var that = this;  

        // 绘制背景海报到canvas

        var postersize = this.setCanvasSize(750);//动态设置画布大小

        const ctx = wx.createCanvasContext('shareCanvas')

        ctx.drawImage(that.data.imgUrl, 0, 0, postersize.w, postersize.h)

        console.log("海报图片路径"+that.data.imgUrl);

        ctx.draw();

       //生成二维码

        qrcode({

          height:80,

          width:80,

          canvasId:"myQrcode",       

          text:'{"ticketCode":"111"}',

          colorDark: "black",

          colorLight: "white",

          callback: (res) => {   

            setTimeout(() => {

                wx.canvasToTempFilePath({

                    canvasId: 'myQrcode',

                    x:0,

                    y:0,                  

                    width: 80,

                    height: 80,

                    success: (e) => {

                        that.setData({ qrcodePath: e.tempFilePath });

                        var res = wx.getSystemInfoSync();

                        var scale = 750 / 180;

                        var width = res.windowWidth / scale;

                        var height = width

                        var leftscale = 750 / 480; // 180为left

                        var left = res.windowWidth / leftscale;

                        var topscale = 750 / 880; // 180为top

                        var top = res.windowWidth / topscale;

                        ctx.drawImage(that.data.qrcodePath, left, top, width, height)

                        ctx.draw(that);    

                        //生成二维码

                        setTimeout(() => { 

                            //获取临时缓存code照片路径，存入data中

                            console.log("二维码照片路径"+that.data.qrcodePath);             

                            wx.canvasToTempFilePath({

                              canvasId: 'shareCanvas',

                              success: function(res) {                      

                                  console.log("开始展示图片");                     

                              },

                              fail:function(res){

                                console.log("失败");                    

                              },

                              complete:function(res){

                                console.log("调用接口结束")                   

                              }

                              })                            

                        },0)

                    }              

                })

            }, 0);

          }

        });

<view class="bg">

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

  <image wx:if="{{img}}" src="{{img}}" class="img" bindtap="previewImg"></image>

</view>

<view class="canvas-box">

  <canvas  canvas-id="myQrcode"/>

</view>