什麼是緩存API？如何與服務人員使用？

Cache API 是瀏覽器提供的一種緩存網絡請求的工具，常與Service Worker 配合使用，以提升網站性能和離線體驗。 1. 它允許開發者手動存儲如腳本、樣式表、圖片等資源；2. 可根據請求匹配緩存響應；3. 支持刪除特定緩存或清空整個緩存；4. 通過Service Worker 監聽fetch 事件實現緩存優先或網絡優先等策略；5. 常用於離線支持、加快重複訪問速度、預加載關鍵資源及後台更新內容；6. 使用時需注意緩存版本控制、存儲限制及與HTTP 緩存機制的區別。

The Cache API is a browser feature that lets websites store network requests — like scripts, stylesheets, images, or even entire pages — for later use. It's especially useful when paired with Service Workers, which can intercept network requests and decide whether to fetch from the network or use a cached response. This combo helps build faster, more reliable web experiences, especially in low-connectivity situations.

What Exactly Can You Do with the Cache API?

The Cache API gives you direct control over caching strategies. You can:

• Store responses from network requests manually
• Match stored responses to future requests
• Delete specific entries or clear the whole cache

This means you're not just relying on the browser's default caching behavior — you get to define when , what , and how long things are cached.

For example:

 caches.open('my-cache-name').then(cache => {
  cache.add('/styles/main.css');
});

This snippet opens a named cache and adds a CSS file to it. You can do this anytime — during installation of a Service Worker, or even dynamically as users interact with your app.

How Does the Cache API Work with Service Workers?

Service Workers act like a proxy between your website and the network. With them, you can listen for events like fetch , then respond by checking the cache first.

Here's a common setup:

 self.addEventListener('fetch', event => {
  event.respondWith(
    caches.match(event.request).then(response => {
      return response || fetch(event.request);
    })
  );
});

In this code, every time the browser makes a request, the Service Worker checks if there's a matching response in the cache. If yes, it returns that; if not, it goes to the network.

You can also update the cache dynamically using strategies like cache-first, network-first, or stale-while-revalidate. These let you balance speed and freshness depending on what kind of content you're serving.

Common Use Cases and When to Use Them

There are several real-world scenarios where combining the Cache API with Service Workers makes sense:

• Offline support : Cache key assets so users can still access parts of your site without internet.
• Faster repeat visits : Serve previously fetched resources instantly instead of waiting for the network.
• Pre-caching : Load critical files when the Service Worker installs, so they're ready before they're needed.
• Background updates : Fetch fresh content while showing the cached version, improving perceived performance.

If you're building a PWA (Progressive Web App), these techniques are essential. But even for regular sites, smart caching can significantly improve load times and user experience.

A Few Gotchas to Keep in Mind

Caching seems straightforward, but some details often trip people up:

• Caches are persistent across page loads but tied to your origin — not all browsers handle cleanup the same way.
• Versioning your caches (like naming them 'cache-v1', 'cache-v2') helps avoid conflicts when you update your app.
• Always test cache eviction logic — don't let old caches stick around forever.
• Be careful with dynamic caching; aggressive caching of too many URLs can eat up storage quickly.

Also, remember that the Cache API works differently than HTTP caching headers. You have more control, but it's up to you to manage expiration and invalidation.

Basically, the Cache API Service Workers combo gives you powerful tools to make your site feel snappier and more resilient — but only if you use them wisely.

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