---
title: FetchEvent
slug: Web/API/FetchEvent
translation_of: Web/API/FetchEvent
---
<p>{{APIRef("Service Workers API")}}{{ SeeCompatTable() }}</p>

<p>使用<code>ServiceWorker</code>技术时,页面的提取动作会在ServiceWorker作用域(<code>ServiceWorkerGlobalScope</code>)中触发fetch事件.</p>

<p>使用 {{domxref("ServiceWorkerGlobalScope.onfetch")}}或addEventListener监听.<br>
 该事件回调会注入<code>FetchEvent</code>参数.它携带了有关请求和结果响应的信息以及方法{{domxref("FetchEvent.respondWith", "FetchEvent.respondWith()")}} ,允许我们向受控页面提供任意响应.</p>

<h2 id="构造函数">构造函数</h2>

<dl>
 <dt>{{domxref("FetchEvent.FetchEvent()")}}</dt>
 <dd>Creates a new <code>FetchEvent</code> object.</dd>
</dl>

<h2 id="属性">属性</h2>

<p><em>Inherits properties from its ancestor, {{domxref("Event")}}</em>.</p>

<dl>
 <dt>{{domxref("FetchEvent.isReload")}} {{readonlyInline}}</dt>
 <dd>Returns a {{jsxref("Boolean")}} that is <code>true</code> if the event was dispatched with the user's intention for the page to reload, and <code>false</code> otherwise. Typically, pressing the refresh button in a browser is a reload, while clicking a link and pressing the back button is not.</dd>
 <dt>{{domxref("FetchEvent.request")}} {{readonlyInline}}</dt>
 <dd>Returns the {{domxref("Request")}} that triggered the event handler.</dd>
 <dt>{{domxref("FetchEvent.clientId")}} {{readonlyInline}}</dt>
 <dd>Returns the id of the client that the current service worker is controlling.</dd>
</dl>

<h3 id="Deprecated_properties">Deprecated properties</h3>

<dl>
 <dt>{{domxref("FetchEvent.client")}} {{readonlyInline}}</dt>
 <dd>Returns the {{domxref("Client")}} that the current service worker is controlling.</dd>
</dl>

<h2 id="方法">方法</h2>

<p><em>Inherits methods from its parent, </em><em>{{domxref("ExtendableEvent")}}</em>.</p>

<dl>
 <dt>{{domxref("FetchEvent.respondWith()")}}</dt>
 <dd>Resolves by returning a {{domxref("Response")}} or a <a href="http://fetch.spec.whatwg.org/#concept-network-error">network error</a>  to <code style="font-style: normal;"><a href="http://fetch.spec.whatwg.org/#concept-fetch">Fetch</a></code>.</dd>
 <dt>{{domxref("ExtendableEvent.waitUntil", "ExtendableEvent.waitUntil()")}}</dt>
 <dd>
 <p>Extends the lifetime of the event.  It is intended to be called in the {{event("install")}} {{event("Event_handlers", "event handler")}} for the {{domxref("ServiceWorkerRegistration.installing", "installing")}} worker and on the {{event("active")}} {{event("Event_handlers", "event handler")}} for the {{domxref("ServiceWorkerRegistration.active", "active")}} worker.</p>
 </dd>
</dl>

<h2 id="示例">示例</h2>

<p>This code snippet is from the <a href="https://github.com/GoogleChrome/samples/blob/gh-pages/service-worker/prefetch/service-worker.js">service worker fetch sample</a> (<a href="https://googlechrome.github.io/samples/service-worker/prefetch/">run the fetch sample live</a>). In an earlier part of the code,  an {{domxref("InstallEvent")}} controls caching of a number of resources. The {{domxref("ServiceWorkerGlobalScope.onfetch")}} event handler then listens for the {{event("fetch")}} event. When fired, {{domxref("FetchEvent.respondWith()")}} returns a promise back to the controlled page. This promise resolves to the first matching URL request in the {{domxref("Cache")}} object. If no match is found (i.e. that resource wasn't cached in the install phase), the code fetches a response from the network.</p>

<p>The code also handles exceptions thrown from the {{domxref("ServiceWorkerGlobalScope.fetch()")}} operation. Note that a HTTP error response (e.g., 404) doesn't trigger an exception. It returns a normal response object that has the appropriate error code set.</p>

<pre class="brush: js">self.addEventListener('fetch', function(event) {
  console.log('Handling fetch event for', event.request.url);

  event.respondWith(
    caches.match(event.request).then(function(response) {
      if (response) {
        console.log('Found response in cache:', response);

        return response;
      }
      console.log('No response found in cache. About to fetch from network...');

      return fetch(event.request).then(function(response) {
        console.log('Response from network is:', response);

        return response;
      }).catch(function(error) {
        console.error('Fetching failed:', error);

        throw error;
      });
    })
  );
});</pre>

<h2 id="规范">规范</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('Service Workers', '#fetch-event-section', 'FetchEvent')}}</td>
   <td>{{Spec2('Service Workers')}}</td>
   <td>Initial definition.</td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容性">浏览器兼容性</h2>

{{Compat("api.FetchEvent")}}

<h2 id="请参见">请参见</h2>

<ul>
 <li>{{jsxref("Promise")}}</li>
 <li><a href="/en-US/docs/Web/API/Fetch_API">Fetch API</a></li>
</ul>