之前用了几种方式

1.后端生成二维码需要加密的字符窜，小程序前端利用二维码组件渲染canvas画出二维码，由于支付宝小程序没有这样的组件，于是去找微信小程序的解决方案，把微信小程序的二维码前端组件搬过来用，调试到不报错了，结果二维码显示不出来，原因很难找，宣告失败！

2.后端生成图片，并把图片的地址URL发到小程序前端，前端调用my.downloadFile 成功，但后端却要生成大量图片，并且二维码都是临时拿来用的，用完就扔，缺点太明显，后端要产生大量垃圾文件和IO操作，使用码的客户端多了，对服务器端有很大压力

3.后端生成图片的Base64字符串，小程序前端直接显示二维码

我最后使用的是第三种方式

要生成二维码，首先是使用.net core 版本的QRCode，然后我们的需求是二维码中间加入一个logo，下面是一个工具类，可实现这个需求

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.IO;
using QRCoder;namespace SchoolWebApi.Utility
{public class RaffQRCode : IQRCode{public byte[] StreamToBytes(Stream stream){byte[] bytes = new byte[stream.Length];stream.Position = 0;stream.Read(bytes, 0, bytes.Length);stream.Close();return bytes;}/// <summary>/// 生成二维码图片/// </summary>/// <param name="logoPath"></param>/// <param name="content"></param>/// <param name="pixel">像素大小</param>/// <returns></returns>public byte[] GetQRCode(string logoPath, string content, int pixel){var generator = new QRCodeGenerator();var codeData = generator.CreateQrCode(content, QRCodeGenerator.ECCLevel.M, true);var qrcode = new QRCode(codeData);var qrImage = qrcode.GetGraphic(pixel, Color.Black, Color.White, true);using (MemoryStream ms = new MemoryStream()){qrImage.Save(ms, ImageFormat.Png);using (MemoryStream ms1 = new MemoryStream()){//把logo图片放到二维码图片正中心var newImage = CombinImage(qrImage, logoPath);newImage.Save(ms1, ImageFormat.Png);return StreamToBytes(ms1);}            }          }/// <summary>   /// 调用此函数后使此两种图片合并，类似相册，有个   /// 背景图，中间贴自己的目标图片   /// </summary>   /// <param name="imgBack">粘贴的源图片</param>   /// <param name="destImg">粘贴的目标图片</param>   public static Image CombinImage(Image imgBack, string destImg){Image img = Image.FromFile(destImg);        //照片图片     if (img.Height != 65 || img.Width != 65){img = KiResizeImage(img, 65, 65, 0);}Graphics g = Graphics.FromImage(imgBack);g.DrawImage(imgBack, 0, 0, imgBack.Width, imgBack.Height);      //g.DrawImage(imgBack, 0, 0, 相框宽, 相框高);    //g.FillRectangle(System.Drawing.Brushes.White, imgBack.Width / 2 - img.Width / 2 - 1, imgBack.Width / 2 - img.Width / 2 - 1,1,1);//相片四周刷一层黑色边框   //g.DrawImage(img, 照片与相框的左边距, 照片与相框的上边距, 照片宽, 照片高);   g.DrawImage(img, imgBack.Width / 2 - img.Width / 2, imgBack.Width / 2 - img.Width / 2, img.Width, img.Height);GC.Collect();return imgBack;}/// <summary>   /// Resize图片   /// </summary>   /// <param name="bmp">原始Bitmap</param>   /// <param name="newW">新的宽度</param>   /// <param name="newH">新的高度</param>   /// <param name="Mode">保留着，暂时未用</param>   /// <returns>处理以后的图片</returns>   public static Image KiResizeImage(Image bmp, int newW, int newH, int Mode){try{Image b = new Bitmap(newW, newH);Graphics g = Graphics.FromImage(b);// 插值算法的质量   g.InterpolationMode = InterpolationMode.HighQualityBicubic;g.DrawImage(bmp, new Rectangle(0, 0, newW, newH), new Rectangle(0, 0, bmp.Width, bmp.Height), GraphicsUnit.Pixel);g.Dispose();return b;}catch{return null;}}}
}

 上面自定义了一个接口 IQRCode

    public interface IQRCode{byte[] GetQRCode(string logoPath,string url, int pixel);}

方便在StartUp中注册

        // This method gets called by the runtime. Use this method to add services to the container.public void ConfigureServices(IServiceCollection services){services.AddSingleton<IQRCode, RaffQRCode>();......

