--- title: FetchObserver slug: Web/API/FetchObserver translation_of: Web/API/FetchObserver ---
{{draft}}{{APIRef("Fetch API")}}{{SeeCompatTable}}

在FetchObserver接口提取API表示观察者对象，它允许您检索关于为获取请求的状态信息。

Properties

FetchObserver接口从其父接口继承属性EventTarget。

{{domxref("FetchObserver.state")}} {{readonlyInline}}
Returns a FetchState enum value indicating the current state of the fetch request.

Event handlers

{{domxref("FetchObserver.onstatechange")}}
Invoked when a {{event("statechange_(cancellable_fetch)", "statechange")}} event fires, i.e. when the state of the fetch request changes.
{{domxref("FetchObserver.onrequestprogress")}}
Invoked when a {{event("requestprogress")}} event fires, i.e. when the request progresses.
{{domxref("FetchObserver.onresponseprogress")}}
Invoked when a {{event("responseprogress")}} event fires, i.e. when the download of the response progresses.

Methods

The FetchSignal interface inherits methods from its parent interface, {{domxref("EventTarget")}}.

Examples

In the following snippet, we create a new {{domxref("FetchController")}} object, get its signal, and then give the signal to the fetch request via the signal parameter of its init object so the controller can control it. Later on we specify an event listener on a cancel button so that when the button is clicked, we abort the fetch request using {{domxref("FetchController.abort()")}}.

We also specify an observe property inside the fetch request init object — this contains a {{domxref("ObserverCallback")}} object, the sole purpose of which is to provide a callback function that runs when the fetch request runs. This returns a {{domxref("FetchObserver")}} object that can be used to retrieve information concerning the status of a fetch request.

Here we use {{domxref("FetchController.responseprogress")}} and {{domxref("FetchController.onstatechange")}} event handlers to respectively fill up a progress bar as more of the reponse downloads, and to determine when the download has completed and display a message to let the user know.

Note that these event handlers are not yet supported anywhere.

var controller = new FetchController();
var signal = controller.signal;

downloadBtn.addEventListener('click', function() {
  fetch(url, {
    signal,
    observe(observer) {
      observer.onresponseprogress = function(e) {
        progress.max = e.total;
        progress.value = e.loaded;
      }

      observer.onstatechange = function() {
        if (observer.state = 'complete') {
          reports.textContent = 'Download complete';
        }
      }
    }
  }).then( ... ) // do something with the response
});

cancelBtn.addEventListener('click', function() {
  controller.abort();
});

You can find a work-in-progress demo showing usage of FetchObserver on GitHub (see the source code and the live example).

Specifications

Not part of a specification yet.

Browser compatibility

{{CompatibilityTable}}

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support

{{CompatNo}}

 {{CompatNo}} {{CompatNo}}[1] {{CompatNo}}

{{CompatNo}}

 {{CompatNo}}
Feature Android Android Webview Edge Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile Chrome for Android
Basic support {{CompatNo}} {{CompatNo}} {{CompatNo}} {{CompatNo}}[1] {{CompatNo}} {{CompatNo}} {{CompatNo}} {{CompatNo}}

[1] Hidden behind a preference in 55+ Nightly. In about:config, you need to create two new boolean prefs — dom.fetchObserver.enabled and dom.fetchController.enabled — and set the values of both to true.

See also