jun Yang"</span>,</span><br><span class="line"> <span class="attr">toLiquid</span>: <span class="keyword">function</span> (<span class="params"></span>) { <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">PersonDrop</span>(<span class="variable language_">this</span>) }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> engine = <span class="keyword">new</span> <span class="title class_">Liquid</span>()</span><br><span class="line"><span class="comment">// 输出: "Yang, Jun"</span></span><br><span class="line">engine.<span class="title function_">parseAndRender</span>(<span class="string">"{{person.name}}"</span>, context).<span class="title function_">then</span>(<span class="function"><span class="params">html</span> =></span> <span class="variable language_">console</span>.<span class="title function_">log</span>(html))</span><br></pre></td></tr></table></figure>
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<blockquote class="note info"><strong class="note-title"><code>toLiquid()</code> 和 <code>valueOf()</code> 的区别</strong><ul>
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<li><code>valueOf()</code> 通常用来定义当前变量如何渲染，<code>toLiquid()</code> 通常用来把一个对象转换为 Drop 或另一个提供给模板的 scope。</li>
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<li><code>valueOf()</code> 是 Drop 才有的方法；而 <code>toLiquid()</code> 可以用在任何 scope 对象上。</li>
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<li><code>valueOf()</code> 是在自己即将被渲染时，用来替代自己；而 <code>toLiquid()</code> 在即将读取它的属性时才会被调用。</li>
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</ul></blockquote>
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<h2 id="特殊-Drop" class="article-heading"><a href="#特殊-Drop" class="headerlink" title="特殊 Drop"></a>特殊 Drop<a class="article-anchor" href="#特殊-Drop" aria-hidden="true"></a></h2><p>LiquidJS 本身实现了几个内置 Drop，以促进模板编写。此部分与 Shopify Liquid 兼容，因为我们需要模板具有可移植性。</p>
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<h3 id="blank" class="article-heading"><a href="#blank" class="headerlink" title="blank"></a>blank<a class="article-anchor" href="#blank" aria-hidden="true"></a></h3><p>用于检查字符串变量是否为 <code>false</code>、<code>null</code>、<code>undefined</code>、空字符串或字符串仅包含空白字符。</p>
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<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">{% unless author == blank %}</span><br><span class="line"> {{author}}</span><br><span class="line">{% endif %}</span><br></pre></td></tr></table></figure>
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<h3 id="empty" class="article-heading"><a href="#empty" class="headerlink" title="empty"></a>empty<a class="article-anchor" href="#empty" aria-hidden="true"></a></h3><p>用于检查数组、字符串或对象是否为空。</p>
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<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">{% if authors == empty %}</span><br><span class="line"> 作者列表为空</span><br><span class="line">{% endif %}</span><br></pre></td></tr></table></figure>
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<blockquote class="note info"><strong class="note-title"><code>empty</code> 的实现</strong><p>对于数组和字符串，LiquidJS 检查它们的 <code>.length</code> 属性。对于对象，LiquidJS 调用 <code>Object.keys()</code> 来检查它是否有键。</p>
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</blockquote>
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<h3 id="nil" class="article-heading"><a href="#nil" class="headerlink" title="nil"></a>nil<a class="article-anchor" href="#nil" aria-hidden="true"></a></h3><p><code>nil</code> Drop 用于检查变量是否未定义或定义为 <code>null</code> 或 <code>undefined</code>，本质上等同于 JavaScript 的 <code>== null</code> 检查。</p>
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<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">{% if notexist == nil %}</span><br><span class="line"> 空变量</span><br><span class="line">{% endif %}</span><br></pre></td></tr></table></figure>
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<h3 id="其他-Drop" class="article-heading"><a href="#其他-Drop" class="headerlink" title="其他 Drop"></a>其他 Drop<a class="article-anchor" href="#其他-Drop" aria-hidden="true"></a></h3><p>仍然有一些特定标签的 Drop，例如 <code>forloop</code>、<code>tablerowloop</code>、<code>block</code>，这些在各自的标签文档中有详细介绍。</p>
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<time class="article-footer-updated" datetime="2024-08-19T19:36:26.430Z" itemprop="dateModified">上次更新：2024-08-19</time>
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