chromium:LoadUrl之后会发生那些事情之CompositedLayer 创建

一 CompositedLayer 综述

实际上，在M44以后，已经不存在实际上的CompositedLayer 这个对象，其实它就是DeprecatedPaintLayer，只是它的m_compositeReason是需要进行合成的。

详细可以查看CompositeReason.h，它详细列出了那些需要合成的类型。

二 CompositedLayer 的创建

在DeprecatedPaintLayerTree创建完毕后，整个文档还并不可见，还需要进行layout计算，layout计算会触发compositeReason的计算，它主要是遍历整个DeprecatedPaintLayerTree，

对其中的每一节点DeprecatedPaintLayer，计算其m_compositeReason属性值。代码调用栈如下：

Call Stack：

#0 blink::CompositingRequirementsUpdater::updateRecursive(

blink::DeprecatedPaintLayer * ancestorLayer, blink::DeprecatedPaintLayer * layer,

blink::CompositingRequirementsUpdater::OverlapMap & overlapMap,

blink::CompositingRequirementsUpdater::RecursionData & currentRecursionData,

bool & descendantHas3DTransform, WTF::Vector<blink::DeprecatedPaintLayer *,

0,WTF::DefaultAllocator> & unclippedDescendants,

blink::IntRect & absoluteDecendantBoundingBox)

#1 blink::CompositingRequirementsUpdater::update(blink::DeprecatedPaintLayer * root)

#2 blink::DeprecatedPaintLayerCompositor::updateIfNeeded()

#3 blink::DeprecatedPaintLayerCompositor::updateIfNeededRecursive()

#4 blink::FrameView::updateLayoutAndStyleForPaintingInternal()

#5 blink::FrameView::updateLayoutAndStyleForPainting()

#6 blink::PageAnimator::updateLayoutAndStyleForPainting(blink::LocalFrame * rootFrame)

#7 blink::PageWidgetDelegate::layout(blink::Page & page, blink::LocalFrame & root)

#8 blink::WebViewImpl::layout()

#9 content::RenderWidgetCompositor::Layout()

#10 cc::LayerTreeHost::Layout()

#11 cc::ThreadProxy::BeginMainFrame()

三 OverlapMap 重叠测试

class OverlapMapContainer {

public:

void add(const IntRect& bounds)

{

m_layerRects.append(bounds);

m_boundingBox.unite(bounds);

}

bool overlapsLayers(const IntRect& bounds) const

{

// Checking with the bounding box will quickly reject cases when

// layers are created for lists of items going in one direction and

// never overlap with each other.

if (!bounds.intersects(m_boundingBox))

return false;

for (unsigned i = 0; i < m_layerRects.size(); i++) {

if (m_layerRects[i].intersects(bounds))

return true;

}

return false;

}

void unite(const OverlapMapContainer& otherContainer)

{

m_layerRects.appendVector(otherContainer.m_layerRects);

m_boundingBox.unite(otherContainer.m_boundingBox);

}

private:

Vector<IntRect, 64> m_layerRects;

IntRect m_boundingBox;

};

class CompositingRequirementsUpdater::OverlapMap {

WTF_MAKE_NONCOPYABLE(OverlapMap);

public:

OverlapMap()

{

// Begin by assuming the root layer will be composited so that there

// is something on the stack. The root layer should also never get a

// finishCurrentOverlapTestingContext() call.

beginNewOverlapTestingContext();

}

void add(DeprecatedPaintLayer* layer, const IntRect& bounds)

{

ASSERT(!layer->isRootLayer());

if (bounds.isEmpty())

return;

// Layers do not contribute to overlap immediately--instead, they will

// contribute to overlap as soon as they have been recursively processed

// and popped off the stack.

ASSERT(m_overlapStack.size() >= 2);

m_overlapStack[m_overlapStack.size() - 2].add(bounds);

}

bool overlapsLayers(const IntRect& bounds) const

{

return m_overlapStack.last().overlapsLayers(bounds);

}

void beginNewOverlapTestingContext()

{

// This effectively creates a new "clean slate" for overlap state.

// This is used when we know that a subtree or remaining set of

// siblings does not need to check overlap with things behind it.

m_overlapStack.append(OverlapMapContainer());

}

void finishCurrentOverlapTestingContext()

{

// The overlap information on the top of the stack is still necessary

// for checking overlap of any layers outside this context that may

// overlap things from inside this context. Therefore, we must merge

// the information from the top of the stack before popping the stack.

//

// FIXME: we may be able to avoid this deep copy by rearranging how

// overlapMap state is managed.

m_overlapStack[m_overlapStack.size() - 2].unite(m_overlapStack.last());

m_overlapStack.removeLast();

}

private:

Vector<OverlapMapContainer> m_overlapStack;

};

主要有两个类，OverlapMap和OverlapMapContainer，OverlapMapContainer保存当前的layer以及其大小，OverlapMap是保存了当前CompositedLayer上一系列其他非CompositedLayer的layers以及其大小，当有新的layer需要被composited时，就会beginNewOverlapTestingContext，进行新的重叠测试。

版权声明：本文为博主原创文章，未经博主允许不得转载。