​

1. 主要实现能够提供给用户绘图的功能；

实现：通过点击，移动事件获取当前坐标，并且在移动的时候将当前的坐标作为下一次起始值的坐标，然后调用绘图功能；

根据四个坐标点绘图。

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>canvas画板</title>
  <style type="text/css">
    #myCanvas {
      border: 1px solid #ccc;
    }
    .canvasBox {
      width: 600px;
      margin: 0 auto;
      text-align: center;
    }
  </style>
</head>
<body>
  <div class="canvasBox">
    <canvas id="myCanvas" height="400" width="500"></canvas>
  </div>
  <script type="text/javascript">
    var canvas = document.getElementById('myCanvas');
    var ctx = myCanvas.getContext('2d');

let clear = false;
let lWidth = 4;


listenToUser(canvas);

function listenToUser(canvas) {
    let painting = false;
    let lastPoint = {x: undefined, y: undefined};

    if (document.body.ontouchstart !== undefined) {
        canvas.ontouchstart = function (e) {
            console.log(e)
            var canvasLeft = e.target.offsetLeft;
            var canvasTop = e.target.offsetTop;
            painting = true;
            let x = e.changedTouches[0].clientX-canvasLeft;
            let y = e.changedTouches[0].clientY-canvasTop;
            lastPoint = {"x": x, "y": y};
            ctx.save();
        };
        canvas.ontouchmove = function (e) {
            if (painting) {
                var canvasLeft = e.target.offsetLeft;
                var canvasTop = e.target.offsetTop;
                let x = e.changedTouches[0].clientX-canvasLeft;
                let y = e.changedTouches[0].clientY-canvasTop;

                let newPoint = {"x": x, "y": y};
                drawLine(lastPoint.x, lastPoint.y, newPoint.x, newPoint.y);
                lastPoint = newPoint;
            }
        };

        canvas.ontouchend = function () {
            painting = false;
        }
    } else {
        canvas.onmousedown = function (e) {
            var canvasLeft = e.target.offsetLeft;
            var canvasTop = e.target.offsetTop;
            painting = true;
            let x = e.clientX-canvasLeft;
            let y = e.clientY-canvasTop;
            lastPoint = {"x": x, "y": y};
            ctx.save();
        };
        canvas.onmousemove = function (e) {
            if (painting) {
                var canvasLeft = e.target.offsetLeft;
                var canvasTop = e.target.offsetTop;
                let x = e.clientX-canvasLeft;
                let y = e.clientY-canvasTop;
                let newPoint = {"x": x, "y": y};
                drawLine(lastPoint.x, lastPoint.y, newPoint.x, newPoint.y,clear);
                lastPoint = newPoint;
            }
        };

        canvas.onmouseup = function () {
            painting = false;
        };

        canvas.mouseleave = function () {
            painting = false;
        }
    }
}


function drawLine(x1, y1, x2, y2) {
    ctx.lineWidth = lWidth;
    ctx.lineCap = "round";
    ctx.lineJoin = "round";
    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);
    ctx.stroke();
    ctx.closePath();

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