下载原理：

 首先引入html2Canvas.js 文件， 通过屏幕快照，截图 ，形成base64的图片，在将base64 转化为二进制文本流，最后用js 形成a标签，然后形成下载功能。

downpic () {
      let fulu = document.getElementById("fulu")
      let collapse = fulu.getElementsByClassName("at-collapse-item__wrap")
      for (let i=0; i<collapse.length; i++){
          collapse[i].style.display= 'block'
      }
      let otbeTarget = document.getElementsByClassName('downbutton')[0]
      otbeTarget.style.display = 'none'
      let picdown = document.querySelector('#picdown')
      html2Canvas(picdown).then(canvas => {
        let contentWidth = canvas.width
        console.log(contentWidth)
        let contentHeight = canvas.height
        let pageHeight = contentWidth / 592.28 * 841.89
        let leftHeight = contentHeight
        let position = 20
        // let imgWidth = 595.28
        let imgWidth = 595.28
        let imgHeight = 592.28 / contentWidth * contentHeight
        let pageData = canvas.toDataURL('image/jpeg', 1.0)
        this.downloadFile('附录.png', pageData)
        otbeTarget.style.display = 'block'
        for (let i=0; i<collapse.length; i++){
          collapse[i].style.display= 'none'
        }
      })
    },

    // 下载
    downloadFile (fileName, content) {
      let aLink = document.createElement('a')
      let blob = this.base64ToBlob(content) // new Blob([content]);
      let evt = document.createEvent('HTMLEvents')
      evt.initEvent('click', true, true)
      aLink.download = fileName
      aLink.href = URL.createObjectURL(blob)
      aLink.dispatchEvent(new MouseEvent('click', {bubbles: true, cancelable: true, view: window}))
    },
    base64ToBlob (code) {
      let parts = code.split(';base64,')
      let contentType = parts[0].split(':')[1]
      let raw = window.atob(parts[1])
      let rawLength = raw.length
      let uInt8Array = new Uint8Array(rawLength)
      for (let i = 0; i < rawLength; ++i) {
        uInt8Array[i] = raw.charCodeAt(i)
      }
      return new Blob([uInt8Array], {type: contentType})
    },