Canvas橡皮筋式画线的实现思路和代码是什么

什么叫橡皮筋式

指画图时橡皮筋一样伸缩自如。。
例子如下

思路

思路很简单，只有橡皮筋式绘制功能要注意，以下总结mousedown，mousemove，mouseup三个阶段的思路
mousedown：记录start位置，drag（记录是否处于拖动状态）设置为true，getImageData(橡皮筋效果关键1)
mousemove：获取拖动时的位置pos，putImageData(对应getImageData，橡皮筋效果关键2)，根据pos与start画直线
mouseup：drag恢复为false
关键就在于putImageData()与getImageData()这两个canvas的方法，putImageData()记录了鼠标点下时的图像，getImageData()对应还原。如果没有执行这两个方法就会出现以下的效果

putImageData()相当于把“扫描”出来的线都擦掉

代码

    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()获取元素位置
            canvasTop = canvas.getBoundingClientRect().top;
    
        let imageData;
     //记录图像数据
        let start = new Map([['x',null],['y',null]]);
    
        let drag = false;
//记录是否处于拖动状态
        canvas.onmousedown = function (e) {
    
            let pos = positionInCanvas(e, canvasLeft, canvasTop);
    
            start.set('x', pos.x);
    
            start.set('y', pos.y);
    
            drag = true;
    
            //记录imageData
            imageData = ctx.getImageData(0,0,canvas.width,canvas.height);

        }

        canvas.onmousemove = function (e) {

            if(drag === true){
    
               let pos = positionInCanvas(e, canvasLeft, canvasTop);
    
               //相当于把扫描出来的线都擦掉，重新画
               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) {
//获取canvas中鼠标点击位置
         return {

                  x:e.clientX - canvasLeft,
                  y:e.clientY - canvasTop
          }
       
        }
        
    /script>