学步园

// Remove all descendant layers from the hierarchy and add them to the new position.
for (RenderObject* curr = renderer()->firstChild(); curr; curr = curr->nextSibling())
curr->moveLayers(m_parent, this);

// Clear out all the clip rects.
clearClipRects();
}

void RenderLayer::addChild(RenderLayer* child, RenderLayer* beforeChild)
{
RenderLayer* prevSibling = beforeChild ? beforeChild->previousSibling() : lastChild();
if (prevSibling) {
child->setPreviousSibling(prevSibling); prevSibling->setNextSibling(child);
} else
setFirstChild(child);

if (beforeChild) { beforeChild->setPreviousSibling(child); child->setNextSibling(beforeChild); }
elsesetLastChild(child); child->setParent(this);

if (child->isOverflowOnly())
dirtyOverflowList();

if (!child->isOverflowOnly() || child->firstChild()) {
// Dirty the z-order list in which we are contained. The stackingContext() can be null in the
// case where we're building up generated content layers. This is ok, since the lists will start
// off dirty in that case anyway.
RenderLayer* stackingContext = child->stackingContext();
if (stackingContext)
stackingContext->dirtyZOrderLists();
}

child->updateVisibilityStatus();
if (child->m_hasVisibleContent || child->m_hasVisibleDescendant)
childVisibilityChanged(true);
}

static void addLayers(RenderObject* obj, RenderLayer* parentLayer, RenderObject*& newObject,
RenderLayer*& beforeChild)
{
if (obj->hasLayer()) {
if (!beforeChild && newObject) {
// We need to figure out the layer that follows newObject. We only do
// this the first time we find a child layer, and then we update the
// pointer values for newObject and beforeChild used by everyone else.
beforeChild = newObject->parent()->findNextLayer(parentLayer, newObject);
newObject = 0;
}
parentLayer->addChild(obj->layer(), beforeChild);
return;
}

for (RenderObject* curr = obj->firstChild(); curr; curr = curr->nextSibling())
addLayers(curr, parentLayer, newObject, beforeChild);
}

void RenderObject::addLayers(RenderLayer* parentLayer, RenderObject* newObject)
{
if (!parentLayer)
return;

RenderObject* object = newObject;
RenderLayer* beforeChild = 0;
WebCore::addLayers(this, parentLayer, object, beforeChild);
}

void RenderObject::removeLayers(RenderLayer* parentLayer)
{
if (!parentLayer)
return;

if (hasLayer()) {
parentLayer->removeChild(layer());
return;
}

for (RenderObject* curr = firstChild(); curr; curr = curr->nextSibling())
curr->removeLayers(parentLayer);
}

void RenderObject::moveLayers(RenderLayer* oldParent, RenderLayer* newParent)
{
if (!newParent)
return;

if (hasLayer()) {
if (oldParent)
oldParent->removeChild(layer());
newParent->addChild(layer());
return;
}

for (RenderObject* curr = firstChild(); curr; curr = curr->nextSibling())
curr->moveLayers(oldParent, newParent);
}

通过上面一组方法我们可以了解到拥有RenderLayer对象的RenderObject对象，按照Render树中最近的原则将含有的RenderLayer对象依Render树对应的父子关系组织RenderLayer树，RenderLayer对象的存在是依附于RenderObject对象而存在。

RenderView对象拥有对应的RenderLayer对象，同时其作为RenderLayer树根。

3、RenderLayer树与Render树的关系
通过RenderContainer::addChild方法回过头再来具体看看Render树自身的构成。
void RenderContainer::addChild(RenderObject* newChild, RenderObject* beforeChild)
{
bool needsTable = false;
//检查是否为Table的情况
if (needsTable) {
......................................................
} else {
// just add it...
insertChildNode(newChild, beforeChild);
}
.........................................................
}

void RenderContainer::insertChildNode(RenderObject* child, RenderObject* beforeChild, bool fullInsert)
{
if (!beforeChild) {
appendChildNode(child);
return;
}

while (beforeChild->parent() != this && beforeChild->parent()->isAnonymousBlock())
beforeChild = beforeChild->parent();

if (beforeChild == m_firstChild)
m_firstChild = child;

RenderObject* prev = beforeChild->previousSibling();
child->setNextSibling(beforeChild);
beforeChild->setPreviousSibling(child);
if(prev) prev->setNextSibling(child);
child->setPreviousSibling(prev);

child->setParent(this);

if (fullInsert) {
// Keep our layer hierarchy updated. Optimize for the common case where we don't have any children
// and don't have a layer attached to ourselves.
RenderLayer* layer = 0;
if (child->firstChild() || child->hasLayer()) {
layer = enclosingLayer(); child->addLayers(layer, child);
}

// if the new child is visible but this object was not, tell the layer it has some visible content
// that needs to be drawn and layer visibility optimization can't be used
if (style()->visibility() != VISIBLE && child->style()->visibility() == VISIBLE && !child->hasLayer()) {
if (!layer)
layer = enclosingLayer();
if (layer)
layer->setHasVisibleContent(true);
}
...........................................................
}

child->setNeedsLayoutAndPrefWidthsRecalc();
if (!normalChildNeedsLayout())
setChildNeedsLayout(true); // We may supply the static position for an absolute positioned child.
.................................................................
}
通过上面代码的了解我们知道通过addChild不仅维护Render树的构成，同时会将拥有的RenderLayer树构建起来。

4、RenderLayer树的作用
RenderLayer树的构建为渲染阶段处理z-index、opacity、overflow、scrollbar等打下一定的基础，在我们了解渲染的处理过程时我们再来深入的了解。

在这里我们初步的了解到在构建Render树的同时会维护一颗RenderLayer树，为分层布局、渲染作准备。

三、总结
其 实WebKit涉及网页布局方面的数据结构还有关于SVG方面的，但通过上面的理解，如果对SVG感兴趣的话，应该对理解SVG有一定的参考作用。 当然数据结构方面还有相当多的内容未提及，这里只是列出一些关键类或结构，以便有个整体的抽象认识，希望能对了解WebKit的网页布局渲染有一定的基础性作用。