原创，转载清注明！
1 什么是mainFrame？
WebCore中，Page的mainFrame只是一个软件上的概念，并不针对具体的网页标签，是webview 创建第一个Frame实例之后，Page的mainFrame存储的是此Frame的引用。它与网页中有没有Frame标签没有关系。
比如网页中这么写：
<html>
<head>
<title>hello world!</title>
</head>
<body>
</body>
</html>
WebCore这么创建Frame并把它设置为Page的mainFrame：
webkitwebframe.cpp: webkit_web_frame_new（）{
Frame::create(viewPriv->corePage, 0, client);
}
Frame的工厂方法：
Frame::create(Page* page, HTMLFrameOwnerElement* ownerElement, FrameLoaderClient* client) {
RefPtr<Frame> frame = adoptRef(new Frame(page, ownerElement, client));
注意：mainFrame没有宿主！在Frame中来说，此为区别普通的Frame和mainFrame的唯一差别。
if (!ownerElement)
page->setMainFrame(frame);
return frame.release();
}
#0 WebCore::Frame::create, Frame.cpp:205
#1 in webkit_web_frame_new (webView=0x6d2030)
at ../../Source/WebKit/gtk/webkit/webkitwebframe.cpp:419
#2 in webkit_web_view_init (webView=0x6d2030)
at ../../Source/WebKit/gtk/webkit/webkitwebview.cpp:3407
2 Frame标签和WebCore的Frame的区别和联系？
Frame标签在WebCore中是用HTMLFrameElement来表示的，它是一个标准的Element。与其他element不同，不同的platform提供了不同的support，比如gtk是这样创建Frame的：
FrameLoaderClientGtk.cpp：
RefPtr<Frame> childFrame = Frame::create(page, ownerElement, new FrameLoaderClient(kitFrame));
此时创建的叫child Frame,相对于mainFrame来说。你看，她有宿主ownerElement--其实就是FrameElement的基类FrameElementBase的实例。请参看下一节。
网页中的Frame标签在WebCore中称为SubFrame，一个Frame标签就会使得WebCore 调用FrameLoaderClient以便new一个Frame,解释如下：
如果网页中这么写：
<html>
<head>
<title>hello world!</title>
</head>
<frameset name="main_frame" cols="100%">
<frame src="00000.gif" id="content_area" name="content_area">
</frameset>
</html>
WebCore是如下图这么工作的，与处理其他Element是一样的：
#0 WebCore::Frame::create (page=, ownerElement=, client=) at ../../Source/WebCore/page/Frame.cpp:205
#1 in WebKit::FrameLoaderClient::createFrame (this ownerElement=) at WebKit/gtk/WebCoreSupport/FrameLoaderClientGtk.cpp:652
#2 in WebCore::SubframeLoader::loadSubframe (this=, ownerElement=, url=..., name=..., referrer=...) at loader/SubframeLoader.cpp:265
#3 in WebCore::SubframeLoader::loadOrRedirectSubframe (this ownerElement=, . lockBackForwardList=true) at loader/SubframeLoader.cpp:240
#4 in WebCore::SubframeLoader::requestFrame (this, ownerElement, =..., =..., lockHistory=true, lockBackForwardList=true) at loader/SubframeLoader.cpp:83
#5 in WebCore::HTMLFrameElementBase::openURL (this, lockHistory=true, lockBackForwardList=true) at ../../Source/WebCore/html/HTMLFrameElementBase.cpp:102
#6 in WebCore::HTMLFrameElementBase::setNameAndOpenURL (this) at ../../Source/WebCore/html/HTMLFrameElementBase.cpp:153
#7 in WebCore::HTMLFrameElementBase::insertedIntoDocument (this) at ../../Source/WebCore/html/HTMLFrameElementBase.cpp:187
#8 in WebCore::ContainerNode::parserAddChild (this, newChild=...) at ../../Source/WebCore/dom/ContainerNode.cpp:690
#9 in WebCore::HTMLConstructionSite::attach<WebCore::Element> (this, rawParent, prpChild=...) at../../Source/WebCore/html/parser/HTMLConstructionSite.cpp:103
3 Frame::create 时的 ownerElement是谁？其实就是HTMLFrameElement的基类HTMLFrameElementBase：
#0 WebCore::HTMLFrameElementBase::HTMLFrameElementBase (this=, tagName=..., document=) at ../../Source/WebCore/html/HTMLFrameElementBase.cpp:56
#1 in WebCore::HTMLFrameElement::HTMLFrameElement (this=, tagName=..., document=) at ../../Source/WebCore/html/HTMLFrameElement.cpp:40
#2 in WebCore::HTMLFrameElement::create (tagName=..., document=) at ../../Source/WebCore/html/HTMLFrameElement.cpp:47
#3 in WebCore::frameConstructor (tagName=..., document=) at DerivedSources/WebCore/HTMLElementFactory.cpp:266
#4 in WebCore::HTMLElementFactory::createHTMLElement (qName=..., document=,
formElement=0x0, createdByParser=true) at DerivedSources/WebCore/HTMLElementFactory.cpp:649
#5 in WebCore::HTMLConstructionSite::createHTMLElement (this=, token=...)
at ../../Source/WebCore/html/parser/HTMLConstructionSite.cpp:384