jQuery源码解析之width()

0

一、在讲之前,先弄清 boxSizing 属性
(1)box-sizing 是默认值 "content-box"

<body>
<script src="jQuery.js"></script>
<div id="pTwo"
     style="width: 55px;
     border:1px red solid;">这是divTwo</div>
<script>
  $("#pTwo").width() //55
</script>
</body>

$().width()的值是 55

(2)box-sizing 是 "border-box"

<div id="pTwo"
     style="width: 55px;
     box-sizing: border-box;
     border:1px red solid;">这是divTwo</div>

$().width()的值是 53

因为 border-box 是包括 border、padding、content 的,而 content-box 只包括 content。
可想而知,jQuery的$().width() 中也包含了对 borderBox 的判断。

  • 注意下div标签的默认值

二、$().width()
作用:
获取目标元素的宽度

源码:

  //源码7033行
  //$.each(obj,callback(index,item){})
  jQuery.each( [ "height", "width" ], function( i, dimension ) {
    //i:0 dimension:height
    //i:1 dimension:width

    //cssHooks是用来定义style方法的
    jQuery.cssHooks[ dimension ] = {
      //读
      //$().width()
      //参数:elem:目标DOM元素/computed:true/extra:"content"
      get: function( elem, computed, extra ) {
        console.log(elem, computed, extra,'extra7040')
        if ( computed ) {

          // 某些元素是有尺寸的信息的,如果我们隐式地显示它们,前提是它必须有一个display值
          // Certain elements can have dimension info if we invisibly show them
          // but it must have a current display style that would benefit

          // 上面这句话的意思是,某个元素用display:none,将它从页面上去掉了,此时是获取不到它的宽度的
          // 如果要获取它的宽度的话,需要隐式地显示它们,比如display:absolute,visible:hidden
          // 然后再去获取它的宽度

          // block:false
          // none:true
          // rdisplayswap的作用是检测 none和table开头的
          return rdisplayswap.test( jQuery.css( elem, "display" ) ) &&

          // 兼容性的考虑,直接看 getWidthOrHeight

          // Support: Safari 8+
          // Table columns in Safari have non-zero offsetWidth & zero
          // getBoundingClientRect().width unless display is changed.
          // Support: IE <=11 only
          // Running getBoundingClientRect on a disconnected node
          // in IE throws an error.

          // display为none的话,elem.getBoundingClientRect().width=0
          // elem.getClientRects() 返回CSS边框的集合
          // https://developer.mozilla.org/zh-CN/docs/Web/API/Element/getClientRects
          ( !elem.getClientRects().length || !elem.getBoundingClientRect().width ) ?
            swap( elem, cssShow, function() {
              return getWidthOrHeight( elem, dimension, extra );
            } ) :
            //$().width()情况
            //dimension:width/extra:"content"
            getWidthOrHeight( elem, dimension, extra );
        }
      },
   };
} );

解析:
(1)box-sizing 是默认值,并且 display 不为 none
rdisplayswap
作用:
检测目标元素的display属性的值 是否为none或以table开头

    // 检测 display 的值是否为 none 或以 table 开头
    // Swappable if display is none or starts with table
    // 除了 "table", "table-cell", "table-caption"
    // except "table", "table-cell", or "table-caption"
    // display 的值,请访问 https://developer.mozilla.org/en-US/docs/CSS/display
    // See here for display values: https://developer.mozilla.org/en-US/docs/CSS/display
    // 源码6698行
var rdisplayswap = /^(none|table(?!-c[ea]).+)/,

如果displaynone的话,就会调用swap()方法,反之,就直接调用getWidthOrHeight()方法

getWidthOrHeight()
作用:
获取widthheight的值

 //获取 width 或 height
  //dimension:width/extra:"content"
  //源码6823行
  function getWidthOrHeight( elem, dimension, extra ) {

