指画图时橡皮筋一样伸缩自如。。
例子如下:point_down:
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>以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持VeVb武林网。