基于clamp.js封装vue指令，处理多行文本的溢出

最近做项目时，遇到了一个需求：要求div里文本在两行显示，div的宽度是固定的，如果溢出的话就显示省略号。单行文本的溢出问题，我们都很熟悉，只要添加以下css属性就ok：

  overflow: hidden;
  white-space: nowrap; //段落中文本不换行
  text-overflow: ellipsis;

但是多行文本的溢出怎么处理呢？

查了资料之后发现还是有办法的

在WebKit浏览器或移动端（绝大部分是WebKit内核的浏览器）的页面实现比较简单，可以直接使用WebKit的CSS扩展属性(WebKit是私有属性)-webkit-line-clamp ；注意：这是一个不规范的属性（unsupported WebKit property），它没有出现在 CSS 规范草案中。-webkit-line-clamp用来限制在一个块元素显示的文本的行数。为了实现该效果，它需要组合其他的WebKit属性。
我们用一下代码即可实现：

    overflow : hidden;
    text-overflow: ellipsis;
    display: -webkit-box;
    -webkit-line-clamp: 2;
    -webkit-box-orient: vertical;

但是这样处理会遇到兼容性问题，在Firefox浏览器上就不起作用。
为了解决兼容性问题，有一个clamp.js[https://www.npmjs.com/package...]很好的解决这个问题。

为了更好的跟Vue相结合，今天我们就封装一个v-clamp的指令，来方便的解决这个问题。

// 注册一个全局自定义指令 `v-clamp`
Vue.directive(‘clamp‘, {
  // 当被绑定的元素插入到 DOM 中时……
  update: function (el, binding) {
      function clamp(element, options) {
          options = options || {};
      
          var self = this,
            win = window,
            opt = {
              clamp: options.clamp || 2,
              useNativeClamp: typeof(options.useNativeClamp) != ‘undefined‘ ? options.useNativeClamp : true,
              splitOnChars: options.splitOnChars || [‘.‘, ‘-‘, ‘–‘, ‘—‘, ‘ ‘], //Split on sentences (periods), hypens, en-dashes, em-dashes, and words (spaces).
              animate: options.animate || false,
              truncationChar: options.truncationChar || ‘…‘,
              truncationHTML: options.truncationHTML
            },
      
            sty = element.style,
            originalText = element.innerHTML,
      
            supportsNativeClamp = typeof(element.style.webkitLineClamp) != ‘undefined‘,
            clampValue = opt.clamp,
            isCSSValue = clampValue.indexOf && (clampValue.indexOf(‘px‘) > -1 || clampValue.indexOf(‘em‘) > -1),
            truncationHTMLContainer;
      
          if (opt.truncationHTML) {
            truncationHTMLContainer = document.createElement(‘span‘);
            truncationHTMLContainer.innerHTML = opt.truncationHTML;
          }
      
      
          // UTILITY FUNCTIONS __________________________________________________________
      
          /**
           * Return the current style for an element.
           * @param {HTMLElement} elem The element to compute.
           * @param {string} prop The style property.
           * @returns {number}
           */
          function computeStyle(elem, prop) {
            if (!win.getComputedStyle) {
              win.getComputedStyle = function(el, pseudo) {
                this.el = el;
                this.getPropertyValue = function(prop) {
                  var re = /(\-([a-z]){1})/g;
                  if (prop == ‘float‘) prop = ‘styleFloat‘;
                  if (re.test(prop)) {
                    prop = prop.replace(re, function() {
                      return arguments[2].toUpperCase();
                    });
                  }
                  return el.currentStyle && el.currentStyle[prop] ? el.currentStyle[prop] : null;
                };
                return this;
              };
            }
      
            return win.getComputedStyle(elem, null).getPropertyValue(prop);
          }
      
          /**
           * Returns the maximum number of lines of text that should be rendered based
           * on the current height of the element and the line-height of the text.
           */
          function getMaxLines(height) {
            var availHeight = height || element.clientHeight,
              lineHeight = getLineHeight(element);
      
            return Math.max(Math.floor(availHeight / lineHeight), 0);
          }
      
          /**
           * Returns the maximum height a given element should have based on the line-
           * height of the text and the given clamp value.
           */
          function getMaxHeight(clmp) {
            var lineHeight = getLineHeight(element);
            return lineHeight * clmp;
          }
      
          /**
           * Returns the line-height of an element as an integer.
           */
          function getLineHeight(elem) {
            var lh = computeStyle(elem, ‘line-height‘);
            if (lh == ‘normal‘) {
              // Normal line heights vary from browser to browser. The spec recommends
              // a value between 1.0 and 1.2 of the font size. Using 1.1 to split the diff.
              lh = parseInt(computeStyle(elem, ‘font-size‘)) * 1.2;
            }
            return parseInt(lh);
          }
      
      
          // MEAT AND POTATOES (MMMM, POTATOES...) ______________________________________
          var splitOnChars = opt.splitOnChars.slice(0),
            splitChar = splitOnChars[0],
            chunks,
            lastChunk;
      
