Content Index API
Experimental:This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
Note: This feature is available in Web Workers.
The Content Index API allows developers to register their offline enabled content with the browser.
Concepts and usage
As it stands, offline web content is not easily discoverable by users. Content indexing allows developers to tell the browser about their specific offline content. This allows users to discover and view what is available, whilst giving developers the ability to add and manage this content. Examples could be a news website prefetching the latest articles in the background, or a content streaming app registering downloaded content.
The Content Index API is an extension to service workers, which allows developers to add URLs and metadata of already cached pages, under the scope of the current service worker. The browser can then use these entries to display offline reading to a user. As a developer you can also display these entries within your application.
Indexed entries do not automatically expire. It's good practice to present an interface for clearing out entries, or periodically remove older entries.
Note: The API supports indexing URLs corresponding to HTML documents. A URL for a cached media file, for example, can't be indexed directly. Instead, you need to provide a URL for a page that displays media, and which works offline.
Interfaces
ContentIndexExperimentalProvides functionality to register content available offline.
ContentIndexEventExperimentalDefines the object used to represent the
contentdeleteevent.
Extensions to other interfaces
The following additions to the ServiceWorker have been specified in the Content Index API spec to provide an entry point for using content indexing.
ServiceWorkerRegistration.indexRead onlyExperimentalReturns a reference to the
ContentIndexinterface for indexing cached pages.contentdeleteevent ExperimentalFired when content is removed by the user agent.
Examples
All the following examples assume a service worker has been registered. For more information see the Service Worker API.
Feature detection and interface access
Here we get a reference to the ServiceWorkerRegistration, then check for the index property, which gives us access to the content index interface.
// reference registration const registration = await navigator.serviceWorker.ready; // feature detection if ("index" in registration) { // Content Index API functionality const contentIndex = registration.index; } Adding to the content index
Here we're declaring an item in the correct format and creating an asynchronous function which uses the add() method to register it with the content index.
// our content const item = { id: "post-1", url: "/posts/amet.html", title: "Amet consectetur adipisicing", description: "Repellat et quia iste possimus ducimus aliquid a aut eaque nostrum.", icons: [ { src: "/media/dark.png", sizes: "128x128", type: "image/png", }, ], category: "article", }; // our asynchronous function to add indexed content async function registerContent(data) { const registration = await navigator.serviceWorker.ready; // feature detect Content Index if (!registration.index) { return; } // register content try { await registration.index.add(data); } catch (e) { console.log("Failed to register content: ", e.message); } } Retrieving items within the current index
The below example shows an asynchronous function that retrieves items within the content index and iterates over each entry, building a list for the interface.
async function createReadingList() { // access our service worker registration const registration = await navigator.serviceWorker.ready; // get our index entries const entries = await registration.index.getAll(); // create a containing element const readingListElem = document.createElement("div"); // test for entries if (entries.length === 0) { // if there are no entries, display a message const message = document.createElement("p"); message.innerText = "You currently have no articles saved for offline reading."; readingListElem.append(message); } else { // if entries are present, display in a list of links to the content const listElem = document.createElement("ul"); for (const entry of entries) { const listItem = document.createElement("li"); const anchorElem = document.createElement("a"); anchorElem.innerText = entry.title; anchorElem.setAttribute("href", entry.url); listElem.append(listItem); } readingListElem.append(listElem); } } Unregistering indexed content
Below is an asynchronous function, that removes an item from the content index.
async function unregisterContent(article) { // reference registration const registration = await navigator.serviceWorker.ready; // feature detect Content Index if (!registration.index) return; // unregister content from index await registration.index.delete(article.id); } All the above methods are available within the scope of the service worker. They are accessible from the WorkerGlobalScope.self property:
// service worker script self.registration.index.add(item); self.registration.index.delete(item.id); const contentIndexItems = self.registration.index.getAll(); The contentdelete event
When an item is removed from the user agent interface, a contentdelete event is received by the service worker.
self.addEventListener("contentdelete", (event) => { console.log(event.id); // logs content index id, which can then be used to determine what content to delete from your cache }); The contentdelete event is only fired when the deletion happens due to interaction with the browser's built-in user interface. It is not fired when the ContentIndex.delete() method is called.
Specifications
| Specification |
|---|
| Content Index |