 然后可以在Controller的构造函数中注入了

        private IQRCode _iQRCode;public GenerateQRCodeController(IQRCode iQRCode){_iQRCode = iQRCode;}

 接着写出返回图片Base64字符串的方法，并在支付宝客户端调用

        /// <summary>/// 返回二维码图片的base64字符窜/// </summary>/// <param name="ali_user_id"></param>/// <returns></returns>[HttpGet][Route("GetGRCodeGenrateImage")]public ActionResult GetGRCodeGenrateImage(string ali_user_id){try{//根据支付宝账号查询用户表var findUser = _tb_school_user.FindByClause(t => t.ali_user_id == ali_user_id);if (findUser != null){var qrCodeFullStr = GetQrCodeString(ali_user_id, findUser);var logoPath = host.WebRootPath + @"\images\logo.jpg";var byteData = _iQRCode.GetQRCode(logoPath, qrCodeFullStr, 4);//创建一个文件流//using (FileStream fs = new FileStream(@"c:\666666666666.png", FileMode.Create))//{//    //将byte数组写入文件中//    fs.Write(byteData, 0, byteData.Length);//}//处理返回提前台image图片//FileContentResult img = new FileContentResult(byteData, "images/png");//return img;return Content(Convert.ToBase64String(byteData));}return Content(string.Empty);}catch (Exception ex){log.Error("错误:" + ex);return Content("图片生成出错");}}

 支付宝小程序前端的代码

const app = getApp();Page({data: {src: '',username: '',studentid: '',QRCodeSrc: '',setInter:'',},GetQRCode : function(){var that = this;my.httpRequest({url: app.globalData.apiurl + '/api/GenerateQRCode/GetGRCodeGenrateImage',method: 'GET',dataType : 'text',data: {ali_user_id: app.globalData.aliuserid },header: {'content-type': 'application/octet-stream',},success: function(res) {var data = res.data                if (res.status == 200) {that.setData ({QRCodeSrc: 'data:image/png;base64,' + data,  //data 为接口返回的base64字符串})}},fail: function(res) {console.log(JSON.stringify(res));my.showToast({type: 'fail',content: '获取二维码失败',duration: 1000});},complete: function(res) {my.hideLoading();}})},//定时刷新二维码refreshQRCode : function(){var that = this;//每分钟刷新that.data.setInter = setInterval(function(){that.GetQRCode();},60000);},imageError: function (e) {console.log('image 发生错误', e.detail.errMsg)},imageLoad: function (e) {console.log('image 加载成功', e);},clearTimeInterval: function (that) {var interval = that.data.interval;clearInterval(interval)},onLoad(query) {var that = this// 页面加载app.getUserInfo().then(user => {console.info(user);//设置头像if (user.avatar.length > 0) {this.setData({src: user.avatar});}else {this.setData({src: '/images/tou.png'});} //设置用户名    if (app.globalData.username) {this.setData({username: app.globalData.username});}else {this.setData({username: user.nickName});}//设置StudentIdif(app.globalData.studentid) {               this.setData({studentid: app.globalData.studentid}); }//页面加载获取二维码this.GetQRCode();//开启定时刷新获取二维码this.refreshQRCode();});},onShow() {// 页面显示   my.onUserCaptureScreen(function() {my.alert({content: '收到用户截屏事件'});});},onReady() {    },onUnload: function () {var that = this;//清除计时器  即清除setInterclearInterval(that.data.setInter)},
});

 重点看GetQRCode 这个方法就可以了，如何在前端接收Base64格式并显示图片