textarea自动撑大

autoTextarea

/**
    * 文本框根据输入内容自适应高度
    * @param                {HTMLElement}        输入框元素
    * @param                {Number}                设置光标与输入框保持的距离(默认0)
    * @param                {Number}                设置最大高度(可选)
    */
    autoTextarea (elem, extra, maxHeight) {
      let _this = this
      let fontSize = this.get_fontsize()
      extra = extra || 0;
      var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
      isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
      addEvent = function (type, callback) {
        elem.addEventListener ?
          elem.addEventListener(type, callback, false) :
          elem.attachEvent('on' + type, callback);
      },
      getStyle = elem.currentStyle ? function (name) {
        var val = elem.currentStyle[name]
        if (name === 'height' && val.search(/px/i) !== 1) {
          var rect = elem.getBoundingClientRect();
          return rect.bottom - rect.top -
            parseFloat(getStyle('paddingTop')) -
            parseFloat(getStyle('paddingBottom')) + 'px'       
        }
        return val
      } : function (name) {
        return getComputedStyle(elem, null)[name]
      },
      minHeight = parseFloat(getStyle('height'))
      elem.style.resize = 'none'

      var change = function () {
        var scrollTop, 
        height,
        padding = 0,
        style = elem.style
        if (elem._length === elem.value.length) return
        elem._length = elem.value.length
        if (!isFirefox && !isOpera) {
          padding = Math.ceil(parseFloat(getStyle('paddingTop'))) + Math.ceil(parseFloat(getStyle('paddingBottom')))
        }
        scrollTop = document.body.scrollTop || document.documentElement.scrollTop
        elem.style.height = minHeight + 'px'
        _this.$refs.input.style.height = minHeight + padding + 'px'
        if (elem.scrollHeight > minHeight) {
          if (maxHeight && elem.scrollHeight > maxHeight) {
            height = maxHeight - padding
            style.overflowY = 'auto'
          } else {
            height = elem.scrollHeight - padding + ((14 / 37.5) * fontSize)   //element.scrollHight 一次滚动就滚动10 这里加上14就等于 字的行高
            style.overflowY = 'hidden'
          };
          style.height = height + extra + 'px'
          _this.$refs.input.style.height = height + extra + padding + 'px'
          scrollTop += parseInt(style.height) - (elem.currHeight || 0)
          if (scrollTop) {
            document.documentElement.scrollTop = scrollTop + 'px'
            elem.currHeight = parseInt(style.height)
          }
        }
      }
      addEvent('propertychange', change)
      addEvent('input', change)
      addEvent('focus', change)
      change()
      _this.change = change
    }