JQ第七部分源码学习

// 功能检测 检测新老版本浏览器的一些默认值之类的
// support 检测出哪些选项不兼容 propHooks来处理不兼容的问题
jQuery.support = (function( support ) {
    var input = document.createElement("input"),
        fragment = document.createDocumentFragment(),
        div = document.createElement("div"),
        select = document.createElement("select"),
        opt = select.appendChild( document.createElement("option") );

    // Finish early in limited environments
    // 后续版本中已经删除 input.type默认值是text 所以这句没意义
    if ( !input.type ) {
        return support;
    }

    input.type = "checkbox";

    // Support: Safari 5.1, iOS 5.1, Android 4.x, Android 2.3
    // Check the default checkbox/radio value ("" on old WebKit; "on" elsewhere)
    // 老版本的webkit下checkOn的值为false 其余都是true
    support.checkOn = input.value !== "";
    
    // 4291行对checkOn做的处理
    // if ( !jQuery.support.checkOn ) {
    //  jQuery.valHooks[ this ].get = function( elem ) {
    //      // Support: Webkit
    //      // "" is returned instead of "on" if a value isn't specified
    //      return elem.getAttribute("value") === null ? "on" : elem.value;
    //  };
    // }

    // Must access the parent to make an option select properly
    // Support: IE9, IE10
    support.optSelected = opt.selected;
    // 4255 默认下拉列表第一项选中
    // if ( !jQuery.support.optSelected ) {
    //     jQuery.propHooks.selected = {
    //         get: function( elem ) {
    //             var parent = elem.parentNode;
    //             if ( parent && parent.parentNode ) {
    //                 parent.parentNode.selectedIndex;
    //             }
    //             return null;
    //         }
    //     };
    // }

    // Will be defined later
    // 这些值是要对DOM节点进行样式操作，因此得等到CSS样式加载完成才能处理
    support.reliableMarginRight = true;
    support.boxSizingReliable = true;
    support.pixelPosition = false;

    // Make sure checked status is properly cloned
    // Support: IE9, IE10
    // 复制的下拉框是否跟以前的有同样的默认选中
    input.checked = true;
    support.noCloneChecked = input.cloneNode( true ).checked;

    // Make sure that the options inside disabled selects aren't marked as disabled
    // (WebKit marks them as disabled)
    // 下拉框禁止选择 列表禁止与否
    select.disabled = true;
    support.optDisabled = !opt.disabled;

    // Check if an input maintains its value after becoming a radio
    // Support: IE9, IE10 默认是on
    // 先设置value再设置type
    input = document.createElement("input");
    input.value = "t";
    input.type = "radio";
    support.radioValue = input.value === "t";

    // #11217 - WebKit loses check when the name is after the checked attribute
    input.setAttribute( "checked", "t" );
    input.setAttribute( "name", "t" );

    fragment.appendChild( input );

    // Support: Safari 5.1, Android 4.x, Android 2.3
    // old WebKit doesn't clone checked state correctly in fragments
    support.checkClone = fragment.cloneNode( true ).cloneNode( true ).lastChild.checked;

    // Support: Firefox, Chrome, Safari
    // Beware of CSP restrictions (https://developer.mozilla.org/en/Security/CSP)
    // 检测onfocusin事件是否可以冒泡（IE下支持，其余不） onfocus没有冒泡 
    support.focusinBubbles = "onfocusin" in window;

    // 检测绘图区背景的克隆改变，原来的会不会跟着改变 
    // IE下会影响 其余基本不会
    div.style.backgroundClip = "content-box";
    div.cloneNode( true ).style.backgroundClip = "";
    support.clearCloneStyle = div.style.backgroundClip === "content-box";

    // Run tests that need a body at doc ready
    jQuery(function() {
        var container, marginDiv,
            // Support: Firefox, Android 2.3 (Prefixed box-sizing versions).
            divReset = "padding:0;margin:0;border:0;display:block;-webkit-box-sizing:content-box;-moz-box-sizing:content-box;box-sizing:content-box",
            body = document.getElementsByTagName("body")[ 0 ];

        if ( !body ) {
            // Return for frameset docs that don't have a body
            return;
        }

        container = document.createElement("div");
        container.style.cssText = "border:0;width:0;height:0;position:absolute;top:0;left:-9999px;margin-top:1px";

        // Check box-sizing and margin behavior.
        body.appendChild( container ).appendChild( div );
        div.innerHTML = "";
        // Support: Firefox, Android 2.3 (Prefixed box-sizing versions).
        div.style.cssText = "-webkit-box-sizing:border-box;-moz-box-sizing:border-box;box-sizing:border-box;padding:1px;border:1px;display:block;width:4px;margin-top:1%;position:absolute;top:1%";

        // Workaround failing boxSizing test due to offsetWidth returning wrong value
        // with some non-1 values of body zoom, ticket #13543
        jQuery.swap( body, body.style.zoom != null ? { zoom: 1 } : {}, function() {
            support.boxSizing = div.offsetWidth === 4;
        });

        // Use window.getComputedStyle because jsdom on node.js will break without it.
        if ( window.getComputedStyle ) {
            // 除了Safari其他浏览器都会把百分比自动转为像素
            support.pixelPosition = ( window.getComputedStyle( div, null ) || {} ).top !== "1%";
            // IE在怪异模式下会减掉padding border当做实际可用width的宽度
            support.boxSizingReliable = ( window.getComputedStyle( div, null ) || { width: "4px" } ).width === "4px";

            // Support: Android 2.3
            // Check if div with explicit width and no margin-right incorrectly
            // gets computed margin-right based on width of container. (#3333)
            // WebKit Bug 13343 - getComputedStyle returns wrong value for margin-right
            marginDiv = div.appendChild( document.createElement("div") );
            marginDiv.style.cssText = div.style.cssText = divReset;
            marginDiv.style.marginRight = marginDiv.style.width = "0";
            div.style.width = "1px";

            support.reliableMarginRight =
                !parseFloat( ( window.getComputedStyle( marginDiv, null ) || {} ).marginRight );
        }

        body.removeChild( container );
    });

    return support;
})( {} );