嵌入式linux webkit,[Webkit分析]Webkit的资源加载流程

网络上的介绍webkit资源加载过程的文档已经不少了，看了之后有一点似懂非懂的感觉，况且看完了就看完了，过几天又还给作者了，所以我还是决定自己对着代码把资源加载的流程走一下，加深一些记忆。

首先了解一下MemoryCache

MemoryCache维护着所有缓存的资源列表，这是一个hash表，hash key是资源的url,

value是CachedResource*，如果你了解MemCached(用于服务器缓存)的话，完全可以用它的概念来理解这里的MemoryCache，完全是一回事。

HTML支持的资源主要包含以下这些类型：

1.HTML页面

2.字体文件

3.图片

4.CSS Shader

5.视频，音频和字幕

6.Script

7.CSS样式表

8.XSL样式表

9.SVG：用来绘制SVG的2D图形

在Webkit当中对这9种类型的资源都有对应的类来表示，这些类都是以CachedResource为父类

CachedResource主要有下面几个子类用来表示具体的资源：

CachedRawResource

CachedFont

CachedImage

CachedShader

CachedTextTrack

CachedScript

CachedCSSStyleSheet

CachedXSLStyleSheet

CachedSVGDocument

资源加载过程当中，首先与外界接触的是CachedResourceLoader类，它会先访问MemoryCache，如果找到对应的资源，则直接返回，否则就定义ResourceRequest，进而调用ResourceLoader去从网络访问资源。

Webkit中的三类Loader：

1.ResourceLoader从网络栈或本地磁盘访问资源

2.CachedResourceLoader从MemoryCache访问资源

3.专有Loader，例如FontLoader, LinkLoader,ImageLoader，它们在Element当中用于与使用者打交道的

ImageLoader加载图片的整个过程：

HTMLImageElement被创建时同时会创建ImageLoader实例，它用来加载图片的。那就从updateFromElement()中看看这个ImageLoader实例m_imageLoader是如何使用的：

void ImageLoader::updateFromElement()

{

Document*

document = m_element->document();

CachedResourceHandlenewImage = 0;

if (!attr.isNull() &&

!stripLeadingAndTrailingHTMLSpaces(attr).isEmpty()) {

CachedResourceRequest request(ResourceRequest(document->completeURL(sourceURI(attr))));

……

if (m_loadManually) {

……

} else {

//从document取cachedResourceLoader，从而调用CachedResourceLoader的requestImage方法，下面看requestImage方法

newImage =

document->cachedResourceLoader()->requestImage(request);

}

// If we do not have an image here, it means that a cross-site

// violation occurred, or that

the image was blocked via Content

// Security Policy, or the page is being dismissed. Trigger an

// error event if the page is not being dismissed.

if (!newImage && !pageIsBeingDismissed(document)) {

m_failedLoadURL = attr;

m_hasPendingErrorEvent = true;

errorEventSender().dispatchEventSoon(this);

} else

clearFailedLoadURL();

} else if

(!attr.isNull()) {

……

}

}

省掉了大部分代码，只看如何请求图片资源的，在CachedResourceLoader当中的资源请求最终都会调用到requestResource

CachedResourceHandleCachedResourceLoader::requestImage(CachedResourceRequest& request)

{

……

return

static_cast(requestResource(CachedResource::ImageResource,

request).get());

}

CachedResourceHandleCachedResourceLoader::requestResource(CachedResource::Type type,

CachedResourceRequest& request)

{

KURL url = request.resourceRequest().url();

……// 查MemoryCache

resource =

memoryCache()->resourceForRequest(request.resourceRequest());

//决定接下来要做的动作

const RevalidationPolicy policy = determineRevalidationPolicy(type,

request.mutableResourceRequest(), request.forPreload(), resource.get(),

request.defer());

switch (policy) {

case Reload:

memoryCache()->remove(resource.get());

// Fall through

case Load:

resource = loadResource(type, request, request.charset());

break;

case Revalidate:

resource = revalidateResource(request, resource.get());

break;

case Use:

if (!shouldContinueAfterNotifyingLoadedFromMemoryCache(resource.get()))

return 0;

memoryCache()->resourceAccessed(resource.get());

break;

}

if (!resource)

return 0;

if (!request.forPreload() || policy != Use)

resource->setLoadPriority(request.priority());

if ((policy !=

Use || resource->stillNeedsLoad()) && CachedResourceRequest::NoDefer

== request.defer()) {

//加载资源

resource->load(this,

request.options());

