html - Assign the value of MediaDevices.enumerateDevices() into global variable in JavaScript - Stack Overflow

admin管理员组

文章数量:1325233

I have a question about the sequence of JavaScript. Let me show you my code first:

Here is my HTML:

<video id="video" width="320" height="320" autoplay></video><br>
<button id="snap">Snap Photo</button><br>
<canvas id="canvas" width="320" height="320"></canvas>
<p id="pngHolder"></p>

And here is my JavaScript:

<script>
var Id;
//List cameras and microphones.
if (!navigator.mediaDevices || !navigator.mediaDevices.enumerateDevices) {
    console.log("enumerateDevices() not supported.");
}
navigator.mediaDevices.enumerateDevices()
.then(function (devices) {
    devices.forEach(function (device) {
        if (device.kind == "videoinput" && device.label.indexOf('back') >= 0) {
            Id = device.deviceId;
            alert("ID 1 : " + Id);
        }
    });
})
.catch(function (err) {
    console.log(err.name + ": " + err.message);
});

// Put event listeners into place
window.addEventListener("DOMContentLoaded", function () {
    // Grab elements, create settings, etc.
    alert("ID 2 : "+ Id);
    var canvas = document.getElementById("canvas"),
        videoObj = {
            video: {
                optional: [{ deviceId: Id }]
            }
        },
        context = canvas.getContext("2d"),
        video = document.getElementById("video"),
        errBack = function (error) {
            console.log("Video capture error: ", error.code);
        };

    // Trigger photo take
    document.getElementById("snap").addEventListener("click", function () {
        context.drawImage(video, 0, 0, 640, 480);
        // Get the image
        var image = convertCanvasToImage(canvas);
        // Actions
        document.getElementById("pngHolder").appendChild(image);
        // Converts canvas to an image
        function convertCanvasToImage(canvas) {
            var image = new Image();
            image.src = canvas.toDataURL("image/png");
            return image;
        }
    });

    //alert("ID 2 : " + Id);
    // Put video listeners into place
    if (navigator.getUserMedia) { // Standard
        navigator.getUserMedia(videoObj, function (stream) {
            video.src = stream;
            video.play();
        }, errBack);
    } else if (navigator.webkitGetUserMedia) { // WebKit-prefixed
        navigator.webkitGetUserMedia(videoObj, function (stream) {
            video.src = window.webkitURL.createObjectURL(stream);
            video.play();
        }, errBack);
    } else if (navigator.mozGetUserMedia) { // Firefox-prefixed
        navigator.mozGetUserMedia(videoObj, function (stream) {
            video.src = window.URL.createObjectURL(stream);
            video.play();
        }, errBack);
    }
}, false);

I want to insert the value of device.deviceId into variable Id that I define on the first of my JavaScript row. It is still a success (it is shown by the alert("ID 1 : " + Id);). But when I try to put it into optional: [{ deviceId: Id }], the Id doesn't have any value.

And also, when I try to run it with browser, I have found that the alert("ID 2 : " + Id); is shown first instead of alert("ID 1 : " + Id);. In fact, I have already put the alert("ID 1 : " + Id); first. I think that's why the variable is still empty.

My question is how can I insert the device.deviceId value to optional: [{ deviceId: Id }] ?

I have a question about the sequence of JavaScript. Let me show you my code first:

Here is my HTML:

<video id="video" width="320" height="320" autoplay></video><br>
<button id="snap">Snap Photo</button><br>
<canvas id="canvas" width="320" height="320"></canvas>
<p id="pngHolder"></p>

And here is my JavaScript:

<script>
var Id;
//List cameras and microphones.
if (!navigator.mediaDevices || !navigator.mediaDevices.enumerateDevices) {
    console.log("enumerateDevices() not supported.");
}
navigator.mediaDevices.enumerateDevices()
.then(function (devices) {
    devices.forEach(function (device) {
        if (device.kind == "videoinput" && device.label.indexOf('back') >= 0) {
            Id = device.deviceId;
            alert("ID 1 : " + Id);
        }
    });
})
.catch(function (err) {
    console.log(err.name + ": " + err.message);
});

