微信小程序实现Canvas画板

这个小demo适用于快递实名签收等业务逻辑

源码如下：

js文件：

Page({
  data: {
    ctx: "", // 保存 canvas 上下文
    pen: 5, // 画笔默认的宽度
    color: "#000", // 画笔默认的颜色
  },
  startX: 0, // 保存 X 坐标
  startY: 0, // 保存 Y 坐标
  onLoad() {
    // 获取到 canvas 的上下文，并且保存在 ctx 里面
    const query = wx.createSelectorQuery();
    query.select("#myCanvas")
      .fields({
        node: true,
        size: true
      })
      .exec(res => {
        const canvas = res[0].node;
        const ctx = canvas.getContext('2d');

        // 注意，这里还有一个非常关键的步骤
        // 在新版的 canvas 接口中需要显式的设置画布的宽高
        // 一定要手动指定画布的宽高
        // 否则画布会被拉伸
        const dpr = wx.getSystemInfoSync().pixelRatio
        canvas.width = res[0].width * dpr
        canvas.height = res[0].height * dpr
        ctx.scale(dpr, dpr);

        this.setData({
          ctx
        });
      })
  },
  touchstart(e) {
    // 拿到手指按下时的坐标
    this.startX = e.changedTouches[0].x;
    this.startY = e.changedTouches[0].y;

    // 进行一些绘制的准备工作（笔粗细、颜色、笔头）
    // 如果使用过微信小程序旧版接口的同学，需要注意
    // 现在很多方法都没有，已经停止维护
    this.data.ctx.lineWidth = this.data.pen; // 笔粗细
    this.data.ctx.strokeStyle = this.data.color; // 画笔颜色
    this.data.ctx.lineCap = 'round'; // 设置笔头

    // 开始绘制
    this.data.ctx.beginPath();

  },
  touchmove(e) {
    // 移动的时候，得到的实时的坐标
    var startX1 = e.changedTouches[0].x;
    var startY1 = e.changedTouches[0].y;
    // 首先将画笔移动到手指按下的地方
    this.data.ctx.moveTo(this.startX, this.startY);
    // 描线，根据实时的坐标进行进行描线
    this.data.ctx.lineTo(startX1, startY1);
    // 绘制直线
    this.data.ctx.stroke();

    // 注意，这个步骤也很重要，需要更新起始坐标
    this.startX = startX1;
    this.startY = startY1;
  },
  touchend() {
    this.data.ctx.closePath();
  },
  // 选择画笔的粗细
  penSelect(e) {
    if (this.data.color === "#ddd") {
      // 从橡皮擦切换过来的
      // 需要还原颜色
      this.setData({
        pen: parseInt(e.currentTarget.dataset.param),
        color: "#000"
      })
    } else {
      this.setData({
        pen: parseInt(e.currentTarget.dataset.param),
      })
    }
  },
  // 设置颜色
  colorSelect(e) {
    this.setData({
      color: e.currentTarget.dataset.param
    })
  },
  // 橡皮擦
  clearCanvas() {
    this.setData({
      color: '#ddd',
      pen: 20
    })
  },
  // 清除画布内容
  clearCanvasAll() {
    const ctx = this.data.ctx;
    const canvas = ctx.canvas;
    ctx.clearRect(0, 0, canvas.width, canvas.height);
  }

})

Wxml文件：

<!-- 最外层的容器 -->
<view class="container">
  <!-- 画布区域 -->
  <canvas type="2d" id="myCanvas" class="canvas-area" disable-scroll="false" bindtouchstart="touchstart" bindtouchmove="touchmove" bindtouchend="touchend"></canvas>
  <!-- 画布工具区域 -->
  <view class='canvas-tools'>
    <!-- 细笔绘制 -->
    <view class='box box1' bindtap='penSelect' data-param='5'></view>
    <!-- 粗笔绘制 -->
    <view class='box box2' bindtap='penSelect' data-param='15'></view>
    <!-- 红色笔 -->
    <view class='box box3' bindtap='colorSelect' data-param='#c03'></view>
    <!-- 黄色笔 -->
    <view class='box box4' bindtap='colorSelect' data-param='#f90'></view>
    <!-- 橡皮 -->
    <view class='box box5' bindtap='clearCanvas'></view>
    <!-- 清除画布 -->
    <view class='box box6' bindtap='clearCanvasAll'></view>
  </view>
</view>

Wxss文件：

.container {
  position: relative;
  width: 100%;
  height: 100%
}

page {
  height: 100%;
}

/* canvas 的宽高 */
.canvas-area {
  width: 100%;
  height: 90%;
  background-color: #ddd;
}

