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2021-09-05 17:06:43 +08:00
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> observer </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">new</span><span class="token plain"> </span><span class="token class-name">IntersectionObserver</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">entries</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> self</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> entries</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">forEach</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter punctuation" style="color:#393A34">{</span><span class="token parameter"> isIntersecting</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> target </span><span class="token parameter punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">isIntersecting</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">target</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">dataset</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">src</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> target</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">src</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> target</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">dataset</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">src</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> target</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"> self</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">unobserve</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">target</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> config</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> images </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token dom variable" style="color:#36acaa">document</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">querySelectorAll</span><span class="token punctuation" style="color:#393A34">(</span><span class="token string" style="color:#e3116c">"[data-src]"</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain">images</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">forEach</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">image</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> observer</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">observe</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">image</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p><strong>scroll</strong></p><p>如果 Intersection Observer 存在兼容性问题，除了可以添加对应 polyfill 之外，也可以考虑降级为监听 scroll、resize、orientationchange 事件的方案。实现思路和 Intersection Observer 一致。具体细节上，需要自行计算图片节点与目标视口的纵向或横向距离，且需使用节流函数来避免性能问题。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="32-图片预加载"></a>3.2 图片预加载<a class="hash-link" href="#32-图片预加载" title="Direct link to heading">#</a></h4><p>图片预加载机制是为了增强用户体验，尽快地加载出图片，使得用户体验更为流畅。</p><p>如果预加载的图片是确切且有限的，可以通过硬编码 link 标签来实现预加载。但是多数情况下，预加载的使用场景是动态的。</p><p><strong>link</strong></p><p><code><link rel="preload"></code> 允许开发者在 HTML 的 head 标签中声明资源请求，指定页面需要预加载的资源，并且在浏览器的主要渲染机制启动之前加载，避免阻塞页面渲染且保证资源尽早可用。</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly html"><pre tabindex="0" class="prism-code language-html codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">link</span><span class="token t
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