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) {

   }
  });
  }
 })
 },
 
})