jQuery提供了一整套非常好用的遍历Dom树节点的API，下面是看源码的过程，看完了后对API进行分级别，将将要的级别移到插件，或者将其删除，保持jQuery的精简。
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学习重点：
1. 正则表达式深化：
var rparentsprev = /^(?:parents|prevUntil|prevAll)/;
???? ?:在正则表达式中表示：非捕获子模式，即使满足parents|prevUntil|prevAll，也不捕获
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2. 同类方法的抽取
??? parent，parents...，并将他们推入栈中（pushStack），回顾pushStack方法定义在core.js中，方法中通过链接的方式设置了一个preObject属性，为end()方法留下可能。
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(function( jQuery ) {
// ?:在正则表达式中表示：非捕获子模式，即使满足parents|prevUntil|prevAll，也不捕获
// /,/用于多个选择器分隔
//
var runtil = /Until$/,
rparentsprev = /^(?:parents|prevUntil|prevAll)/,
// Note: This RegExp should be improved, or likely pulled from Sizzle
rmultiselector = /,/,
// 以任何字符开始，并且后续字符是非# [ . ,的正则表达式
isSimple = /^.[^:#\[\.,]*$/,
slice = Array.prototype.slice,
POS = jQuery.expr.match.globalPOS,
// methods guaranteed to produce a unique set when starting from a unique set
guaranteedUnique = {
children: true,
contents: true,
next: true,
prev: true
};
jQuery.fn.extend({
find: function( selector ) {
var self = this,
i, l;
//如果是简单的字符串，则交由filter函数，通过contains函数判断是否需要过滤掉集合同的数据（其中contains方法在sizzle中）
if ( typeof selector !== "string" ) {
return jQuery( selector ).filter(function() {
for ( i = 0, l = self.length; i < l; i++ ) {
if ( jQuery.contains( self[ i ], this ) ) {
return true;
}
}
});
}
//将当前对象的所有的selector压入栈中
var ret = this.pushStack( "", "find", selector ),
length, n, r;
for ( i = 0, l = this.length; i < l; i++ ) {
length = ret.length;
// 调用Sizzle进行对象的查找，并将结果保存到ret中，其中Sizzle的定义为：var Sizzle = function( selector, context, results, seed )
jQuery.find( selector, this[i], ret );
// 将重复的搜索对象删除，保证唯一
if ( i > 0 ) {
// Make sure that the results are unique
for ( n = length; n < ret.length; n++ ) {
for ( r = 0; r < length; r++ ) {
if ( ret[r] === ret[n] ) {
ret.splice(n--, 1);
break;
}
}
}
}
}
return ret;
},
// 只要包含target中的任何一个对象，即添加到返回列表，filter出来
has: function( target ) {
var targets = jQuery( target );
return this.filter(function() {
for ( var i = 0, l = targets.length; i < l; i++ ) {
if ( jQuery.contains( this, targets[i] ) ) {
return true;
}
}
});
},
// 将列表推到stack中，方便end()回退
not: function( selector ) {
return this.pushStack( winnow(this, selector, false), "not", selector);
},
// 将列表推到stack中，方便end()回退
filter: function( selector ) {
return this.pushStack( winnow(this, selector, true), "filter", selector );
},
// 判断是否是选择器中选择结果的一员
is: function( selector ) {
return !!selector && (
typeof selector === "string" ?
// If this is a positional selector, check membership in the returned set
// so $("p:first").is("p:last") won't return true for a doc with two "p".
POS.test( selector ) ?
jQuery( selector, this.context ).index( this[0] ) >= 0 :
jQuery.filter( selector, this ).length > 0 :
this.filter( selector ).length > 0 );
},
// TODO：这个API可以删除，最近的元素
closest: function( selectors, context ) {
var ret = [], i, l, cur = this[0];
// Array (deprecated as of jQuery 1.7)
if ( jQuery.isArray( selectors ) ) {
var level = 1;
while ( cur && cur.ownerDocument && cur !== context ) {
for ( i = 0; i < selectors.length; i++ ) {
if ( jQuery( cur ).is( selectors[ i ] ) ) {
ret.push({ selector: selectors[ i ], elem: cur, level: level });
}
}
cur = cur.parentNode;
level++;
}
return ret;
}
// String
var pos = POS.test( selectors ) || typeof selectors !== "string" ?
jQuery( selectors, context || this.context ) :
0;
for ( i = 0, l = this.length; i < l; i++ ) {
cur = this[i];
while ( cur ) {
if ( pos ? pos.index(cur) > -1 : jQuery.find.matchesSelector(cur, selectors) ) {
ret.push( cur );
break;
} else {
cur = cur.parentNode;
if ( !cur || !cur.ownerDocument || cur === context || cur.nodeType === 11 ) {
break;
}
}
}
}
ret = ret.length > 1 ? jQuery.unique( ret ) : ret;
return this.pushStack( ret, "closest", selectors );
},
// Determine the position of an element within
// the matched set of elements
// 获取元素的索引
index: function( elem ) {
// No argument, return index in parent
if ( !elem ) {
return ( this[0] && this[0].parentNode ) ? this.prevAll().length : -1;
}
// index in selector
if ( typeof elem === "string" ) {
return jQuery.inArray( this[0], jQuery( elem ) );
}
// Locate the position of the desired element
return jQuery.inArray(
// If it receives a jQuery object, the first element is used
elem.jquery ? elem[0] : elem, this );
},
// 将元素添加进来
add: function( selector, context ) {
// 对selector进行简单的判断
var set = typeof selector === "string" ?
