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Native js and canvas simulated electrocardiogram code sharing

小云云Original: 2018-02-05 13:33:042043browse

The html page that simulates electrocardiogram is made using native js+canvas. Since it is packaged and put on github together with the project, it uses the single-page mode of vue.js. In fact, you do not need to use any additional frameworks and styles. You can also complete this demo, now let’s dismantle this project together!

1: Create a canvas on the page. To make the "line" of the electrocardiogram move on our page, canvas is essential. Because the project is relatively simple, the DOM elements on the page have been written so far. The main workload is concentrated in the js part

<p class="heartBeat">
 　　<canvas id="can">Canvas画板</canvas>
 </p>

2: Define several variables and assign values. These variables will be needed for calculation during runtime

var can = document.getElementById(&#39;can&#39;),//画布对象
    pan,//获取2D图像API接口
    index = 0,//用来接收setinerval的值
    flag = true,//用来控制心电图折线的运行方向
    wid = document.body.clientWidth,//获取浏览器宽度
    hei = document.body.clientHeight,//获取浏览器高度
    x = 0,//心电图的“点”在画布上的x轴坐标，从0开始
    y = hei/2;//心电图的“点”在画布上的y轴坐标，从页面y轴居中位置开始

3: Initialize the canvas and set various properties for the canvas

function start(){
    can.height = hei;//设置画布高度
    can.width = wid;//设置画布宽度
    pan = can.getContext("2d");//获取2D图像API接口
    pan.strokeStyle = "#08b95a";//设置画笔颜色
    pan.lineJoin = "round";//设置画笔轨迹基于圆点拼接
    pan.lineWidth = 9;//设置画笔粗细
    pan.beginPath();//开始一条画笔的路径
    pan.moveTo(x,y);//定位我们的“落笔点”
    index = setInterval(move,1);//让我们的画笔动起来
  };

As you can see, we have not yet involved the "painting" action here. We just initialized the canvas size so that the canvas fills the screen. At the same time, we defined the color, thickness, pen point and other operations of the brush, and then used the setInterval method. Let the brush keep moving along the route we calculated. If you are not very familiar with the setInterval method, I suggest you take a look at the usage of setInterVal, which will not be described here. Because we want the electrocardiogram to loop infinitely and execute automatically, we encapsulate it here as the start() function, so that when the electrocardiogram moves to the far right of the screen, we re-execute the start() function to make the electrocardiogram infinite. It’s a cycle

4. Let the electrocardiogram move! It can be said that the previous steps are not difficult. The real core code is to make our electrocardiogram move and move along the route we want. Now we will make the electrocardiogram really come alive

function move(){
    x++;//x轴是始终运动的，所以x一直自增
    if(x < 100){
      //前100px，我们不希望做垂直运动，让点只沿垂直方向运动即可，所以不做任何操作
    }else{
      if(x >= wid - 100){
      //最后的100px，同样希望心电图只做水平运动，不会上下波动，所以不做任何操作
      }else{
        //为了让心电图看起来更加逼真，我们希望心电图在运动时每次的波峰和波谷都是随机的，这样更类似于人类的心跳，所以我们给它一个随机值z
        var z = Math.random()*280;

        if(y <= z){
          //画布的坐标是从左上角开始计算的，也就是最左上角的点的坐标是（0,0）,y是当前画笔所在坐标的y轴，假如y小于z，就代表y已经到达波峰位置，准备开始向波谷运动
          flag = true
        }
        if((hei - y) <= z){
          //假如当前画笔在y轴的坐标y距离浏览器底部hei的差值已经小于随机值z，代表当前的画笔已经运行到波谷位置，准备转向波峰位置运动
          flag = false
        }
        if(flag){
          //假如flag为true，代表画笔仍然向波谷位置前进，需要花点功夫理解的是，因为画布左上角的点的坐标是（0,0），所以y的值越大，画笔在y轴的位置越靠近浏览器底部，所以向波谷运动时，y的值是不断增加的，同时为了让波峰波谷更陡峭，我这里设置y += 5，
          y+=5
        }else{
          //假如flag为false，表示向波峰运动，y的值是不断减小的
          y-=5
        }
      }
    }
    if(x == wid){
      //当画笔运动到浏览器右侧边缘，停止绘图
      pan.closePath();
      //清除循环
      clearInterval(index);
      //将index置零，准备下一次循环
      index = 0;
      //重新定位画笔到屏幕左侧上下居中的位置
      x = 0;
      y = hei/2;
      flag = true;
      //重新进行下一次心电图的绘制
      start();
    }
    //lineTo和stroke函数负责描绘运动轨迹
    pan.lineTo(x,y);
    pan.stroke();
  }

5: Note, the electrocardiogram can actually be run at this point, but it should be noted that set your body height to 100%, otherwise the canvas may not be able to fill the entire page

html,body{
   width: 100%;
   height: 100%;
   margin: 0;
  }

Complete project code:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>模拟心电图</title>
  <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no">
  <style>
    html,body{
      width: 100%;
      height: 100%;
      margin: 0;
    }
  </style>
</head>
<body>
<p id="canvas">
  <canvas id="can">Canvas画板</canvas>
</p>

<script src="js/vue.min.js"></script>
<script>
  var can = document.getElementById(&#39;can&#39;),
    pan,
    index = 0,
    flag = true,
    wid = document.body.clientWidth,
    hei = document.body.clientHeight,
    x = 0,
    y = hei/2;
  start();
  function start(){
    can.height = hei;
    can.width = wid;
    pan = can.getContext("2d");//获取2D图像API接口
    pan.strokeStyle = "#08b95a";//设置画笔颜色
    pan.lineJoin = "round";//设置画笔轨迹基于圆点拼接
    pan.lineWidth = 9;//设置画笔粗细
    pan.beginPath();
    pan.moveTo(x,y);
    index = setInterval(move,1);
  };
  function move(){
    x++;
    if(x < 100){

    }else{
      if(x >= wid - 100){

      }else{
        var z = Math.random()*280;
        if(y <= z){
          flag = true
        }
        if((hei - y) <= z){
          flag = false
        }
        if(flag){
          y+=5
        }else{
          y-=5
        }
      }
    }
    if(x == wid){
      pan.closePath();
      clearInterval(index);
      index = 0;
      x = 0;
      y = hei/2;
      flag = true;
      start();
    }
    pan.lineTo(x,y);
    pan.stroke();
  }

 /* */

</script>
</body>
</html>

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