video标签自动播放音视频并绘制波形图

html中的<video>标签可以用来播放常见的html" title=音视频>音视频格式，支持的格式包括:MP3、Ogg、WAV、AAC、MP4、WebM、AVI等，当然支持的格式也和浏览器和操作系统有关。这里以一个可以自动播放html" title=音视频>音视频并绘制波形图的页面为例说明一下<video>标签的用法。

video标签想自动播放，需要设置三个可选属性分别是muted、autoplay、controls，muted负责让html" title=音视频>音视频播放静音，autoplay让html" title=音视频>音视频自动播放，controls属性负责显示对应的控制菜单。除了通过html页面设置标签属性之外我们还可以通过js脚本来设置对应的属性，设置方法如下所示:

html"><!DOCTYPE html>
<html>
<head>
  <title>播放html" title=音视频>音视频</title>
  <meta charset="UTF-8">
</head>
<body>
  <video id="myVideo" autoplay muted controls>
    <source src="./mysong.mp3" type="audio/mpeg">
    <!--source src="video.mp4" type="video/mp4"-->
    Your browser does not support the video tag.
  </video>
  <script>
	var videoElement = document.getElementById('myVideo');
	// 自动静音播放
	videoElement.muted = true; 
	videoElement.autoplay = true;
	videoElement.controls = true;
  </script>
</body>
</html>

浏览器为了防止页面自动播放音频干扰用户，不允许在用户没有进行交互操作的时候，网页自动以非静音的模式播放html" title=音视频>音视频。所以autoplay属性必须搭配muted属性一块使用。
如果想要绘制html" title=音视频>音视频播放过程中的音频波形图，我们需要拦截对应的音频上下文，分析绘制对应的音频数据。对应的实现如下所示:

html"><!DOCTYPE html>
<html>
<head>
  <title>绘制音频波形图</title>
  <meta charset="UTF-8">
</head>
<body>
  <h1>绘制音频波形图</h1>
  
  <video id="myVideo" controls>
    <source src="./mysong.mp3" type="audio/mpeg">
    Your browser does not support the video tag.
  </video>
  <canvas id="waveformCanvas"></canvas>
<script>
  // 获取video元素和canvas元素
  let video,analyser,ctx,canvas,audioContext,timerID,analyserNode;
  video = document.getElementById('myVideo');
  //播放的时候调用初始化操作
  video.addEventListener("play",initWaveDraw);
  //获取画布元素
  canvas = document.getElementById('waveformCanvas');
  ctx = canvas.getContext('2d');
 
  function initWaveDraw(){
	  // 创建音频上下文
	  if(!audioContext)
	  {
		  audioContext = new(window.AudioContext || window.webkitAudioContext)();
		  analyser = audioContext.createAnalyser();
		  analyser.connect(audioContext.destination);
		  analyserNode = audioContext.createMediaElementSource(video);
		  analyserNode.connect(analyser);
		  timerID = setInterval(drawWaveform,200); 
	  }
	    
  }
 
  // 绘制波形图
  function drawWaveform() {
    // 获取波形数据
    var bufferLength = analyser.fftSize;
	console.log("drawing wave");
	
    var dataArray = new Uint8Array(bufferLength);
    analyser.getByteTimeDomainData(dataArray);

    // 清空画布
    ctx.clearRect(0, 0, canvas.width, canvas.height);

    // 绘制波形图
    ctx.lineWidth = 2;
    ctx.strokeStyle = 'rgb(0, 255, 255)';
    ctx.beginPath();
    var sliceWidth = canvas.width * 1.0 / bufferLength;
    var x = 0;
    for (var i = 0; i < bufferLength; i++) {
      var v = dataArray[i] / 128.0;
      var y = v * canvas.height / 2;
      if (i === 0) {
        ctx.moveTo(x, y);
      } else {
        ctx.lineTo(x, y);
      }
      x += sliceWidth;
    }
    ctx.lineTo(canvas.width, canvas.height / 2);
    ctx.stroke();
  }
</script>	
</body>
</html>

显示效果如下图所示:

在这里插入图片描述

如果想让播放器自动播放音频并放音，我们可以采用一些迂回策略，首先让播放器自动静音播放，然后设置一个定时器检测用户是否和页面发生了操作交互，如果产生了交互就播放音频并绘制波形图。(浏览器默认没有交互的时候不允许播放声音) 对应的实现如下所示:

html"><!DOCTYPE html>
<html>
<head>
  <title>绘制音频波形图</title>
  <meta charset="UTF-8">
</head>
<body>
  <h1>绘制音频波形图</h1>
  
  <video id="myVideo" controls>
    <source src="./mysong.mp3" type="audio/mpeg">
    Your browser does not support the video tag.
  </video>
  <canvas id="waveformCanvas"></canvas>
<script>
  // 获取video元素和canvas元素
  let video,analyser,ctx,canvas,audioContext,timerID, checktimerID, analyserNode;
  var hasUserInteracted = false;
  video = document.getElementById('myVideo');
  //获取画布元素
  canvas = document.getElementById('waveformCanvas');
  ctx = canvas.getContext('2d');
  
  video.muted = true; 
  video.autoplay = true;
  video.controls = true;
  
  // 监听键盘按下事件
  function handleUserInteraction(){
    console.log("user has interacted");
	hasUserInteracted = true;
  }
  document.addEventListener('click', handleUserInteraction);
 
  checktimerID = setInterval(checkMouseClick, 1000);
  setTimeout(function(){ video.addEventListener("volumechange",handleUserInteraction);},2000);
  
  //定时检测鼠标事件,开启带声音的播放
	function checkMouseClick(){
		if(hasUserInteracted)
		{			
		   initWaveDraw();
		   video.muted = false; 
		   video.play();				
		   clearInterval(checktimerID);
		}
	}
 
  function initWaveDraw(){
	  // 创建音频上下文
	  if(!audioContext)
	  {
		  audioContext = new(window.AudioContext || window.webkitAudioContext)();
		  analyser = audioContext.createAnalyser();
		  analyser.connect(audioContext.destination);
		  analyserNode = audioContext.createMediaElementSource(video);
		  analyserNode.connect(analyser);
		  timerID = setInterval(drawWaveform,200); 
	  }
	    
  }
 
  // 绘制波形图
  function drawWaveform() {
    // 获取波形数据
    var bufferLength = analyser.fftSize;
	console.log("drawing wave");
	
    var dataArray = new Uint8Array(bufferLength);
    analyser.getByteTimeDomainData(dataArray);

    // 清空画布
    ctx.clearRect(0, 0, canvas.width, canvas.height);

    // 绘制波形图
    ctx.lineWidth = 2;
    ctx.strokeStyle = 'rgb(0, 255, 255)';
    ctx.beginPath();
    var sliceWidth = canvas.width * 1.0 / bufferLength;
    var x = 0;
    for (var i = 0; i < bufferLength; i++) {
      var v = dataArray[i] / 128.0;
      var y = v * canvas.height / 2;
      if (i === 0) {
        ctx.moveTo(x, y);
      } else {
        ctx.lineTo(x, y);
      }
      x += sliceWidth;
    }
    ctx.lineTo(canvas.width, canvas.height / 2);
    ctx.stroke();
  }
</script>	
</body>
</html>