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FetchEvent

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这是一个实验中的功能
此功能某些浏览器尚在开发中,请参考浏览器兼容性表格以得到在不同浏览器中适合使用的前缀。由于该功能对应的标准文档可能被重新修订,所以在未来版本的浏览器中该功能的语法和行为可能随之改变。

The parameter passed into the ServiceWorkerGlobalScope.onfetch handler, FetchEvent represents a fetch action that is dispatched on the ServiceWorkerGlobalScope of a ServiceWorker. It contains information about the request and resulting response, and provides the FetchEvent.respondWith() method, which allows us to provide an arbitrary response back to the controlled page.

构造函数

FetchEvent.FetchEvent()
Creates a new FetchEvent object.

属性

Inherits properties from its ancestor, Event.

FetchEvent.isReload 只读
Returns a Boolean that is true if the event was dispatched with the user's intention for the page to reload, and false otherwise. Typically, pressing the refresh button in a browser is a reload, while clicking a link and pressing the back button is not.
FetchEvent.request 只读
Returns the Request that triggered the event handler.
FetchEvent.clientId 只读
Returns the id of the client that the current service worker is controlling.

Deprecated properties

FetchEvent.client 只读
Returns the Client that the current service worker is controlling.

方法

Inherits methods from its parent, ExtendableEvent.

FetchEvent.respondWith()
Resolves by returning a Response or a network error  to Fetch.
ExtendableEvent.waitUntil()

Extends the lifetime of the event.  It is intended to be called in the install EventHandler for the installing worker and on the active EventHandler for the active worker.

示例

This code snippet is from the service worker fetch sample (run the fetch sample live). In an earlier part of the code,  an InstallEvent controls caching of a number of resources. The ServiceWorkerGlobalScope.onfetch event handler then listens for the fetch event. When fired, FetchEvent.respondWith() returns a promise back to the controlled page. This promise resolves to the first matching URL request in the 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.

The code also handles exceptions thrown from the 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.

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;
      });
    })
  );
});

规范

Specification Status Comment
Service Workers
FetchEvent
Working Draft Initial definition.

浏览器兼容性

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 40.0 44.0 (44.0)[1] 未实现 24 未实现
Feature Android Android Webview Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support 未实现 ? 44.0 (44.0) (Yes) 未实现 ? 未实现 44.0

[1] Service workers (and Push) have been disabled in the Firefox 45 and 52 Extended Support Releases (ESR.)

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