// We don't support immediate loads, but we do support immediate

failure.

if (resource->errorOccurred()) {

if (resource->inCache())

memoryCache()->remove(resource.get());

return 0;

}

}

……

return resource;

}

CachedResourceLoader::requestResource(…)暂且不关心资源已经在MemeoryCache中的情况，这里只看资源从网络栈请求的情况下是如何走的。资源的加载最终会走到resource->load(this, request.options());，而在这里调用的应该CachedImage::load(…),稍微留言一下这里的两个参数，第一个是this,意指CachedResourceLoaderr指针，第二个是一些参数，暂不关心。

void CachedImage::load(CachedResourceLoader* cachedResourceLoader,

const ResourceLoaderOptions& options)

{

if (!cachedResourceLoader

|| cachedResourceLoader->autoLoadImages())

CachedResource::load(cachedResourceLoader, options);

else

setLoading(false);

}

又调用了CachedResource::load(…)，在此函数中我只关心那一行最长的代码：

m_loader =

platformStrategies()->loaderStrategy()->resourceLoadScheduler()->scheduleSubresourceLoad(cachedResourceLoader->frame(),

this, request, request.priority(), options);

搜遍了整个代码发现有两个地方有：scheduleSubresourceLoad，一处是在WebCore当中，另一处是在Webkit2的WebProcess当中实现的，这里暂且不针对webkit2的多进程进行分析，所以就直接看WebCore当中的实现：

PassRefPtrResourceLoadScheduler::scheduleSubresourceLoad(Frame* frame, CachedResource*

resource, const ResourceRequest& request, ResourceLoadPriority priority,

const ResourceLoaderOptions& options)

{

RefPtrloader = SubresourceLoader::create(frame, resource, request, options);

if (loader)

scheduleLoad(loader.get(), priority);

return loader.release();

}

这个函数很简单，创建loader并进行调度就结束了。其实就是将资源请求放到队列当中，然后再决定是意即调度还是让他慢慢排队等待被调度，看看scheduleLoad：

void ResourceLoadScheduler::scheduleLoad(ResourceLoader*

resourceLoader, ResourceLoadPriority priority)

{

……

HostInformation* host =

hostForURL(resourceLoader->url(), CreateIfNotFound);

bool hadRequests =

host->hasRequests();

host->schedule(resourceLoader,

priority);

if (priority >

ResourceLoadPriorityLow || !resourceLoader->url().protocolIsInHTTPFamily()

|| (priority == ResourceLoadPriorityLow && !hadRequests)) {

// Try to request important resources immediately.

servePendingRequests(host, priority);

return;

}

notifyDidScheduleResourceRequest(resourceLoader);

// Handle asynchronously

so early low priority requests don't

// get scheduled before

later high priority ones.

scheduleServePendingRequests();

}

不管是意即被调度还是过一会再被调度，最终都会调用到函数ResourceLoadScheduler::servePendingRequests

void ResourceLoadScheduler::servePendingRequests(HostInformation*

host, ResourceLoadPriority minimumPriority)

{

LOG(ResourceLoading,

"ResourceLoadScheduler::servePendingRequests HostInformation.m_name='%s'",

host->name().latin1().data());

for (int priority =

ResourceLoadPriorityHighest; priority >= minimumPriority; --priority) {

HostInformation::RequestQueue& requestsPending =

host->requestsPending(ResourceLoadPriority(priority));

while (!requestsPending.isEmpty()) {

RefPtrresourceLoader = requestsPending.first();

……

resourceLoader->start();

}

}

}

－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－从这里开始要上演从网络请求资源的过程了。－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－

ResourceLoader是直接被ResourceLoadScheduler使用的类，他将ResourceLoadScheduler和ResourceHandle联系起来了，在ResourceLoadScheduler当中会按一定的优先级调用每一个资源请求job,而每一个请求job被封成了ResourceLoader实例去请求，当调度到某一个ResourceLoader时，就会调用ResourceLoader::start()方法，而在ResourceLoader::start()当中创建了ResourceHandle,如下面代码段：

void ResourceLoader::start()

{

ASSERT(!m_handle);

ASSERT(!m_request.isNull());

ASSERT(m_deferredRequest.isNull());

#if ENABLE(WEB_ARCHIVE) || ENABLE(MHTML)

if (m_documentLoader->scheduleArchiveLoad(this, m_request))

return;

#endif

if

(m_documentLoader->applicationCacheHost()->maybeLoadResource(this,

m_request, m_request.url()))

return;

if (m_defersLoading) {

m_deferredRequest = m_request;

return;

