【jQuery源码】preFilter

  1 preFilter: {
  2         "ATTR": function( match ) {
  3             //属性名解码
  4             match[1] = match[1].replace( runescape, funescape );
  5
  6             // Move the given value to match[3] whether quoted or unquoted
  7             //属性解码
  8             match[3] = ( match[3] || match[4] || match[5] || "" ).replace( runescape, funescape );
  9
 10             //若判断符为~=，则在属性值两边加上空格
 11             if ( match[2] === "~=" ) {
 12                 match[3] = " " + match[3] + " ";
 13             }
 14
 15             //返回前4个元素
 16             return match.slice( 0, 4 );
 17         },
 18
 19         "CHILD": function( match ) {
 20             /* matches from matchExpr["CHILD"]
 21                 1 type (only|nth|...)
 22                 2 what (child|of-type)
 23                 3 argument (even|odd|\d*|\d*n([+-]\d+)?|...)
 24                 4 xn-component of xn+y argument ([+-]?\d*n|)
 25                 5 sign of xn-component
 26                 6 x of xn-component
 27                 7 sign of y-component
 28                 8 y of y-component
 29             */
 30             //match[1]：(only|first|last|nth|nth-last)
 31             //match[2]: (child|last-child|of-type|last-of-type)
 32             //match[3]: (even|odd)
 33             //match[4]、match[5]: an+b中的a,b
 34
 35             //match[1]变成小写
 36             match[1] = match[1].toLowerCase();
 37
 38             //如果match[1]是"nth"
 39             if ( match[1].slice( 0, 3 ) === "nth" ) {
 40                 // nth-* requires argument
 41                 //若选择器括号内没有有效参数，则抛出异常
 42                 if ( !match[3] ) {
 43                     Sizzle.error( match[0] );
 44                 }
 45
 46                 // numeric x and y parameters for Expr.filter.CHILD
 47                 // remember that false/true cast respectively to 0/1
 48                 /*
 49                      * 下面先以nth-child()为例介绍一下语法，以便更好的理解下面代码的作用
 50                      * nth-child允许的几种使用方式如下：
 51                      *    :nth-child(even)
 52                      *    :nth-child(odd)
 53                      *    :nth-child(3n)
 54                      *    :nth-child(+2n+1)
 55                      *    :nth-child(2n-1)
 56                      * 下面代码中赋值号左侧的match[4]、match[5]用于分别记录括号内n前及n后的数值，包括正负号
 57                      * 对于:nth-child(even)和:nth-child(odd)来说，match[4]为空，
 58                      *     所以返回 2 * (match[3] === "even" || match[3] === "odd")的计算结果
 59                      *     因为在js中true=1,false=0，所以(match[3] === "even" || match[3] === "odd")等于1
 60                      *     因此，2 * (match[3] === "even" || match[3] === "odd")的计算结果为2
 61                      *
 62                      * 等号右侧的“+”的作用是强制类型转换，将之后的字符串转换成数值类型
 63                  */
 64                 match[4] = +( match[4] ? match[5] + (match[6] || 1) : 2 * ( match[3] === "even" || match[3] === "odd" ) );
 65                 match[5] = +( ( match[7] + match[8] ) || match[3] === "odd" );
 66
 67             // other types prohibit arguments
 68             //若非nth起头的其它CHILD类型选择器带有括号说明，则抛出异常
 69             } else if ( match[3] ) {
 70                 Sizzle.error( match[0] );
 71             }
 72
 73             return match;
 74         },
 75
 76         "PSEUDO": function( match ) {
 77             var excess,
 78                 unquoted = !match[6] && match[2];
 79
 80             if ( matchExpr["CHILD"].test( match[0] ) ) {
 81                 return null;
 82             }
 83
 84             // Accept quoted arguments as-is
 85             if ( match[3] ) {
 86                 match[2] = match[4] || match[5] || "";
 87
 88             // Strip excess characters from unquoted arguments
 89             } else if ( unquoted && rpseudo.test( unquoted ) &&
 90                 // Get excess from tokenize (recursively)
 91                 (excess = tokenize( unquoted, true )) &&
 92                 // advance to the next closing parenthesis
 93                 (excess = unquoted.indexOf( ")", unquoted.length - excess ) - unquoted.length) ) {
 94
 95                 // excess is a negative index
 96                 match[0] = match[0].slice( 0, excess );
 97                 match[2] = unquoted.slice( 0, excess );
 98             }
 99
100             // Return only captures needed by the pseudo filter method (type and argument)
101             return match.slice( 0, 3 );
102         }
103     },