    // Start with computed style
    var styles = getStyles( elem ),
      val = curCSS( elem, dimension, styles ),
      //判断 box-sizing 的值是否 是 border-box
      //如果启用了 box-sizing,js 的 width 是会算上 margin、border、padding的
      //如果不启用的话,js 的 width 只会算 content
      //jQuery 的 width 自始至终都是算的 content
      isBorderBox = jQuery.css( elem, "boxSizing", false, styles ) === "border-box",

      valueIsBorderBox = isBorderBox;

    //火狐兼容性处理,可不看
    // Support: Firefox <=54
    // Return a confounding non-pixel value or feign ignorance, as appropriate.
    if ( rnumnonpx.test( val ) ) {
      if ( !extra ) {
        return val;
      }
      val = "auto";
    }

    // 通过getComputedStyle检查style属性,并返回可靠的style属性,这样可以防止浏览器返回不可靠的值
    // Check for style in case a browser which returns unreliable values
    // for getComputedStyle silently falls back to the reliable elem.style
    valueIsBorderBox = valueIsBorderBox &&
      ( support.boxSizingReliable() || val === elem.style[ dimension ] );
    console.log(valueIsBorderBox,'valueIsBorderBox6853')
    // Fall back to offsetWidth/offsetHeight when value is "auto"
    // This happens for inline elements with no explicit setting (gh-3571)
    // Support: Android <=4.1 - 4.3 only
    // Also use offsetWidth/offsetHeight for misreported inline dimensions (gh-3602)
    if ( val === "auto" ||
      !parseFloat( val ) && jQuery.css( elem, "display", false, styles ) === "inline" ) {

      val = elem[ "offset" + dimension[ 0 ].toUpperCase() + dimension.slice( 1 ) ];
      console.log(val,'val6862')
      // offsetWidth/offsetHeight provide border-box values
      valueIsBorderBox = true;
    }
    // Normalize "" and auto
    // 55px
    val = parseFloat( val ) || 0;
    console.log(val,extra,'val6869')
    // Adjust for the element's box model
    return ( val +
      boxModelAdjustment(
        //DOM节点
        elem,
        //width
        dimension,
        //content
        extra || ( isBorderBox ? "border" : "content" ),
        //true/false
        valueIsBorderBox,
        //styles
        styles,
        //55
        // Provide the current computed size to request scroll gutter calculation (gh-3589)
        val
      )
    ) + "px";
  }

getWidthOrHeight() 里面有好多方法,我们一一来解析:

getStyles( elem )
作用:
获取该 DOM 元素的所有 css 属性的值

  //获取该DOM元素的所有css属性的值
  //源码6501行
  var getStyles = function( elem ) {
    // 兼容性处理,旨在拿到正确的view
    // Support: IE <=11 only, Firefox <=30 (#15098, #14150)
    // IE throws on elements created in popups
    // FF meanwhile throws on frame elements through "defaultView.getComputedStyle"
    var view = elem.ownerDocument.defaultView;

    if ( !view || !view.opener ) {
      view = window;
    }
    //获取所有CSS属性的值
    return view.getComputedStyle( elem );
  };

可以看到,本质是调用了getComputedStyle()方法。

curCSS( elem, dimension, styles )
作用:
获取元素的当前属性的值

// 获取元素的当前属性的值
  // elem, "position"
  // elem,width,styles
  // 源码6609行
  function curCSS( elem, name, computed ) {

    var width, minWidth, maxWidth, ret,

      // Support: Firefox 51+
      // Retrieving style before computed somehow
      // fixes an issue with getting wrong values
      // on detached elements
      style = elem.style;
    //获取elem所有的样式属性
    computed = computed || getStyles( elem );
    // console.log(computed,'computed6621')
    // getPropertyValue is needed for:
    //   .css('filter') (IE 9 only, #12537)
    //   .css('--customProperty) (#3144)
    if ( computed ) {
      //返回元素的属性的当前值
      //position:static
      //top:0px
      //left:0px
      ret = computed.getPropertyValue( name ) || computed[ name ];
      console.log(ret,'ret6627')
      //如果目标属性值为空并且目标元素不在目标元素所在的文档内(感觉这种情况好奇怪)
      if ( ret === "" && !jQuery.contains( elem.ownerDocument, elem ) ) {
        //使用jQuery.style方法来获取目标元素的属性值
        ret = jQuery.style( elem, name );
      }

