微信小程序wx.createCanvasContext停止维护了，文档又说的不是太清楚，研究并记录下来如何生成实例的。

并且推荐另一种画海报的方式：微信小程序生成海报_工具人小胡的博客-CSDN博客_小程序海报

<view>
<text bindtap="up_img" >上传图片</text>
<view style="top:-6000px; position: absolute;">
<canvas type="2d" style="width:5000px;height:5000px" id="shareBox" ></canvas>
</view>
</view>

Page({
data:{}
    init(res) {
        const canvas = res[0].node
        const ctx = canvas.getContext('2d')
        const dpr = wx.getSystemInfoSync().pixelRatio
        canvas.width = res[0].width * dpr
        canvas.height = res[0].height * dpr
        this.setData({
          canvas,
          ctx
        });
        ctx.scale(dpr, dpr);

      },
      getimg(img_file) { //压缩图片
        return new Promise((resolve, reject) => {
          wx.getImageInfo({
            src: img_file,
            success: succ => {
              let ratio = succ.height / succ.width
              if (this.data.originHeight == 0) {
                this.data.originHeight = succ.height * 0.7;
                this.data.originWidth = succ.height * ratio;
              } else {
                this.data.originHeight = this.data.originHeight * 0.7
                this.data.originWidth = this.data.originHeight * ratio
              }
              let Img = this.data.canvas.createImage(); //创建img对象
              Img.src = img_file
              Img.onload = () => {

                this.data.ctx.drawImage(Img, 0, 0, Math.ceil(this.data.originWidth), Math.ceil(this.data.originHeight));

                wx.canvasToTempFilePath({ //将canvas生成图片
                  canvas: this.data.canvas,
                  x: 0,
                  y: 0,
                  width: this.data.originWidth,
                  height: this.data.originHeight,
                  destWidth: this.data.originWidth,
                  destHeight: this.data.originHeight,
                  success: function (res) {

                    wx.getFileInfo({
                      filePath: res.tempFilePath,
                      success(ree) {
                        if (ree.size > 1000000) {
                          reject();
                        } else {
                          wx.hideLoading();
                          resolve(res.tempFilePath)
                        }
                        return
                      }
                    })

                    return
                  },
                }, this)
              }



            }
          })


        })
      },
      up_img(e) {
        wx.chooseImage({
          sizeType: ['compressed'],
          sourceType: ['album', 'camera'],
          success: res => {
            if (res.tempFiles[0].size > 1000000) {
              wx.showLoading({
                title: '上传中',
              })
              let getImg = () => {
                this.getimg(res.tempFilePaths[0]).catch(err => {
                  getImg()
                }).then(res => {
                  if (res) {
                    this.get_push_img(res)
                  }

                })
              }
            }else{
              
              wx.showLoading({
                title: '上传中',
              })
            this.get_push_img(res.tempFilePaths[0])
            }
          }
        })
          
        },
          get_push_img(file) {
            return new Promise((resolve, reject) => {

              wx.uploadFile({
                url: `/files/upload/up`, //仅为示例，非真实的接口地址
                filePath: file,
                name: 'file',
                formData: {
                  token: this.data.token_obj.token,
                  article: 1
                },
                success(res) {

                  let data = JSON.parse(res.data)
                  if (data.code == 200) {
                    $Message({
                      content: '上传成功',
                      type: 'success'
                    });
                    resolve(data.result[0].temp_path)
                  } else {
                    reject();
                  }
                },
                fail: function (err) {
                  console.log(err)
                }
              })

            })
          },
          onl oad: function (options) {
            const query = wx.createSelectorQuery()
            query.select('#shareBox')
              .fields({
                id: true,
                node: true,
                size: true
              })
              .exec(this.init.bind(this));
          }

})