摘要：最近做項目時，遇到了一個需求要求里文本在兩行顯示，的寬度是固定的，如果溢出的話就顯示省略號。用來限制在一個塊元素顯示的文本的行數。為了實現該效果，它需要組合其他的屬性。為了更好的跟相結合，今天我們就封裝一個的指令，來方便的解決這個問題。

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

查了資料之后發現還是有辦法的

在WebKit瀏覽器或移動端（絕大部分是WebKit內核的瀏覽器）的頁面實現比較簡單，可以直接使用WebKit的CSS擴展屬性(WebKit是私有屬性)-webkit-line-clamp ；注意：這是一個 不規范的屬性（unsupported WebKit property），它沒有出現在 CSS 規范草案中。-webkit-line-clamp用來限制在一個塊元素顯示的文本的行數。 為了實現該效果，它需要組合其他的WebKit屬性。

我們用一下代碼即可實現：

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

但是這樣處理會遇到兼容性問題，在Firefox瀏覽器上就不起作用。

為了解決兼容性問題，有一個clamp.js[https://www.npmjs.com/package...]很好的解決這個問題。

為了更好的跟Vue相結合，今天我們就封裝一個v-clamp的指令，來方便的解決這個問題。

// 注冊一個全局自定義指令 `v-clamp`
Vue.directive("clamp", {
  // 當被綁定的元素插入到 DOM 中時……
  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}) 


  }
})

其實很簡單，僅僅是把clamp.js中的函數搬移了過來。然后就可以像這樣來使用：

  
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