}

// 这里创建了ResourceHandle实例

if (!m_reachedTerminalState)

m_handle = ResourceHandle::create(m_frame->loader()->networkingContext(),

m_request, this, m_defersLoading, m_options.sniffContent == SniffContent);

}

ResourceHandle创建即会运行，下面是ResourceHandle的创建函数：

PassRefPtrResourceHandle::create(NetworkingContext* context, const ResourceRequest&

request, ResourceHandleClient* client, bool defersLoading, bool

shouldContentSniff)

{

BuiltinResourceHandleConstructorMap::iterator protocolMapItem =

builtinResourceHandleConstructorMap().find(request.url().protocol());

if (protocolMapItem != builtinResourceHandleConstructorMap().end())

return

protocolMapItem->value(request, client);

RefPtrnewHandle(adoptRef(new ResourceHandle(context, request, client, defersLoading,

shouldContentSniff)));

if (newHandle->d->m_scheduledFailureType

!= NoFailure)

return newHandle.release();

//这里调用了newHandle 的start()方法

if (newHandle->start())

return newHandle.release();

return 0;

}

直接来看ResourceHandle的start方法，此函数在ResourceHandleQt.cpp当中实现：

bool

ResourceHandle::start()

{

printf("ResourceHandle::startn");

// If NetworkingContext is invalid then we

are no longer attached to a Page,

// this must be an attempted load from an

unload event handler, so let's just block it.

if (d->m_context &&

!d->m_context->isValid())

return false;

if (!d->m_user.isEmpty() ||

!d->m_pass.isEmpty()) {

// If credentials were specified for

this request, add them to the url,

// so that they will be passed to

QNetworkRequest.

KURL urlWithCredentials(firstRequest().url());

urlWithCredentials.setUser(d->m_user);

urlWithCredentials.setPass(d->m_pass);

d->m_firstRequest.setURL(urlWithCredentials);

}

ResourceHandleInternal *d = getInternal();

//下面这行创建了QNetworkReplyHandler实例，并且指定了参数是AsynchronousLoad(异步加载)

d->m_job = new

QNetworkReplyHandler(this, QNetworkReplyHandler::AsynchronousLoad,

d->m_defersLoading);

return true;

}

查看QNetworkReplyHandler的构造函数：

QNetworkReplyHandler::QNetworkReplyHandler(ResourceHandle*

handle, LoadType loadType, bool deferred)

: QObject(0)

, m_resourceHandle(handle)

, m_loadType(loadType)

, m_redirectionTries(gMaxRedirections)

, m_queue(this, deferred)

{

const ResourceRequest &r =

m_resourceHandle->firstRequest();

if (r.httpMethod() == "GET")

m_method =

QNetworkAccessManager::GetOperation;

else if (r.httpMethod() ==

"HEAD")

m_method =

QNetworkAccessManager::HeadOperation;

else if (r.httpMethod() ==

"POST")

m_method = QNetworkAccessManager::PostOperation;

else if (r.httpMethod() == "PUT")

m_method =

QNetworkAccessManager::PutOperation;

else if (r.httpMethod() ==

"DELETE")

m_method =

QNetworkAccessManager::DeleteOperation;

else

m_method = QNetworkAccessManager::CustomOperation;

m_request =

r.toNetworkRequest(m_resourceHandle->getInternal()->m_context.get());

// 注意这一句： 传递的参数是一个函数

m_queue.push(&QNetworkReplyHandler::start);

}

m_queue是什么东西，我先不管他，从这里给的参数是一个函数，就能知道这个函数某个时间点肯定会被调用，那我就假设他进入队列后马上就被调用了，所以跳过来看：

void

QNetworkReplyHandler::start()

{

printf("QNetworkReplyHandler::startn");

ResourceHandleInternal* d =

m_resourceHandle->getInternal();

if (!d || !d->m_context)

return;

// ###这里发送请求了哦

QNetworkReply* reply =

sendNetworkRequest(d->m_context->networkAccessManager(),

d->m_firstRequest);

if (!reply)

return;

// ###这里的QNetworkReplyWrapper里面注册了数据ready的回调

m_replyWrapper = adoptPtr(new

QNetworkReplyWrapper(&m_queue, reply, m_resourceHandle->shouldContentSniff()

&& d->m_context->mimeSniffingEnabled(), this));

if (m_loadType == SynchronousLoad) {

m_replyWrapper->synchronousLoad();

// If supported, a synchronous request

will be finished at this point, no need to hook up the signals.

