jQuery1.8 css模块评析

jQuery早期的核心设施早都是DE大神那里拿过来的,什么选择器，事件系统，精确计算样式……现在CSS模块集中了DE大神的两个伟大的HACK

一个是用于在标准浏览器下转换百分比值为更有用的像素值

// A tribute to the "awesome hack by Dean Edwards"

            // Chrome < 17 and Safari 5.0 uses "computed value" instead of "used value" for margin-right

            // Safari 5.1.7 (at least) returns percentage for a larger set of values, but width seems to be reliably pixels

            // this is against the CSSOM draft spec: http://dev.w3.org/csswg/cssom/#resolved-values

            if ( rnumnonpx.test( ret ) && rmargin.test( name ) ) {

                width = style.width;

                minWidth = style.minWidth;

                maxWidth = style.maxWidth;

 

                style.minWidth = style.maxWidth = style.width = ret;

                ret = computed.width;

 

                style.width = width;

                style.minWidth = minWidth;

                style.maxWidth = maxWidth;

            }

一个是在IE下刷新原始值为当前的计算值(像素值)。

// From the awesome hack by Dean Edwards

    // http://erik.eae.net/archives/2007/07/27/18.54.15/#comment-102291

 

    // If we're not dealing with a regular pixel number

    // but a number that has a weird ending, we need to convert it to pixels

    // but not position css attributes, as those are proportional to the parent element instead

    // and we can't measure the parent instead because it might trigger a "stacking dolls" problem

    if ( rnumnonpx.test( ret ) && !rposition.test( name ) ) {

 

        // Remember the original values

        left = style.left;

        rsLeft = elem.runtimeStyle && elem.runtimeStyle.left;

 

        // Put in the new values to get a computed value out

        if ( rsLeft ) {

            elem.runtimeStyle.left = elem.currentStyle.left;

        }

        style.left = name === "fontSize" ? "1em" : ret;

        ret = style.pixelLeft + "px";

 

        // Revert the changed values

        style.left = left;

        if ( rsLeft ) {

            elem.runtimeStyle.left = rsLeft;

        }

    }

 

    return ret === "" ? "auto" : ret;

浏览器内部肯定有一套获取精确的像素值（对于某些样式属性来说）的机制，被DE大神发现了它们的触发条件。

此外,jquery还提供两个单独的API,width, height获取元素的长宽,它们甚至可以取得隐藏元素的长宽。出于性能的考量，浏览器对隐藏元素的样式是统统不计算的，有一套默认值返回给你，对于长宽就是0。对于动画来说，这有点不方便，比如show, slideDown等特效！因此jQuery等偷偷让它们显示出来，取得精确值，再隐藏回来，这个是由jquery.swap方法来干。不过到了jquery1.8中，是否要调用这swap方法的条件严格了，

rdisplayswap = /^(none|table(?!-c[ea]).+)/,

jQuery.each([ "height", "width" ], function( i, name ) {

    jQuery.cssHooks[ name ] = {

        get: function( elem, computed, extra ) {

            if ( computed ) {

                // certain elements can have dimension info if we invisibly show them

                // however, it must have a current display style that would benefit from this

                if ( elem.offsetWidth === 0 && rdisplayswap.test( curCSS( elem, "display" ) ) ) {

                    return jQuery.swap( elem, cssShow, function() {

                        return getWidthOrHeight( elem, name, extra );

                    });

                } else {

                    return getWidthOrHeight( elem, name, extra );

                }

            }

        },

 

        set: function( elem, value, extra ) {}

    };

});

rdisplayswap这个正则意思是，以 none 或者 table （但后面不是 -ce 或 -ca）开头的display值。换言之，下面这几种display值会进入swap分支

none table-column table-column-group table-footer-group table-header-group table-row table-row-group

此外，我们在设置宽高时，jquery会根据当前的box-sizing选择盒子模型，于是有了augmentWidthOrHeight这个方法。

为了不用用户自己写CSS3私有实现的前缀，jquery还会贴心地帮你补上它们，于是有了vendorPropName方法。

由于现在-webkit-的私有属性基本上事实的标准，opera也开始支持-webkit-前缀，我们可以看到cssPrefixes = [ "Webkit", "O", "Moz", "ms" ]是这样排列的！

有些样式，必须添加单位才能效，而jquery搞了个cssNumber，帮你在这些样式之外的样式补上“px”。总之，jquery在背后帮你做了许多事情，你写一行代码，jQuery帮你写了剩下的五十行或三百行！

 

 

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