FetchEvent - Web API 接口参考编辑

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。

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

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

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.

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

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