// 如果是同步，直接在这里完成就返回

return;

}

// 异步情况会走到这里，先启动一个定时器，如果有必要注册一个进度更新回调

double timeoutInSeconds =

d->m_firstRequest.timeoutInterval();

if (timeoutInSeconds > 0 &&

timeoutInSeconds < (INT_MAX / 1000))

m_timeoutTimer.start(timeoutInSeconds *

1000, this);

if

(m_resourceHandle->firstRequest().reportUploadProgress())

connect(m_replyWrapper->reply(),

SIGNAL(uploadProgress(qint64, qint64)), this, SLOT(uploadProgress(qint64,

qint64)));

}

sendNetworkRequest到底做了些什么呢？我也不知道，因为我对 QNetworkAccessManager类不熟悉，所以暂且不分析细节，暂时就认为他发送了http请求，然后就马上退出了(因为是异步请求)。

好了，请求发出去了，数据怎么样得到呢？在调用完sendNetworkRequest()函数后，创建了一个QNetworkReplyWrapper实例，而QNetworkReplyWrapper类就是专用来处理数据reply的。这里只需要知道他是处理reply就行，内容细节不研究。

数据历经磨难，终于来到了我们的眼前，最后会调用到void

QNetworkReplyHandler::forwardData()，此时我们就知道有数据来了，而当数据全部接收结束后，又会调用到void

QNetworkReplyHandler::finish()，来看看forwardData()

void

QNetworkReplyHandler::forwardData()

{

ASSERT(m_replyWrapper &&

m_replyWrapper->reply() && !wasAborted() &&

!m_replyWrapper->wasRedirected());

printf("QNetworkReplyHandler::forwardDatan");

ResourceHandleClient* client =

m_resourceHandle->client();

if (!client)

return;

qint64 bytesAvailable =

m_replyWrapper->reply()->bytesAvailable();

char* buffer = new char[8128 + 1]; //

smaller than 8192 to fit within 8k including overhead.

while (bytesAvailable > 0 &&

!m_queue.deferSignals()) {

buffer[8128] = 0x00;

qint64 readSize =

m_replyWrapper->reply()->read(buffer, 8128);

if (readSize <= 0)

break;

bytesAvailable -= readSize;

//我在这里加了打印，输出结果就是我们请求的html文档内容

printf("bytesAvailable =

%d, readSize %dn", bytesAvailable, readSize);

printf("%sn",

buffer);

printf("didReceiveData

%dn", readSize);

// FIXME:

// -1 means we do not provide any data

about transfer size to inspector so it would use

// Content-Length headers or content

size to show transfer size.

//这里上报数据，这里的client是谁呢？回去看void ResourceLoader::start()

client->didReceiveData(m_resourceHandle,

buffer, readSize, -1);

}

delete[] buffer;

if (bytesAvailable > 0)

m_queue.requeue(&QNetworkReplyHandler::forwardData);

}

在void ResourceLoader::start()当中通过ResourceHandle::create创建ResourceHandle实例时传入的第3个参数是this,而这个参数就是这里所创建的这个ResourceHandle的client，既然明确了这个client是谁，来看他的didReceiveData()方法，ResourceLoader有2个didReceiveData()方法，不过最终都会调用到didReceiveDataOrBuffer()方法：

void

ResourceLoader::didReceiveDataOrBuffer(const char* data, int length,

PassRefPtrprpBuffer, long long encodedDataLength,

DataPayloadType dataPayloadType)

{

// This method should only get data+length

*OR* a SharedBuffer.

ASSERT(!prpBuffer || (!data &&

!length));

// Protect this in this delegate method

since the additional processing can do

// anything including possibly derefing

this; one example of this is Radar 3266216.

RefPtrprotector(this);

RefPtrbuffer =

prpBuffer;

// 数据存入buffer当中先

addDataOrBuffer(data, length, buffer.get(),

dataPayloadType);

// FIXME: If we get a resource with more

than 2B bytes, this code won't do the right thing.

// However, with today's computers and

networking speeds, this won't happen in practice.

// Could be an issue with a giant local file.

