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canvas简单连线动画的实现代码

建站知识 2021-07-02 23:00www.168986.cn长沙网站建设

前言canvas动画入门系列之简单连线动画。虽然简单，但连线动画应用场景还挺多，做了个小demo，一通百通。

step1绘制点

创建个标签<canvas id="canvas"></canvas>
设置几个点的坐标

   const points = [
        [200, 100], //上
        [300, 200], //右
        [100, 200], //左
        [200, 100], //上
        [200, 300], //下
        [100, 200], //左
        [300, 200], //右
        [200, 300]
      ];
      const canvas = document.querySelector("canvas");
      const ctx = canvas.getContext("2d");

然后把点给画出来

points.forEach(([x, y]) => {
          drawDot(x, y);
        });
function drawDot(x1, y1, r) {
          ctx.save();
          ctx.beginPath(); //不写会和线连起来
          ctx.fillStyle = "red";
          //绘制成矩形
          ctx.arc(x1, y1, r ? r : 2, 0, 2  Math.PI);
          ctx.fill();
          ctx.restore();
        }

step2绘制线条

我们封装一个方法,传入起点终点，绘制一根线条

function drawLine(x1, y1, x2, y2) {
          ctx.save();
          ctx.beginPath(); //不写每次都会重绘上次的线
          ctx.lineCap = "round";
          ctx.lineJoin = "round";
          var grd = ctx.createLinearGradient(x1, y1, x2, y2);

          ctx.moveTo(x1, y1);
          ctx.lineTo(x2, y2);
          ctx.closePath();
          ctx.strokeStyle = "rgba(255,255,255,1)";
          ctx.stroke();
          ctx.restore();
        }

step3线条动画

这里面需要计算两点之间的斜率，然后x坐标每次挪动±1单位，已知斜率和x偏移，即可计算出y的偏移。值得注意的是，这个坐标系和数学中的xy坐标系有点不一样，y轴是反的。然后可以引入额外的参数speed控制速度

function lineMove(points) {
          if (points.length < 2) {
              
            return;
          }
          const [[x1, y1], [x2, y2]] = points;
          let dx = x2 - x1;
          let dy = y2 - y1;
          if (Math.abs(dx) < 1 && Math.abs(dy) < 1) {
            points = points.slice(1);
            lineMove(points);
            return;
          }
          let x = x1,
            y = y1; //线条绘制过程中的终点
          if (dx === 0) {
            (x = x2), (y += (speed  dy) / Math.abs(dy));
          } else if (dy === 0) {
            x += (speed  dx) / Math.abs(dx);
            y = y2;
          } else if (Math.abs(dx) >= 1) {
            let rate = dy / dx;
            x += (speed  dx) / Math.abs(dx);
            y += (speed  rate  dx) / Math.abs(dx);
          }
          drawLine(x1, y1, x, y);
          points[0] = [x, y];
          window.requestAnimationFrame(function() {
            lineMove(points);
          });
        }

主要代码就这么多，先看效果

完整代码

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <meta http-equiv="X-UA-Compatible" content="ie=edge" />
  <title>canvas-连线动画</title>
</head>

<body>
  <canvas id="canvas" width="400" height="400"></canvas>
  <script>
    //起点10，20 终点150，200
    const points = [
      [200, 100], //上
      [300, 200], //右
      [100, 200], //左
      [200, 100], //上
      [200, 300], //下
      [100, 200], //左
      [300, 200], //右
      [200, 300]
    ];
    const canvas = document.querySelector("canvas");
    const ctx = canvas.getContext("2d");
    // const img = new Image();
    const speed = 10; //速度
    // img.onload = function() {
    // canvas.width = img.width;
    // canvas.height = img.height;
    animate(ctx);
    // };

    // img.src = "./imgs/demo.png";
    function animate(ctx) {
      // ctx.drawImage(img, 0, 0);
      ctx.fillRect(0, 0, canvas.width, canvas.height);
      points.forEach(([x, y]) => {
        drawDot(x, y);
      });
      lineMove(points);
    }
    function lineMove(points) {
      if (points.length < 2) {
        return;
      }
      const [[x1, y1], [x2, y2]] = points;
      let dx = x2 - x1;
      let dy = y2 - y1;
      if (Math.abs(dx) < 1 && Math.abs(dy) < 1) {
        points = points.slice(1);
        lineMove(points);
        return;
      }
      let x = x1,
        y = y1; //线条绘制过程中的终点
      if (dx === 0) {
        (x = x2), (y += (speed  dy) / Math.abs(dy));
      } else if (dy === 0) {
        x += (speed  dx) / Math.abs(dx);
        y = y2;
      } else if (Math.abs(dx) >= 1) {
        let rate = dy / dx;
        x += (speed  dx) / Math.abs(dx);
        y += (speed  rate  dx) / Math.abs(dx);
      }
      drawLine(x1, y1, x, y);
      points[0] = [x, y];
      window.requestAnimationFrame(function () {
        lineMove(points);
      });
    }

    function drawLine(x1, y1, x2, y2) {
      ctx.save();
      ctx.beginPath(); //不写每次都会重绘上次的线
      ctx.lineCap = "round";
      ctx.lineJoin = "round";
      var grd = ctx.createLinearGradient(x1, y1, x2, y2);

      ctx.moveTo(x1, y1);
      ctx.lineTo(x2, y2);
      ctx.closePath();
      ctx.strokeStyle = "rgba(255,255,255,1)";
      ctx.stroke();
      ctx.restore();
    }

    function drawDot(x1, y1, r) {
      ctx.save();
      ctx.beginPath(); //不写会和线连起来
      ctx.fillStyle = "red";
      //绘制成矩形
      ctx.arc(x1, y1, r ? r : 2, 0, 2  Math.PI);
      ctx.fill();
      ctx.restore();
    }
  </script>
</body>

</html>

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