如何在Chrome中为从MediaRecorder录制的音频添加预定义的长度

这是一个chrome错误。

FF确实公开了录制媒体的持续时间，如果您确实将录制媒体的currentTime设置为大于其实际duration，则该属性在chrome中可用。。。

var recorder,
  chunks = [],
  ctx = new AudioContext(),
  aud = document.getElementById('aud');
function exportAudio() {
  var blob = new Blob(chunks);
  aud.src = URL.createObjectURL(new Blob(chunks));
  aud.onloadedmetadata = function() {
    // it should already be available here
    log.textContent = ' duration: ' + aud.duration;
    // handle chrome's bug
    if (aud.duration === Infinity) {
      // set it to bigger than the actual duration
      aud.currentTime = 1e101;
      aud.ontimeupdate = function() {
        this.ontimeupdate = () => {
          return;
        }
        log.textContent += ' after workaround: ' + aud.duration;
        aud.currentTime = 0;
      }
    }
  }
}
function getData() {
  var request = new XMLHttpRequest();
  request.open('GET', 'https://upload.wikimedia.org/wikipedia/commons/4/4b/011229beowulf_grendel.ogg', true);
  request.responseType = 'arraybuffer';
  request.onload = decodeAudio;
  request.send();
}

function decodeAudio(evt) {
  var audioData = this.response;
  ctx.decodeAudioData(audioData, startRecording);
}
function startRecording(buffer) {
  var source = ctx.createBufferSource();
  source.buffer = buffer;
  var dest = ctx.createMediaStreamDestination();
  source.connect(dest);
  recorder = new MediaRecorder(dest.stream);
  recorder.ondataavailable = saveChunks;
  recorder.onstop = exportAudio;
  source.start(0);
  recorder.start();
  log.innerHTML = 'recording...'
  // record only 5 seconds
  setTimeout(function() {
    recorder.stop();
  }, 5000);
}
function saveChunks(evt) {
  if (evt.data.size > 0) {
    chunks.push(evt.data);
  }
}
// we need user-activation
document.getElementById('button').onclick = function(evt){
  getData();
  this.remove();
}

<button id="button">start</button>
<audio id="aud" controls></audio><span id="log"></span>

因此，这里的建议是在错误报告中添加星号，这样chromium的团队就需要一些时间来修复它，即使这种变通方法可以奏效。。。

感谢@Kaiido识别错误并提供有效修复。

我准备了一个名为getblob-duration的npm包，您可以安装它来获得一个很好的Promise包装函数来完成脏工作。

用法如下：

// Returns Promise<Number>
getBlobDuration(blob).then(function(duration) {
  console.log(duration + ' seconds');
});

或ECMAScript 6:

// yada yada async
const duration = await getBlobDuration(blob)
console.log(duration + ' seconds')

Chrome中的一个错误是这种行为背后的根本原因，该错误于2016年检测到，但今天（2019年3月）仍然存在。在某些情况下，audioElement.duration将返回Infinity。

Chrome Bug此处和此处的信息

以下代码提供了避免该错误的解决方法

用法：创建audioElement，并一次性调用此函数，提供audioElement的参考。当返回的promise解析时，audioElement.duration属性应该包含正确的值。（它还修复了videoElements的相同问题）

/**
 *  calculateMediaDuration() 
 *  Force media element duration calculation. 
 *  Returns a promise, that resolves when duration is calculated
 **/
function calculateMediaDuration(media){
  return new Promise( (resolve,reject)=>{
    media.onloadedmetadata = function(){
      // set the mediaElement.currentTime  to a high value beyond its real duration
      media.currentTime = Number.MAX_SAFE_INTEGER;
      // listen to time position change
      media.ontimeupdate = function(){
        media.ontimeupdate = function(){};
        // setting player currentTime back to 0 can be buggy too, set it first to .1 sec
        media.currentTime = 0.1;
        media.currentTime = 0;
        // media.duration should now have its correct value, return it...
        resolve(media.duration);
      }
    }
  });
}
// USAGE EXAMPLE :  
calculateMediaDuration( yourAudioElement ).then( ()=>{ 
  console.log( yourAudioElement.duration ) 
});

感谢@colxi提供的实际解决方案，我添加了一些验证步骤（因为解决方案运行良好，但存在长音频文件问题）。

我花了大约4个小时才让它与长音频文件一起工作，结果验证是的修复方法

        function fixInfinity(media) {
          return new Promise((resolve, reject) => {
            //Wait for media to load metadata
            media.onloadedmetadata = () => {
              //Changes the current time to update ontimeupdate
              media.currentTime = Number.MAX_SAFE_INTEGER;
              //Check if its infinite NaN or undefined
              if (ifNull(media)) {
                media.ontimeupdate = () => {
                  //If it is not null resolve the promise and send the duration
                  if (!ifNull(media)) {
                    //If it is not null resolve the promise and send the duration
                    resolve(media.duration);
                  }
                  //Check if its infinite NaN or undefined //The second ontime update is a fallback if the first one fails
                  media.ontimeupdate = () => {
                    if (!ifNull(media)) {
                      resolve(media.duration);
                    }
                  };
                };
              } else {
                //If media duration was never infinity return it
                resolve(media.duration);
              }
            };
          });
        }
        //Check if null
        function ifNull(media) {
          if (media.duration === Infinity || media.duration === NaN || media.duration === undefined) {
            return true;
          } else {
            return false;
          }
        }
    //USAGE EXAMPLE
            //Get audio player on html
            const AudioPlayer = document.getElementById('audio');
            const getInfinity = async () => {
              //Await for promise
              await fixInfinity(AudioPlayer).then(val => {
                //Reset audio current time
                AudioPlayer.currentTime = 0;
                //Log duration
                console.log(val)
              })
            }

我封装了webm持续时间修复包来解决webm长度问题，它可以在nodejs和web浏览器中使用，以支持2GB以上的视频文件，而不会占用太多内存。

用法如下：

import fixWebmDuration from 'webm-duration-fix';
const mimeType = 'video/webm';codecs=vp9';
const blobSlice: BlobPart[] = [];
mediaRecorder = new MediaRecorder(stream, {
  mimeType
});
mediaRecorder.ondataavailable = (event: BlobEvent) => {
  blobSlice.push(event.data);
}
mediaRecorder.onstop = async () => {  
    // fix blob, support fix webm file larger than 2GB
    const fixBlob = await fixWebmDuration(new Blob([...blobSlice], { type: mimeType }));
    // to write locally, it is recommended to use fs.createWriteStream to reduce memory usage
    const fileWriteStream = fs.createWriteStream(inputPath);
    const blobReadstream = fixBlob.stream();
    const blobReader = blobReadstream.getReader();
    while (true) {
      let { done, value } = await blobReader.read();
      if (done) {
        console.log('write done.');
        fileWriteStream.close();
        break;
      }
      fileWriteStream.write(value);
      value = null;
    }
    blobSlice = [];
};

//如果你想完全修改视频文件，可以使用这个包"webmFixDuration"；，其他方法仅应用于视频标签的显示级别。使用此方法，修改完整的视频文件

webmFixDuration github示例

 mediaRecorder.onstop = async () => {
        const duration = Date.now() - startTime;
        const buggyBlob = new Blob(mediaParts, { type: 'video/webm' });
    
        const fixedBlob = await webmFixDuration(buggyBlob, duration);
        displayResult(fixedBlob);
      };