canvas = wx.createCanvasContext(canvasID)
// 1. 绘制图片至canvas
canvas.drawImage(imgPath, 0, 0, imgWidth, imgHeight)
// 绘制完成后执行回调，API 1.7.0
canvas.draw(false, () => {
  // 2. 获取图像数据， API 1.9.0
  wx.canvasGetImageData({
    canvasId: canvasID,
    x: 0,
    y: 0,
    width: imgWidth,
    height: imgHeight,
    success(res) {
      // 3. png编码
      let pngData = upng.encode([res.data.buffer], res.width, res.height)
      // 4. base64编码
      let base64 = wx.arrayBufferToBase64(pngData)
      // ...
    }
  })
})

saveSign () {
        return new Promise((resolve, reject) => {
        // 首先把canvas转成临时文件，并获取地址
          wx.canvasToTempFilePath({
            x: 0,
            y: 0,
            width: this.width,
            height: this.height,
            canvasId: 'canvas',
            success: res1 => {
              const fsm = wx.getFileSystemManager()
              // 读取地址，并解码成base64
              fsm.readFile({
                filePath: res1.tempFilePath,
                encoding: 'base64',
                success: res2 => {
                  resolve(res2.data)
                },
                fail: err2 => {
                  reject(err2)
                }
              })
            },
            fail: err1 => {
              reject(err1)
            }
          })
        })
      }