.box {
  width: 100rpx;
  height: 100rpx;
  border-radius: 50%;
  background-color: rebeccapurple;
}

.canvas-tools {
  position: fixed;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 10%;
  display: flex;
  justify-content: space-around;
  align-items: center;
}

.box1 {
  background-color: #9cc;
  background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAgCAYAAAAbifjMAAAABGdBTUEAALGPC/xhBQAAACBjSFJNAAB6JgAAgIQAAPoAAACA6AAAdTAAAOpgAAA6mAAAF3CculE8AAAABmJLR0QA/wD/AP+gvaeTAAAAB3RJTUUH3gUEFTU3owPJ+gAAArpJREFUSMeVlU9IVFEUxn/nvjdTaTQZYyaaZlEpIfhvkVSMSAmBFRZpYYugbcs2QbWpXUFKC3chLdpFUFHQpqxE2rQ0ahEp5BRm6TgqODP3tpg78nzznukHF+7lnfOd73zn8q7gw8jISGFbCzQCLvAGmANIJBKr4l3CcRqIAhVAFngRFKRCqu8ALtkCMeAyEPHFFBN40Am0Ajm7TgDNayrwMEeBi4ADGLu2ARf+24JFE9AR4kmdv40ggnPWAz/2Ad2BCjyMVbZSGHqtqSs5fgVdwIE1CFqBo6sUeKpvAc4TPplCTJ81uEhBM9Ae0KKf8DjQUGjD+7EH2G73WeAV8A54DoySHydAJXDGr6DK5/AjK/Uj8NKSP/EVi3sJOoH9dv8XGALmPQnTwCCQsudGIFEg2Ex+9gVj5m2CH0kgbfdRa3hEAW3AEU9gHDgUQFAPlHnOHUCLIn/v454PJcANOxUFiJV83Y6xgAqg3wVOBlRrB54BY4AGDpP/wfjRrYAHHnO8qLZ99oUkp4EhZd29AyyzfuSAe8B9ZSUOAgN2vx48BO4CWac1vsTS3HSurffKh9mpHzsRaRFRIqIIWko5T8t2H7w6/np4Pjk+itw4tRdgl85lmsqq6ytr2rpuupFNdcba74XOZWeS42O3k5/Hvjpu9AvwTW6dbcAY0yMij0VEKaUcUcoxBrRe3ZExWhutsyLiGGOuiciAW3WsH7RRynGiNbW1amtpKQALi4tMTkz4SRT5W4gx2lHKwS0prwOjcdyIKd9TTywWAyCVSvF7SRWpWFGjNcpxcTPpGYwxOI5L+s9PJLOQH3I6zXJ6Bp3TxWYYMMYSTL4dBmMQEabeRxClVipkMpnQORoDogQ3szAL1nH/TRLCYQAl4IoIxs5srYRABgE3p/OPj5gNpeefLA3up++/CueNwgD8A0fm3rIoh3Y0AAAAJXRFWHRkYXRlOmNyZWF0ZQAyMDE2LTA5LTE3VDE1OjIwOjM4KzA4OjAwDAwhawAAACV0RVh0ZGF0ZTptb2RpZnkAMjAxNC0wNS0wNFQyMTo1Mzo1NSswODowMMTKtgsAAABNdEVYdHNvZnR3YXJlAEltYWdlTWFnaWNrIDcuMC4xLTYgUTE2IHg4Nl82NCAyMDE2LTA5LTE3IGh0dHA6Ly93d3cuaW1hZ2VtYWdpY2sub3Jn3dmlTgAAABh0RVh0VGh1bWI6OkRvY3VtZW50OjpQYWdlcwAxp/+7LwAAABl0RVh0VGh1bWI6OkltYWdlOjpIZWlnaHQAMTA2Mx0uiBoAAAAXdEVYdFRodW1iOjpJbWFnZTo6V2lkdGgANTM3XiCV0QAAABl0RVh0VGh1bWI6Ok1pbWV0eXBlAGltYWdlL3BuZz+yVk4AAAAXdEVYdFRodW1iOjpNVGltZQAxMzk5MjExNjM1VZFQ9AAAABJ0RVh0VGh1bWI6OlNpemUAMTguNEtCh1extQAAAF90RVh0VGh1bWI6OlVSSQBmaWxlOi8vL2hvbWUvd3d3cm9vdC9zaXRlL3d3dy5lYXN5aWNvbi5uZXQvY2RuLWltZy5lYXN5aWNvbi5jbi9zcmMvMTE2MDIvMTE2MDI0NC5wbmcC2t7tAAAAAElFTkSuQmCC);
  background-repeat: no-repeat;
  background-position: center;
}