      // A tribute to the "awesome hack by Dean Edwards"
      // Android Browser returns percentage for some values,
      // but width seems to be reliably pixels.
      // This is against the CSSOM draft spec:
      // https://drafts.csswg.org/cssom/#resolved-values
      //当属性设置成数值时,安卓浏览器会返回一些百分比,但是宽度是像素显示的
      //这违反了CSSOM草案规范
      //所以以下方法是修复不规范的width属性的
      if ( !support.pixelBoxStyles() && rnumnonpx.test( ret ) && rboxStyle.test( name ) ) {

        // Remember the original values
        width = style.width;
        minWidth = style.minWidth;
        maxWidth = style.maxWidth;

        // Put in the new values to get a computed value out
        style.minWidth = style.maxWidth = style.width = ret;
        ret = computed.width;

        // Revert the changed values
        style.width = width;
        style.minWidth = minWidth;
        style.maxWidth = maxWidth;
      }
    }

    return ret !== undefined ?
      // 兼容性,IE下返回的zIndex的值是数字,
      // 而使用jQuery获取的属性都是返回字符串
      // Support: IE <=9 - 11 only
      // IE returns zIndex value as an integer.
      ret + "" :
      ret;
  }

可以看到,curCSS本质是调用了computed.getPropertyValue( name )方法,也就是说我们可以这样去获取目标元素的属性值:

let a=document.getElementById("pTwo")
a.ownerDocument.defaultView.getComputedStyle(a).getPropertyValue('width')
//55px

目标元素的所属 view,调用getComputedStyle()方法,获取目标元素的所有 CSS 属性,再调用getPropertyValue('width'),获取目标width的属性值,为 55px

注意:无论box-sizing的值是border-box还是content-box,上面的方法获取的width值都是55px,这是不符合 CSS3 盒子模型的,所以 jQuery 拿到该值后,还会继续处理。

boxModelAdjustment
因为这里讨论的是情况一,所以boxModelAdjustment()会直接返回 0

综上:当box-sizing 是默认值,并且 display 不为 none时,返回的width是:

parseFloat(a.ownerDocument.defaultView.getComputedStyle(a).getPropertyValue('width')) //55

(2)box-sizing 值为 border-box

<div id="pTwo"
     style="width: 55px;
     margin-left:2px;
     padding-left: 2px;
     box-sizing: border-box;
     border:1px red solid;">这是divTwo</div>
$("#pTwo").width() //51
document.getElementById("pTwo").style.width //55px

可以看到,原生 js 获取 width 是不遵循 CSS3 盒子规范的。

borderBox 的判断在getWidthOrHeight()方法中,直接看过去:

  //获取 width 或 height
  //dimension:width/extra:"content"
  //源码6823行
  function getWidthOrHeight( elem, dimension, extra ) {
    xxx
    ...
    var styles = getStyles( elem ),
      //true
      isBorderBox = jQuery.css( elem, "boxSizing", false, styles ) === "border-box",
      //true
      valueIsBorderBox = isBorderBox;
    xxx
    ...
    valueIsBorderBox = valueIsBorderBox &&
      //val值是通过a.ownerDocument.defaultView.getComputedStyle(a).getPropertyValue('width')得出的
      //但又通过js原生的style.width来取值并与val相比较
      ( support.boxSizingReliable() || val === elem.style[ dimension ] );
    console.log(val === elem.style[ dimension ],'valueIsBorderBox6853')
 