// Put event listeners into place
window.addEventListener("DOMContentLoaded", function () {
    // Grab elements, create settings, etc.
    alert("ID 2 : "+ Id);
    var canvas = document.getElementById("canvas"),
        videoObj = {
            video: {
                optional: [{ deviceId: Id }]
            }
        },
        context = canvas.getContext("2d"),
        video = document.getElementById("video"),
        errBack = function (error) {
            console.log("Video capture error: ", error.code);
        };

    // Trigger photo take
    document.getElementById("snap").addEventListener("click", function () {
        context.drawImage(video, 0, 0, 640, 480);
        // Get the image
        var image = convertCanvasToImage(canvas);
        // Actions
        document.getElementById("pngHolder").appendChild(image);
        // Converts canvas to an image
        function convertCanvasToImage(canvas) {
            var image = new Image();
            image.src = canvas.toDataURL("image/png");
            return image;
        }
    });

    //alert("ID 2 : " + Id);
    // Put video listeners into place
    if (navigator.getUserMedia) { // Standard
        navigator.getUserMedia(videoObj, function (stream) {
            video.src = stream;
            video.play();
        }, errBack);
    } else if (navigator.webkitGetUserMedia) { // WebKit-prefixed
        navigator.webkitGetUserMedia(videoObj, function (stream) {
            video.src = window.webkitURL.createObjectURL(stream);
            video.play();
        }, errBack);
    } else if (navigator.mozGetUserMedia) { // Firefox-prefixed
        navigator.mozGetUserMedia(videoObj, function (stream) {
            video.src = window.URL.createObjectURL(stream);
            video.play();
        }, errBack);
    }
}, false);

I want to insert the value of device.deviceId into variable Id that I define on the first of my JavaScript row. It is still a success (it is shown by the alert("ID 1 : " + Id);). But when I try to put it into optional: [{ deviceId: Id }], the Id doesn't have any value.

And also, when I try to run it with browser, I have found that the alert("ID 2 : " + Id); is shown first instead of alert("ID 1 : " + Id);. In fact, I have already put the alert("ID 1 : " + Id); first. I think that's why the variable is still empty.

My question is how can I insert the device.deviceId value to optional: [{ deviceId: Id }] ?

• javascript
• html
• mobile
• camera
• webrtc

Share Improve this question Follow edited Apr 6, 2016 at 14:35 Yusril Maulidan Raji asked Apr 6, 2016 at 14:16 Yusril Maulidan RajiYusril Maulidan Raji 1,95222 gold badges2222 silver badges5050 bronze badges

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 5

navigator.mediaDevices.enumerateDevices and DOMContentLoaded are racing, and the latter is winning, so you're using Id before it's set.

To solve this use a temporary haveId promise:

var haveId = navigator.mediaDevices.enumerateDevices()
  .then(devices => devices.find(d => d.kind == "videoinput" &&
                                d.label.indexOf("back") >= 0));

// Put event listeners into place
window.addEventListener("DOMContentLoaded", function () {
  haveId.then(id => {
    // Grab elements, create settings, etc.
    alert("ID 2 : "+ id);
    var canvas = document.getElementById("canvas"),
    videoObj = { video: { deviceId: id } }, // <-- adapter.js constraints

Promise-chains create dependencies, and this way the getUserMedia code wont proceed until both things have happened.

The second problem is you're mixing new and outdated Chrome-specific constraints. Either use adapter.js until Chrome catches up, or in a pinch, use the Chrome-only sourceId (but that wont work in any other browser).

navigator.mediaDevices.enumerateDevices() is asynchronous. It returns a promise (think callback, but fancier).

You should trigger your call to getUserMedia from there or wait for both DOMContentLoaded and enumerateDevices and then execute getUserMedia.

本文标签： htmlAssign the value of MediaDevicesenumerateDevices() into global variable in JavaScriptStack Overflow