The MediaDevices.ondevicechange
property is an EventHandler
which specifies a function to be called when the devicechange
event occurs on a MediaDevices
instance. This happens whenever the set of media devices available to the user agent and, by extension, to the web site or app has changed. You can at any time use enumerateDevices()
to get the updated list of available devices.
MediaDevices.ondevicechange = eventHandler;
A function you provide which accepts as input a Event
object describing the devicehange
event that occurred. There is no information about the change included in the event object; to get the updated list of devices, you'll have to use enumerateDevices()
.
In this example, we create a function called updateDeviceList()
, which is called once when MediaDevices.getUserMedia()
successfully obtains a stream, and then is called any time the device list changes. It displays in the browser window two lists: one of audio devices and one of video devices, with both the device's label (name) and whether it's an input or an output device. Because the example provides a handler for the devicechange
event, the list is refreshed any time a media device is attached to or removed from the device running the sample.
We set up global variables that contain references to the <ul>
elements that are used to list the audio and video devices:
let audioList = document.getElementById("audioList"); let videoList = document.getElementById("videoList");
Now let's take a look at updateDeviceList()
itself. This method is called any time we want to fetch the current list of media devices and then update the displayed lists of audo and video devices using that information.
function updateDeviceList() { navigator.mediaDevices.enumerateDevices() .then(function(devices) { audioList.innerHTML = ""; videoList.innerHTML = ""; devices.forEach(function(device) { let elem = document.createElement("li"); let [kind, type, direction] = device.kind.match(/(\w+)(input|output)/i); elem.innerHTML = "<strong>" + device.label + "</strong> (" + direction + ")"; if (type === "audio") { audioList.appendChild(elem); } else if (type === "video") { videoList.appendChild(elem); } }); }); }
updateDeviceList()
consists entirely of a call to the function enumerateDevices()
on the MediaDevices
object referenced in the navigator.mediaDevices
property, as well as the code that's run when the promise
returned by enumerateDevices()
is fulfilled. The fulfillment handler is called when the device list is ready. The list is passed into the fulfillment handler as an array of MediaDeviceInfo
objects, each describing one media input or output device.
A forEach()
loop is used to scan through all the devices. For each device, we create a new <li>
object to be used to display it to the user.
The line let [kind, type, direction] = device.kind.match(/(\w+)(input|output)/i);
deserves special notice. This uses destructuring assignment (a new feature of ECMAScript 6) to assign the values of the first three items in the array returned by String.match()
to the variables kind
, type
, and direction
. We do this because the value of MediaDeviceInfo.kind
is a single string that includes both the media type and the direction the media flows, such as "audioinput" or "videooutput". This line, then, pulls out the type ("audio" or "video") and direction ("input" or "output") so they can be used to construct the string displayed in the list.
Once the string is assembled, containing the device's name in bold and the direction in parentheses, it's appended to the appropriate list by calling appendChild()
on either audioList
or videoList
, as appropriate based on the device type.
We call updateDeviceList()
in two places. The first is in the getUserMedia()
promise's fulfillment handler, to initially fill out the list when the stream is opened. The second is in the event handler for devicechange
:
navigator.mediaDevices.ondevicechange = function(event) { updateDeviceList(); }
With this code in place, each time the user plugs in a camera, microphone, or other media device, or turns one on or off, we call updateDeviceList()
to redraw the list of connected devices.
Specification | Status | Comment |
---|---|---|
Media Capture and Streams The definition of 'ondevicechange' in that specification. | Editor's Draft | Initial specification. |
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|
Basic support | (Yes) | 51 (51) | ? | ? | ? |
Feature | Android | Android Webview | Firefox Mobile (Gecko) | IE Phone | Opera Mobile | Safari Mobile | Chrome for Android |
---|---|---|---|---|---|---|---|
Basic support | ? | ? | ? | No support | ? | ? | (Yes) |
[1] Support for the devicechange
event and for MediaDevices.ondevicechange
landed in Firefox 51, but only for Mac, and disabled by default. It can be enabled by setting the preference media.ondevicechange.enabled
to true
. Support for this event was added for Linux and Windows—and it was enabled by default—starting in Firefox 52.
devicechange
event and its type, Event
.MediaDevices.enumerateDevices()
MediaDeviceInfo
© 2005–2017 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/MediaDevices/ondevicechange