微信小程序实现张图片合成为一张并下载

Page({ /** * 页面的初始数据 */ data: { imgUrl: "项目中图片地址", //图片链接 img: '' // 合成后图片路径 }, /** * 生命周期函数--监听页面加载 */ onLoad: function (options) { let that = this; wx.downloadFile({  url: '线上图片地址',  success(res) {  // 绘制背景海报到canvas  var postersize = that.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)  var re = wx.getSystemInfoSync();  var scale = 750 / 180;  var width = re.windowWidth / scale;  var height = width  var leftscale = 750 / 480; // 180为left  var left = re.windowWidth / leftscale;  var topscale = 750 / 880; // 180为top  var top = re.windowWidth / topscale;  ctx.drawImage(res.tempFilePath, left, top, width, height)  ctx.draw()  setTimeout(() => {   // code_url = this.canvasToTempImage();    //获取临时缓存合成照片路径，存入data中   wx.canvasToTempFilePath({   canvasId: 'shareCanvas',   success: function (res) {    var tempFilePath = res.tempFilePath;    that.setData({    img: tempFilePath    })    console.log(tempFilePath)   },   fail: function (res) {    console.log(res);   }   });  }, 1000);  } })  }, /** * 生命周期函数--监听页面初次渲染完成 */ onReady: function () {  }, /** * 生命周期函数--监听页面显示 */ onShow: function () {  }, /** * 生命周期函数--监听页面隐藏 */ onHide: function () {  }, /** * 生命周期函数--监听页面卸载 */ onUnload: function () {  }, /** * 页面相关事件处理函数--监听用户下拉动作 */ onPullDownRefresh: function () {  }, /** * 页面上拉触底事件的处理函数 */ onReachBottom: function () {  }, /** * 用户点击右上角分享 */ onShareAppMessage: function () {  }, //适配不同屏幕大小的canvas setCanvasSize: function (width) { var size = {}; try {  var res = wx.getSystemInfoSync();  var scale = 750 / width;//不同屏幕下canvas的适配比例；设计稿是750宽  // var scale = 1  var width = res.windowWidth / scale;  var height = res.windowHeight / scale;;  size.w = width;  size.h = height; } catch (e) {  // Do something when catch error  console.log("获取设备信息失败" + e); } return size; }, //点击图片进行预览，长按保存分享图片 previewImg: function (e) { var img = this.data.img; let _this = this; //保存二维码到相册 wx.saveImageToPhotosAlbum({  filePath: img,  success: function (res) {  wx.showModal({   content: '保存成功',   confirmText: '确认',   showCancel: false,   success: function (res) {   }  });  },  fail: function (res) {  wx.showModal({   content: '保存失败',   confirmText: '确认',   showCancel: false,   success: function (res) {   }  });  } }) }, })