canvas 橡皮筋式线条制图运用方式

  <canvas id="canvas" width="600" height="400" style="border: 1px solid black;"> </canvas>
    <script type="text/javascript">
        let canvas = document.getElementById('canvas'),
            ctx = canvas.getContext('2d'),
            canvasLeft = canvas.getBoundingClientRect().left, //getBoundingClientRect()获得元素部位
            canvasTop = canvas.getBoundingClientRect().top;
        let imageData; //纪录图象数据信息
        let start = new Map([['x',null],['y',null]]);
        let drag = false;//纪录是不是处在拖拽情况
        canvas.onmousedown = function (e) {
            let pos = positionInCanvas(e, canvasLeft, canvasTop);
            start.set('x', pos.x);
            start.set('y', pos.y);
            drag = true;
            //纪录imageData
            imageData = ctx.getImageData(0,0,canvas.width,canvas.height);
        }
        canvas.onmousemove = function (e) {
            if(drag === true){
               let pos = positionInCanvas(e, canvasLeft, canvasTop);
               //非常于把扫描仪出来的线都擦掉，再次画
               ctx.putImageData(imageData, 0, 0);
               ctx.beginPath();
               ctx.moveTo(start.get('x'), start.get('y'));
               ctx.lineTo(pos.x, pos.y);
               ctx.stroke();
            }

        }
        canvas.onmouseup = function  (e) {
            drag = false;
        }
        function positionInCanvas (e, canvasLeft, canvasTop) {//获得canvas中电脑鼠标点一下部位
         return {
                  x:e.clientX - canvasLeft,
                  y:e.clientY - canvasTop
          }       
        }    

    </script>