.box2 {
  background-color: #09c;
  background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAwAAAAeCAMAAAAvtQ9FAAAABGdBTUEAALGPC/xhBQAAACBjSFJNAAB6JgAAgIQAAPoAAACA6AAAdTAAAOpgAAA6mAAAF3CculE8AAACf1BMVEUAAAB8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiN8TiNINiWHUyJ+TyN9TyN9TyN9TyN9TyN+TyOPVyJBMyYoKCgpKChaPyVkQyVjQyVjQyVjQyVjQyVTPCYmJygoKCgfHx8eHh4fHx4gHx4gHx4gHx4gHx4gHx4eHh4eHh4fHx8TExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExITExIHBwcHBwcHBwcHBwd8TiMTExITExITExITExIHBwf3ECwAAAAAznRSTlMAAAAARX4EAAAAAFLkqwIAAAAAVOv/vQkAAAA54v//4CEAAAARvP1qAAAAAGj9/9o1AAAAFMb/2WgXAE70//7BLI///6O+/+fY///84f///dz///HI/9Wl/6hy/f///G056+YxDr7/sggAcv1hAAAg3P/JEwBMTLj//v7//pxKS+j7+//++vvi1f///83V///O1f//ztX//87V///O1f//zSidjOsAAAABYktHRNQJuwuFAAAAB3RJTUUH3gUEFTU21AT5bAAAAS9JREFUGNNjYGBgZGJmYWVjZwABDk4ubh5ePn4QW0BQSFhEVExcAsSRlJKWkZWTV1BUAnKUVVTV1DU0tbR1dBkY9PQNDI2MTUzNzC0sGaysbWzPnbOzd3B0cmZwcXU7d+6cu4enl7cPg6+f/zkQCAgMCmYICQ0Dc86FR0QyREWfg4KYWIa4eBgnIZEhKRnGSUllSEvPgLAzs7IZcnLzIJz8gkKGouISCKe0rJyhorIKzK6uqa1jqG9obAJxmlta2xjaOzq7QBLdPb19DP0TJk6aPHnylKnTps9gmDlr9py58+bNX7Bw0WKGJUuXLV+xcuWq1WvWrmNYv2Hjps1btmzdtn3HToZdu89DwZ69DPv2X4CCAwfxcA4dvggFR44yHDt+CQpOnGQ4dfoyFJw5CwBZdsIQZAP16AAAACV0RVh0ZGF0ZTpjcmVhdGUAMjAxNi0wOS0xN1QxNToyMDozOCswODowMAwMIWsAAAAldEVYdGRhdGU6bW9kaWZ5ADIwMTQtMDUtMDRUMjE6NTM6NTQrMDg6MDBivb2/AAAATXRFWHRzb2Z0d2FyZQBJbWFnZU1hZ2ljayA3LjAuMS02IFExNiB4ODZfNjQgMjAxNi0wOS0xNyBodHRwOi8vd3d3LmltYWdlbWFnaWNrLm9yZ93ZpU4AAAAYdEVYdFRodW1iOjpEb2N1bWVudDo6UGFnZXMAMaf/uy8AAAAZdEVYdFRodW1iOjpJbWFnZTo6SGVpZ2h0ADEwNjMdLogaAAAAF3RFWHRUaHVtYjo6SW1hZ2U6OldpZHRoADQyNaj3r4sAAAAZdEVYdFRodW1iOjpNaW1ldHlwZQBpbWFnZS9wbmc/slZOAAAAF3RFWHRUaHVtYjo6TVRpbWUAMTM5OTIxMTYzNCKWYGIAAAASdEVYdFRodW1iOjpTaXplADEyLjdLQs+hF00AAABfdEVYdFRodW1iOjpVUkkAZmlsZTovLy9ob21lL3d3d3Jvb3Qvc2l0ZS93d3cuZWFzeWljb24ubmV0L2Nkbi1pbWcuZWFzeWljb24uY24vc3JjLzExNjAyLzExNjAyNDMucG5nsPoC/QAAAABJRU5ErkJggg==);
  background-repeat: no-repeat;
  background-position: center;
}

.box3 {
  background-color: #c03;
}

.box4 {
  background-color: #f90;
}

.box5 {
  background-color: #ccc;
  background-image: url(data:image/png;base64,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);
  background-repeat: no-repeat;
  background-position: center;
}

.box6 {
  background-color: #ccc;
}