// 提交数据

if (m_options.sendLoadCallbacks ==

SendCallbacks && m_frame)

frameLoader()->notifier()->didReceiveData(this,

buffer ? buffer->data() : data, buffer ? buffer->size() : length,

static_cast(encodedDataLength));

}

数据是怎么提交上去的呢？

frameLoader()->notifier()->didReceiveData(…)

frameLoader()->notifier()返回的是ResourceLoadNotifier，那就去找找ResourceLoadNotifier:: didReceiveData(…)

void

ResourceLoadNotifier::didReceiveData(ResourceLoader* loader, const char* data,

int dataLength, int encodedDataLength)

{// 更新进度

if (Page* page = m_frame->page())

page->progress()->incrementProgress(loader->identifier(), data,

dataLength);

// 继续上报数据

dispatchDidReceiveData(loader->documentLoader(),

loader->identifier(), data, dataLength, encodedDataLength);

}

到这里还没有走出ResourceLoadNotifier，继续往下跟，看看ResourceLoadNotifier:: dispatchDidReceiveData

void

ResourceLoadNotifier::dispatchDidReceiveData(DocumentLoader* loader, unsigned

long identifier, const char* data, int dataLength, int encodedDataLength)

{

m_frame->loader()->client()->dispatchDidReceiveContentLength(loader,

identifier, dataLength);

InspectorInstrumentation::didReceiveData(m_frame, identifier, data,

dataLength, encodedDataLength);

}

InspectorInstrumentation::didReceiveData可以不用关心。看到这里让我感觉有点奇怪了，为什么m_frame->loader()->client()->dispatchDidReceiveContentLength(loader,

identifier, dataLength);没有送data参数，而只是上报了长度，难道这里不是真正上报数据吗？于是我查看了void

FrameLoaderClientQt::dispatchDidReceiveContentLength，发现这个函数是空实现，看来这里确实没有真正上报数据，那真正上报数据在哪里做的呢？肯定是上面某个环节出问题了。

经常验证证明从QnetworkReplyHandler::forwardData(…)当中调用client->didReceiveData(…)时，这个client不是ResourceLoader，而是SubResourceLoader，SubResourceLoader是ResourceLoader的子类，ResourceLoader只是一个接口类，实例了大部分通用功能，但并没有实现上报数据的功能，所以数据上报完成在SubresourceLoader::didReceiveDataOrBuffer(…)当中，其中调用了父类的ResourceLoader::didReceiveDataOrBuffer(…),原因是数据保存、上报进度、还有inspector相关的功能都是在父类当中实现，子类完全可以不用关心。

void

SubresourceLoader::didReceiveDataOrBuffer(const char* data, int length,

PassRefPtrprpBuffer, long long encodedDataLength,

DataPayloadType dataPayloadType)

{    // 1.参数检查

……

printf("SubresourceLoader::didReceiveDataOrBuffern");

// Reference the object in this method

since the additional processing can do

// anything including removing the last

reference to this object; one example of this is 3266216.

RefPtrprotect(this);

RefPtrbuffer =

prpBuffer;

// 2.调用父类didReceiveDataOrBuffer函数

ResourceLoader::didReceiveDataOrBuffer(data, length, buffer,

encodedDataLength, dataPayloadType);

// 3.提交数据

if (!m_loadingMultipartContent) {

if (ResourceBuffer* resourceData =

this->resourceData())

m_resource->addDataBuffer(resourceData);

else

m_resource->addData(buffer ?

buffer->data() : data, buffer ? buffer->size() : length);

}

}

看来可以继续往上跟了。看最后几行代码发现调用了m_resource->addDataBuffer / addData，m_resource是CachedResource的指针，CachedResource是接口类，所以这里的实例肯定是他的某个子类，是哪个子类要看具体请求的资源是什么了，因为我这里测试的页面是一个空白页，所以对应的子类应该是CachedRawResource，那这里应该会调用到了CachedRawResource::addDataBuffer(…)。

下面再来理一理往上调用的类层次关系：

CachedRawResource::addDataBuffer

CachedRawResource ::notifyClientsDataWasReceived

{

while

(CachedRawResourceClient* c = w.next())

c->dataReceived(this,

data, length);

}

//这里的c就是DocumentLoader

DocumentLoader::dataReceived

DocumentLoader::commitLoad

FrameLoaderClientQt::committedLoad

DocumentLoader::commitData

DocumentWriter::addData

DecodedDataDocumentParser::appendBytes

HTMLDocumentParser::append

从HTMLDocumentParser::append开始已经属于解析部分的内容的。

哎呀，不好。我发现我的分析思路跑偏了，我刚开始是在分析ImageLoader如何加载图片数据，但数据上报时却当html页面去分析了，思路有点混乱了，不过不要紧，都是资源，请求过程大同小异，等有时间再来分析Image的显示过程，一并将数据的上报过程走一下。