微信小程序canvas当前画布指定区域的内容导出生成指定大小的图

详情：https://developers.weixin.qq.com/miniprogram/dev/api/canvas/wx.canvasToTempFilePath.html

index.wxml

<canvas class="canvas" canvas-id="myQrcode"></canvas>

index.js

saveImg: function() {
    wx.showModal({
      title: '提示',
      content: '确定保存图片到相册吗？',
      success(res) {
        if (res.confirm) {
          wx.canvasToTempFilePath({
            canvasId: 'myQrcode',
            success(res) {
              wx.saveImageToPhotosAlbum({
                filePath: res.tempFilePath,
                success(res) {
                  wx.showToast({
                    title: '保存成功',
                    icon: 'success'
                  })
                },
                fail() {
                  wx.showModal({
                    title: '警告',
                    content: '若不打开授权，则无法将图片保存在相册中！',
                    showCancel: false,
                    success() {
                      wx.openSetting({
          
                      })
                    }
                  })
                }
              })
            },
            
          })
        }else if (res.cancel) {
  
        }
      }
    })
  }