          /**
           * Gets an element‘s last child. That may be another node or a node‘s contents.
           */
          function getLastChild(elem) {
            //Current element has children, need to go deeper and get last child as a text node
            if (elem.lastChild.children && elem.lastChild.children.length > 0) {
              return getLastChild(Array.prototype.slice.call(elem.children).pop());
            }
            //This is the absolute last child, a text node, but something‘s wrong with it. Remove it and keep trying
            else if (!elem.lastChild || !elem.lastChild.nodeValue || elem.lastChild.nodeValue === ‘‘ || elem.lastChild.nodeValue == opt.truncationChar) {
              elem.lastChild.parentNode.removeChild(elem.lastChild);
              return getLastChild(element);
            }
            //This is the last child we want, return it
            else {
              return elem.lastChild;
            }
          }
      
          /**
           * Removes one character at a time from the text until its width or
           * height is beneath the passed-in max param.
           */
          function truncate(target, maxHeight) {
            if (!maxHeight) {
              return;
            }
      
            /**
             * Resets global variables.
             */
            function reset() {
              splitOnChars = opt.splitOnChars.slice(0);
              splitChar = splitOnChars[0];
              chunks = null;
              lastChunk = null;
            }
      
            var nodeValue = target.nodeValue.replace(opt.truncationChar, ‘‘);
      
            //Grab the next chunks
            if (!chunks) {
              //If there are more characters to try, grab the next one
              if (splitOnChars.length > 0) {
                splitChar = splitOnChars.shift();
              }
              //No characters to chunk by. Go character-by-character
              else {
                splitChar = ‘‘;
              }
      
              chunks = nodeValue.split(splitChar);
            }
      
            //If there are chunks left to remove, remove the last one and see if
            // the nodeValue fits.
            if (chunks.length > 1) {
              // console.log(‘chunks‘, chunks);
              lastChunk = chunks.pop();
              // console.log(‘lastChunk‘, lastChunk);
              applyEllipsis(target, chunks.join(splitChar));
            }
            //No more chunks can be removed using this character
            else {
              chunks = null;
            }
      
            //Insert the custom HTML before the truncation character
            if (truncationHTMLContainer) {
              target.nodeValue = target.nodeValue.replace(opt.truncationChar, ‘‘);
              element.innerHTML = target.nodeValue + ‘ ‘ + truncationHTMLContainer.innerHTML + opt.truncationChar;
            }
      
            //Search produced valid chunks
            if (chunks) {
              //It fits
              if (element.clientHeight <= maxHeight) {
                //There‘s still more characters to try splitting on, not quite done yet
                if (splitOnChars.length >= 0 && splitChar !== ‘‘) {
                  applyEllipsis(target, chunks.join(splitChar) + splitChar + lastChunk);
                  chunks = null;
                }
                //Finished!
                else {
                  return element.innerHTML;
                }
              }
            }
            //No valid chunks produced
            else {
              //No valid chunks even when splitting by letter, time to move
              //on to the next node
              if (splitChar === ‘‘) {
                applyEllipsis(target, ‘‘);
                target = getLastChild(element);
      
                reset();
              }
            }
      
            //If you get here it means still too big, let‘s keep truncating
            if (opt.animate) {
              setTimeout(function() {
                truncate(target, maxHeight);
              }, opt.animate === true ? 10 : opt.animate);
            } else {
              return truncate(target, maxHeight);
            }
          }
      
          function applyEllipsis(elem, str) {
            elem.nodeValue = str + opt.truncationChar;
          }
      
      
          // CONSTRUCTOR ________________________________________________________________
      
          if (clampValue == ‘auto‘) {
            clampValue = getMaxLines();
          } else if (isCSSValue) {
            clampValue = getMaxLines(parseInt(clampValue));
          }
      
          var clampedText;
          if (supportsNativeClamp && opt.useNativeClamp) {
            sty.overflow = ‘hidden‘;
            sty.textOverflow = ‘ellipsis‘;
            sty.webkitBoxOrient = ‘vertical‘;
            sty.display = ‘-webkit-box‘;
            sty.webkitLineClamp = clampValue;
      
            if (isCSSValue) {
              sty.height = opt.clamp + ‘px‘;
            }
          } else {
            var height = getMaxHeight(clampValue);
            if (height <= element.clientHeight) {
              console.log(getLastChild(element));
              clampedText = truncate(getLastChild(element), height);
            }
          }
      
          return {
            ‘original‘: originalText,
            ‘clamped‘: clampedText
          };
        }

       clamp(el,{clamp: 2}) 


  }
})

其实很简单，仅仅是把clamp.js中的函数搬移了过来。然后就可以像这样来使用：

  <div class="txt" v-clamp>很抱歉！没有搜索到相关模板很抱歉！没有搜索到相关模板很抱歉！没有搜索到相关模板很抱歉！没有搜索到相关模板</div>