sizzle源码分析 （2）ID 类 tag querySelectorAll 快速匹配

不是所有的选择器都需要去分词，生成相应的匹配函数，这样流程比较复杂，当浏览器具备原生的方法去匹配元素是，没有理由不优先匹配，下面看看进入Sizzle后，它是怎么优先匹配这些元素的：

 

function Sizzle( selector, context, results, seed ) {
    /*
    执行$("ul.list>li span:eq(1)")时，递归第二次时，
    selector
    "ul.list>li span"

    */
    var match, elem, m, nodeType,
        // QSA vars
        i, groups, old, nid, newContext, newSelector;

    if ( ( context ? context.ownerDocument || context : preferredDoc ) !== document ) {
        setDocument( context );
    }

    context = context || document;
    results = results || [];

    if ( !selector || typeof selector !== "string" ) {
        return results;
    }

    if ( (nodeType = context.nodeType) !== 1 && nodeType !== 9 ) {
        return [];
    }

    if ( documentIsHTML && !seed ) {
        // Shortcuts
        if ( (match = rquickExpr.exec( selector )) ) {//先看是不是简单的ID选择器 TAG选择器 类选择器
            // Speed-up: Sizzle("#ID")
            if ( (m = match[1]) ) {
                if ( nodeType === 9 ) {
                    elem = context.getElementById( m );
                    // Check parentNode to catch when Blackberry 4.6 returns
                    // nodes that are no longer in the document #6963
                    if ( elem && elem.parentNode ) {
                        // Handle the case where IE, Opera, and Webkit return items
                        // by name instead of ID
                        if ( elem.id === m ) {
                            results.push( elem );
                            return results;
                        }
                    } else {
                        return results;
                    }
                } else {
                    // Context is not a document
                    if ( context.ownerDocument && (elem = context.ownerDocument.getElementById( m )) &&
                        contains( context, elem ) && elem.id === m ) {
                        results.push( elem );
                        return results;
                    }
                }

            // Speed-up: Sizzle("TAG")
            } else if ( match[2] ) {
                push.apply( results, context.getElementsByTagName( selector ) );
                return results;

            // Speed-up: Sizzle(".CLASS")
            } else if ( (m = match[3]) && support.getElementsByClassName && context.getElementsByClassName ) {
                push.apply( results, context.getElementsByClassName( m ) );
                return results;
            }
        }

        // QSA path 是否支持document.querySelectorAll
        if ( support.qsa && (!rbuggyQSA || !rbuggyQSA.test( selector )) ) {
            nid = old = expando;
            newContext = context;
            newSelector = nodeType === 9 && selector;

            // qSA works strangely on Element-rooted queries
            // We can work around this by specifying an extra ID on the root
            // and working up from there (Thanks to Andrew Dupont for the technique)
            // IE 8 doesn't work on object elements
            if ( nodeType === 1 && context.nodeName.toLowerCase() !== "object" ) {
                groups = tokenize( selector );

                if ( (old = context.getAttribute("id")) ) {
                    nid = old.replace( rescape, "\$&" );
                } else {
                    context.setAttribute( "id", nid );
                }
                nid = "[id='" + nid + "'] ";

                i = groups.length;
                while ( i-- ) {
                    groups[i] = nid + toSelector( groups[i] );
                }
                newContext = rsibling.test( selector ) && context.parentNode || context;
                newSelector = groups.join(",");
            }

            if ( newSelector ) {
                try {
                    push.apply( results,
                        newContext.querySelectorAll( newSelector )
                    );
                    return results;
                } catch(qsaError) {
                } finally {
                    if ( !old ) {
                        context.removeAttribute("id");
                    }
                }
            }
        }
    }

    // All others
    //去除选择器首位空格，并进入select函数
    return select( selector.replace( rtrim, "$1" ), context, results, seed );
}

先看：rquickExpr

rquickExpr = /^(?:#([w-]+)|(w+)|.([w-]+))$/,

当执行：match = rquickExpr.exec( selector )，产生一个长度为4的数组：其中match[1]ID匹配项，match[2]为tag匹配项，match[3]为类匹配项
比如：当执行$("div")时，
match=["div", undefined, "div", undefined]
接下来就判断具体符合哪种选择器了，然后用原生方法直接获得。

如果不是上述三种选择器，那么开始试探querySelectorAll：

// QSA path 是否支持document.querySelectorAll
        if ( support.qsa && (!rbuggyQSA || !rbuggyQSA.test( selector )) ) {

然后就就开始try尝试：

if ( newSelector ) {
                try {
                    push.apply( results,
                        newContext.querySelectorAll( newSelector )
                    );
                    return results;
                } catch(qsaError) {
                } finally {
                    if ( !old ) {
                        context.removeAttribute("id");
                    }
                }
            }

如果成功就返回results