HTML5 选择前置摄像头，选择后置摄像头

最近发现我写的都是乱七八糟的，觉得应该给大家带点福利，于是写了这篇

背景：最近想做个web应用，需要用到摄像头，但是发现默认一直是前置摄像头，拍照很麻烦，于是找了很多文章，居然没有人提到，只好翻墙去找外文，终于看到了HTML5的一些定义，研究以后……挺简单的，注意这句

MediaStreamTrack.getSources(gotSources);
有了这句就搞定了
html就不解释了，js里有一个兼容各个浏览器的

navigator.getUserMedia = navigator.getUserMedia ||
      navigator.webkitGetUserMedia || navigator.mozGetUserMedia;
然后是获取设备，就是上面说的需要注意的这句……研究好久才知道是这么用的……
最后选择好设备后，绑定设备，刷新就好了

源码如下，敏感部分我修改掉了，不过这个应该可以运行的

<div>
    <label for='audioSource'>Audio source: </label><select id='audioSource'></select>
        <label for='videoSource'>Video source: </label><select id='videoSource'></select>
    <video id="me" width="160" height="120" autoplay></video>
    <video id="you" width="160" height="120" autoplay></video>
</div>

<script type="text/javascript">
    var videoElement = document.querySelector("video#me");
    var audioSelect = document.querySelector("select#audioSource");
    var videoSelect = document.querySelector("select#videoSource");
    var startButton = document.querySelector("button#start");

    navigator.getUserMedia = navigator.getUserMedia ||
      navigator.webkitGetUserMedia || navigator.mozGetUserMedia;

    function gotSources(sourceInfos) {
        for (var i = 0; i != sourceInfos.length; ++i) {
            var sourceInfo = sourceInfos[i];
            var option = document.createElement("option");
            option.value = sourceInfo.id;
            if (sourceInfo.kind === 'audio') {
                option.text = sourceInfo.label || 'microphone ' + (audioSelect.length + 1);
                audioSelect.appendChild(option);
            } else if (sourceInfo.kind === 'video') {
                option.text = sourceInfo.label || 'camera ' + (videoSelect.length + 1);
                videoSelect.appendChild(option);
            } else {
                console.log('Some other kind of source: ', sourceInfo);
            }
        }
    }

    if (typeof MediaStreamTrack === 'undefined') {
        alert('This browser does not support MediaStreamTrack.\n\nTry Chrome Canary.');
    } else {
        MediaStreamTrack.getSources(gotSources);
    }

    function successCallback(stream) {
        window.stream = stream; // make stream available to console
        videoElement.src = window.URL.createObjectURL(stream);
        videoElement.play();
    }

    function errorCallback(error) {
        console.log("navigator.getUserMedia error: ", error);
    }

    function start() {
        if (!!window.stream) {
            videoElement.src = null;
            window.stream.stop();
        }
        var audioSource = audioSelect.value;
        var videoSource = videoSelect.value;
        var constraints = {
            audio: {
                optional: [{ sourceId: audioSource }]
            },
            video: {
                optional: [{ sourceId: videoSource }]
            }
        };
        navigator.getUserMedia(constraints, successCallback, errorCallback);
    }

    audioSelect.onchange = start;
    videoSelect.onchange = start;

    start();
    </script>

　　不知道这里能显示不，可以选择当前设备的音频和视频，然后切换以后会实时体现的