jQuery选择器源码解读（八）：addCombinator函数

function addCombinator(matcher, binator, base) {
 var dir = binator.dir, checkNonElements = base
   && dir === "parentNode", doneName = done++;

 return binator.first ?
 // Check against closest ancestor/preceding element
 function(elem, context, xml) {
  while ((elem = elem[dir])) {
   if (elem.nodeType === 1 || checkNonElements) {
    return matcher(elem, context, xml);
   }
  }
 } :

 // Check against all ancestor/preceding elements
 function(elem, context, xml) {
  var data, cache, outerCache, dirkey = dirruns + " " + doneName;

  // We can't set arbitrary data on XML nodes, so they don't
  // benefit from dir caching
  if (xml) {
   while ((elem = elem[dir])) {
    if (elem.nodeType === 1 || checkNonElements) {
     if (matcher(elem, context, xml)) {
      return true;
     }
    }
   }
  } else {
   while ((elem = elem[dir])) {
    if (elem.nodeType === 1 || checkNonElements) {
     outerCache = elem[expando] || (elem[expando] = {});
     if ((cache = outerCache[dir])
       && cache[0] === dirkey) {
      if ((data = cache[1]) === true
        || data === cachedruns) {
       return data === true;
      }
     } else {
      cache = outerCache[dir] = [ dirkey ];
      cache[1] = matcher(elem, context, xml)
        || cachedruns;
      if (cache[1] === true) {
       return true;
      }
     }
    }
   }
  }
 };
}

Expr.relative : {
 ">" : {
  dir : "parentNode",
  first : true
 },
 " " : {
  dir : "parentNode"
 },
 "+" : {
  dir : "previousSibling",
  first : true
 },
 "~" : {
  dir : "previousSibling"
 }
}

function(elem, context, xml) {
 while ((elem = elem[dir])) {
  if (elem.nodeType === 1 || checkNonElements) {
   return matcher(elem, context, xml);
  }
 }
}

//Check against all ancestor/preceding elements
function(elem, context, xml) {
 var data, cache, outerCache, dirkey = dirruns + " " + doneName;

 // We can't set arbitrary data on XML nodes, so they don't
 // benefit from dir caching
 if (xml) {
  while ((elem = elem[dir])) {
   if (elem.nodeType === 1 || checkNonElements) {
    if (matcher(elem, context, xml)) {
     return true;
    }
   }
  }
 } else {
  while ((elem = elem[dir])) {
   if (elem.nodeType === 1 || checkNonElements) {
    outerCache = elem[expando] || (elem[expando] = {});
    if ((cache = outerCache[dir])
      && cache[0] === dirkey) {
     if ((data = cache[1]) === true
       || data === cachedruns) {
      return data === true;
     }
    } else {
     cache = outerCache[dir] = [ dirkey ];
     cache[1] = matcher(elem, context, xml)
       || cachedruns;
     if (cache[1] === true) {
      return true;
     }
    }
   }
  }
 }
};

while ((elem = elem[dir])) {
 if (elem.nodeType === 1 || checkNonElements) {
  // 若elem节点的expando属性不存在，则赋予空对象，并同时赋予outerCache
  // 若elem节点的expando属性存在，则将其值赋予outerCache
  outerCache = elem[expando] || (elem[expando] = {});
  /*
   * 若outCache[dir]有值，且其第一个元素等于当前的dirkey，
   *     则说明当前位置选择器在本次执行过程中已检测过该节点，执行if内的语句，从缓存中直接获取结果
   * 若outCache[dir]不存在，或第一个元素不等于当前的dirkey，
   *     则说明当前位置选择器在本次执行过程中还未检测过该节点，执行else内的语句，匹配节点并将结果放入缓存
   */
  if ((cache = outerCache[dir])
    && cache[0] === dirkey) {
   // 若缓存中检测结果等于true或cachedruns的值，则返回检测结果（非true皆为false），
   // 否则继续循环获取上一个符合位置关系的节点进行匹配
   if ((data = cache[1]) === true
     || data === cachedruns) {
    return data === true;
   }
  } else {
   // 将数组[ dirkey ]赋予outerCache[dir]及cache
   cache = outerCache[dir] = [ dirkey ];
   // 将匹配成功，将true赋予cache[1]，否则将cachedruns的值赋予cache[1]
   cache[1] = matcher(elem, context, xml)
     || cachedruns;
   // 若匹配结果为true，则返回true，否则继续循环获取上一个符合位置关系的节点进行匹配
   if (cache[1] === true) {
    return true;
   }
  }
 }
}