    // 55
    val = parseFloat( val ) || 0;
    // Adjust for the element's box model
    return ( val +
      //borderBox走这里
      boxModelAdjustment(
        //DOM节点
        elem,
        //width
        dimension,
        //content
        extra || ( isBorderBox ? "border" : "content" ),
        //true/false
        valueIsBorderBox,
        //styles
        styles,
        //55
        // Provide the current computed size to request scroll gutter calculation (gh-3589)
        val
      )
    ) + "px";
  }

boxModelAdjustment():
作用:
集中处理borderBox的情况

//参数说明:
  //elem:DOM节点/dimension:width/box:content/isBorderBox:true/false/styles:styles/computedVal:55
  //源码6758行
  function boxModelAdjustment( elem, dimension, box, isBorderBox, styles, computedVal ) {
    var i = dimension === "width" ? 1 : 0,
      extra = 0,
      delta = 0;

    // 如果 boxSizing 的属性值,而不是 borderBox 的话,就直接返回 0
    // Adjustment may not be necessary
    if ( box === ( isBorderBox ? "border" : "content" ) ) {
      console.log('content1111','content6768')
      return 0;
    }
    //小技巧
    //i 的初始值是 0/1
    //然后 cssExpand = [ "Top", "Right", "Bottom", "Left" ]
    for ( ; i < 4; i += 2 ) {

      // Both box models exclude margin
      if ( box === "margin" ) {
        //var cssExpand = [ "Top", "Right", "Bottom", "Left" ];
        //width 的话,就是 marginRight/marginLeft
        //height 的话,就是 marginTop/marginBottom
        //jQuery.css( elem, box + cssExpand[ i ], true, styles ) 的意思就是
        //返回 marginRight/marginLeft/marginTop/marginBottom 的数字,并给 delta 加上
        delta += jQuery.css( elem, box + cssExpand[ i ], true, styles );
      }

      // If we get here with a content-box, we're seeking "padding" or "border" or "margin"
      // 如果不是 borderBox 的话
      if ( !isBorderBox ) {

        // Add padding
        // 添加 padding-xxx
        delta += jQuery.css( elem, "padding" + cssExpand[ i ], true, styles );

        // For "border" or "margin", add border
        if ( box !== "padding" ) {
          delta += jQuery.css( elem, "border" + cssExpand[ i ] + "Width", true, styles );

          // But still keep track of it otherwise
        } else {
          extra += jQuery.css( elem, "border" + cssExpand[ i ] + "Width", true, styles );
        }

        // If we get here with a border-box (content + padding + border), we're seeking "content" or
        // "padding" or "margin"
      } else {
        // 去掉 padding
        // For "content", subtract padding
        if ( box === "content" ) {
          //width,去掉paddingLeft,paddingRight的值
          delta -= jQuery.css( elem, "padding" + cssExpand[ i ], true, styles );
        }

        // For "content" or "padding", subtract border
        // 去掉 borderXXXWidth
        if ( box !== "margin" ) {
          //width,去掉borderLeftWidth,borderRightWidth的值
          delta -= jQuery.css( elem, "border" + cssExpand[ i ] + "Width", true, styles );
        }
      }
    }

    // Account for positive content-box scroll gutter when requested by providing computedVal
    if ( !isBorderBox && computedVal >= 0 ) {

      // offsetWidth/offsetHeight is a rounded sum of content, padding, scroll gutter, and border
      // Assuming integer scroll gutter, subtract the rest and round down
      delta += Math.max( 0, Math.ceil(
        //就是将dimension的首字母做个大写
        elem[ "offset" + dimension[ 0 ].toUpperCase() + dimension.slice( 1 ) ] -
        computedVal -
        delta -
        extra -
        0.5
      ) );
    }

    return delta;
  }

可以看到,isBorderBox 为 true 的话,会执行下面两段代码:

if ( box === "content" ) {
   //width,去掉paddingLeft,paddingRight的值
   delta -= jQuery.css( elem, "padding" + cssExpand[ i ], true, styles );
}
 if ( box !== "margin" ) {
    //width,去掉borderLeftWidth,borderRightWidth的值
    delta -= jQuery.css( elem, "border" + cssExpand[ i ] + "Width", true, styles );
}

去除了paddingLeftpaddingRightborderLeftWidthborderRightWidth,并最终返回值

二、$().width(xxx)
作用:
设置目标元素的宽度

源码:


  //源码7033行
  //$.each(obj,callback(index,item){})
  jQuery.each( [ "height", "width" ], function( i, dimension ) {
    //i:0 dimension:height
    //i:1 dimension:width

    //cssHooks是用来定义style方法的
    jQuery.cssHooks[ dimension ] = {
      //写
      //$().width(55)
      //elem:DOM节点,value:55,extra:content
      set: function( elem, value, extra ) {
        var matches,
          styles = getStyles( elem ),
          isBorderBox = jQuery.css( elem, "boxSizing", false, styles ) === "border-box",
          //-4
          subtract = extra && boxModelAdjustment(
            elem,
            dimension,
            extra,
            isBorderBox,
            styles
          );

        // 如果是 borderBox 的话,通过 offset 计算的尺寸是不准的,
        // 所以要假设成 content-box 来获取 border 和 padding
        // Account for unreliable border-box dimensions by comparing offset* to computed and
        // faking a content-box to get border and padding (gh-3699)
        //true true 'static'
        //调整 subtract
        if ( isBorderBox && support.scrollboxSize() === styles.position ) {
          subtract -= Math.ceil(
            elem[ "offset" + dimension[ 0 ].toUpperCase() + dimension.slice( 1 ) ] -
            parseFloat( styles[ dimension ] ) -
            boxModelAdjustment( elem, dimension, "border", false, styles ) -
            0.5
          );
          console.log(subtract,'subtract7169')
        }
        // 如果需要进行值调整,则转换为像素
        // Convert to pixels if value adjustment is needed
        //如果是 borderBox 并且 value 的单位不是 px,则会转换成像素
        if ( subtract && ( matches = rcssNum.exec( value ) ) &&
          ( matches[ 3 ] || "px" ) !== "px" ) {
          elem.style[ dimension ] = value;
          value = jQuery.css( elem, dimension );
        }
        //59px
        return setPositiveNumber( elem, value, subtract );
      }
  };
} );

解析:
(1)整体上看,实际上两个 if ,最后再 return 一个setPositiveNumber()方法

(2)注意subtract ,如果有 borderBox 属性,并且 borderWidth、padding 有值的话,subtract 一般为负数,比如下面的例子,subtract = -4

<div id="pTwo"
     style="width: 55px;
     margin-left:2px;
     padding-left: 2px;
     box-sizing: border-box;
     /*box-sizing: content-box;*/
     /*display: none;*/
     border:1px red solid;">
  这是divTwo
</div>

$("#pTwo").width(55)

反之则会是 0

(3)两个 if 我试了下,都会去执行,所以直接看的setPositiveNumber ()

setPositiveNumber:
作用:
设置真正的 width 值

  function setPositiveNumber( elem, value, subtract ) {
    // 标准化相对值
    // Any relative (+/-) values have already been
    // normalized at this point

    //[
    // "55px",
    // undefined,
    // "55",
    // "px",
    // index: 0,
    // input: "55px",
    // groups: undefined,
    // index: 0
    // input: "55px"
    // ]
    var matches = rcssNum.exec( value );
    console.log(matches,( subtract || 0 ),'matches6760')
    return matches ?
      //(0,55-(-4))+'px'
      //Math.max(a,b) 返回两个指定的数中带有较大的值的那个数
      // Guard against undefined "subtract", e.g., when used as in cssHooks
      Math.max( 0, matches[ 2 ] - ( subtract || 0 ) ) + ( matches[ 3 ] || "px" ) :
      value;
  }

如果是 borderBox,width 会设置成 59px(虽然表面上开发者设置的是$("#pTwo").width(55)),反之,则是 55px

总结:
1、$().width()
(1)不是borderBox
$().width()=parseFloat(elem.ownerDocument.defaultView.getComputedStyle(elem).getPropertyValue('width'))

(2)是borderBox()
在(1)的基础上执行boxModelAdjustment()方法,去除 borderWidth、padding

2、$().width(xxx)
(1)不是borderBox
width=xxx
(2)是borderBox
width=xxx+ setPositiveNumber()


(完)

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