jQuery( selector, context ) :
jQuery.makeArray( selector && selector.nodeType ? [ selector ] : selector ),
all = jQuery.merge( this.get(), set );
return this.pushStack( isDisconnected( set[0] ) || isDisconnected( all[0] ) ?
all :
jQuery.unique( all ) );
},
andSelf: function() {
return this.add( this.prevObject );
}
});
// A painfully simple check to see if an element is disconnected
// from a document (should be improved, where feasible).
function isDisconnected( node ) {
return !node || !node.parentNode || node.parentNode.nodeType === 11;
}
// 对同类结果组合改变的API进行统一的抽象
jQuery.each({
// 获取单个父结点
parent: function( elem ) {
var parent = elem.parentNode;
return parent && parent.nodeType !== 11 ? parent : null;
},
// 获取所有父结点
parents: function( elem ) {
return jQuery.dir( elem, "parentNode" );
},
parentsUntil: function( elem, i, until ) {
return jQuery.dir( elem, "parentNode", until );
},
next: function( elem ) {
return jQuery.nth( elem, 2, "nextSibling" );
},
prev: function( elem ) {
return jQuery.nth( elem, 2, "previousSibling" );
},
nextAll: function( elem ) {
return jQuery.dir( elem, "nextSibling" );
},
prevAll: function( elem ) {
return jQuery.dir( elem, "previousSibling" );
},
nextUntil: function( elem, i, until ) {
return jQuery.dir( elem, "nextSibling", until );
},
prevUntil: function( elem, i, until ) {
return jQuery.dir( elem, "previousSibling", until );
},
siblings: function( elem ) {
return jQuery.sibling( ( elem.parentNode || {} ).firstChild, elem );
},
children: function( elem ) {
return jQuery.sibling( elem.firstChild );
},
contents: function( elem ) {
return jQuery.nodeName( elem, "iframe" ) ?
elem.contentDocument || elem.contentWindow.document :
jQuery.makeArray( elem.childNodes );
}
}, function( name, fn ) {
jQuery.fn[ name ] = function( until, selector ) {
var ret = jQuery.map( this, fn, until );
if ( !runtil.test( name ) ) {
selector = until;
}
if ( selector && typeof selector === "string" ) {
ret = jQuery.filter( selector, ret );
}
ret = this.length > 1 && !guaranteedUnique[ name ] ? jQuery.unique( ret ) : ret;
if ( (this.length > 1 || rmultiselector.test( selector )) && rparentsprev.test( name ) ) {
ret = ret.reverse();
}
return this.pushStack( ret, name, slice.call( arguments ).join(",") );
};
});
jQuery.extend({
filter: function( expr, elems, not ) {
if ( not ) {
expr = ":not(" + expr + ")";
}
// 完全是通过调用sizzle进行过滤，支持not表示“包含内”或者是“包含外”
return elems.length === 1 ?
jQuery.find.matchesSelector(elems[0], expr) ? [ elems[0] ] : [] :
jQuery.find.matches(expr, elems);
},
// cur属于一个递归的调用过程，不断的将递归的结果放到cur里面，并添加到matched中，这个设计模式用得好
dir: function( elem, dir, until ) {
var matched = [],
cur = elem[ dir ];
while ( cur && cur.nodeType !== 9 && (until === undefined || cur.nodeType !== 1 || !jQuery( cur ).is( until )) ) {
if ( cur.nodeType === 1 ) {
matched.push( cur );
}
cur = cur[dir];
}
return matched;
},
// 获取第几个结点
nth: function( cur, result, dir, elem ) {
result = result || 1;
var num = 0;
for ( ; cur; cur = cur[dir] ) {
if ( cur.nodeType === 1 && ++num === result ) {
break;
}
}
return cur;
},
// 递归获取兄弟结点，返回数组
sibling: function( n, elem ) {
var r = [];
for ( ; n; n = n.nextSibling ) {
if ( n.nodeType === 1 && n !== elem ) {
r.push( n );
}
}
return r;
}
});
// Implement the identical functionality for filter and not
function winnow( elements, qualifier, keep ) {
// Can't pass null or undefined to indexOf in Firefox 4
// Set to 0 to skip string check
qualifier = qualifier || 0;
// grep函数在core.js中，若callback返回true，则elem保留
// 如果qualifier为一个函数，则遍历elements，调用qualifier，若返回true，对象elem保留
if ( jQuery.isFunction( qualifier ) ) {
return jQuery.grep(elements, function( elem, i ) {
var retVal = !!qualifier.call( elem, i, elem );
return retVal === keep;
});
// 如果qualifier为一个dom结点，则直接比较elem比较
} else if ( qualifier.nodeType ) {
return jQuery.grep(elements, function( elem, i ) {
return ( elem === qualifier ) === keep;
});
// 如果qualifier为string字符串，则
} else if ( typeof qualifier === "string" ) {
var filtered = jQuery.grep(elements, function( elem ) {
return elem.nodeType === 1;
});
if ( isSimple.test( qualifier ) ) {
// 如果以任何字符开始，并且后续字符是非# [ . ,选择器，则直接返回使用sizzle过滤的结果，说明是单层级的
return jQuery.filter(qualifier, filtered, !keep);
} else {
// 其实这里的逻辑，加上下面的逻辑不就一样了吗？都是返回一个列表
qualifier = jQuery.filter( qualifier, filtered );
}
}
// 如果elem在qualifier中，则添加到结果中
return jQuery.grep(elements, function( elem, i ) {
return ( jQuery.inArray( elem, qualifier ) >= 0 ) === keep;
});
}
})( jQuery );