canvas.wxml

<view class="container">
    <view class="canvas_area">
        <canvas
            canvas-id="myCanvas"
            class="myCanvas"
            disable-scroll="{{false}}"
            bindtouchstart="touchStart"
            bindtouchmove="touchMove"
            bindtouchend="touchEnd"
        ></canvas>        
    </view>
    <view class="canvas_tools">
        <!--细笔-->
        <view class="box box1" bindtap="penSelect" data-param="5">细</view>
        <!--粗笔-->
        <view class="box box2" bindtap="penSelect" data-param="15">粗</view>
        <!--颜色-->
        <view class="box box3" bindtap="colorSelect" data-param="#cc0033"></view>
        <view class="box box4" bindtap="colorSelect" data-param="#ff9900"></view>
        <!--导出-->
        <view class="box box5" bindtap="handleOperate">操作</view>
    </view>
</view>

canvas.wxss

page{
    height: 100%;
}

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

.canvas_area {
    width: 100%;
    height: 100%;
    background-color: lightblue;
}

.myCanvas {
    width: 100%;
    height: 100%;
}

.canvas_tools {
    position: absolute;
    left: 0;
    bottom: 20rpx;
    width: 100%;
    height: 100rpx;
    display: flex;
    flex-direction: row;
    justify-content: space-around;
}

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

.box1, .box2, .box5 {
    color: #fff;
    text-align: center;
    line-height: 100rpx;
}

.box3 {
    background-color: #cc0033;
}

.box4 {
    background-color: #ff9900;
}

canvas.js

Page({
  // 定义坐标变量
  startX: 0,
  startY: 0,
  /**
   * 页面的初始数据
   */
  data: {
    pen: 2, // 笔的粗细
    color: "#00ff00", // 笔的颜色
  },
  /**
   * 选取笔的粗细
   */
  penSelect(e) {
    this.setData({
      pen: parseInt(e.currentTarget.dataset.param)
    });
    this.clearCanvas();
  },
  /**
   * 触笔颜色选择
   */
  colorSelect (e) {
    this.setData({
      color: e.currentTarget.dataset.param
    });
  },
  /**
  * 清除画布
  */
  clearCanvas() {
    console.log("............");
    this.context.draw();
  },
  /**
   * 触摸起始事件
   */
  touchStart(e) {
    // 获取当前的坐标位置
    this.startX = e.changedTouches[0].x;
    this.startY = e.changedTouches[0].y;
    // 创建绘图上下文对象
    this.context = wx.createCanvasContext('myCanvas', this);
    // 设置颜色
    this.context.setStrokeStyle(this.data.color);
    // 设置笔触
    this.context.setLineWidth(this.data.pen);
    // 设置笔边（圆角）
    this.context.setLineCap("round");
    // 开始绘制
    this.context.beginPath();
  },
  /**
   * 触摸的移动事件
   */
  touchMove(e) {
    // 获取移动后的新坐标
    let startX1 = e.changedTouches[0].x;
    let startY1 = e.changedTouches[0].y;
    // 设置画笔移动到起始点
    this.context.moveTo(this.startX, this.startY);
    // 绘制一条道x1,y1的直线
    this.context.lineTo(startX1, startY1);
    // 需要进行路径描边
    this.context.stroke();
    // 重新设置坐标点
    this.startX = startX1;
    this.startY = startY1;
    // 绘制
    // this.context.draw();
    wx.drawCanvas({
      canvasId: "myCanvas",
      reserve: true,
      actions: this.context.getActions() // 获取绘图动作数组
    });
  },
  /**
   * touchStart
   */
  touchEnd() {

  },
  /**
   * 导出
   */
  handleOperate() {
    wx.canvasToTempFilePath({
      canvasId: 'myCanvas',
      quality: 0,
      success: (res) => {
        console.log(res);
        // 保存相册
        wx.saveImageToPhotosAlbum({
          filePath: res.tempFilePath,
          success: () => {
            wx.showToast({
              title: '保存成功'
            })
          },
          fail: (err) => {
            // 授权失败
            if (err.errMsg) {
              wx.showModal({
                title: '提示',
                content: '您好,请先授权，在保存此图片!',
                showCancel: false,
                success(res) {
                  if (res.confirm) {
                    wx.openSetting({
                      complete: (settingdata) => {
                        // 授权成功或者失败操作
                        if (settingdata.authSetting['scope.writePhotosAlbum']) {
                          wx.showToast({
                            title: '授权成功'
                          })
                        } else {
                          wx.showToast({
                            title: '授权失败'
                          })
                        }
                      },
                    })
                  }
                }
              })
            }
          }
        });
      }
    })
  }
})

效果图