通过前面小节的讲解我们知道，surfaceflinger在接收到Vsync信号之后，会执行一系列的函数，这些函数做了哪些事情呢？在之前提到SurfaceFlinger.cpp中：
void SurfaceFlinger::onMessageReceived(int32_t what) {
case MessageQueue::INVALIDATE: {
/*处理事务，实际大部分做了一些标志位的操作*/
bool refreshNeeded = handleMessageTransaction();
/*切换各个层的buffer*/
refreshNeeded |= handleMessageInvalidate();
/*发出一个界面更新的信号，最终导致handleMessageRefresh被调用*/
refreshNeeded |= mRepaintEverything;
case MessageQueue::REFRESH: {
handleMessageRefresh();
以上函数都做了哪些工作呢？下面是一个草图： 
我们猜测一下handleMessageRefresh函数会做什么事情。在android系统中存在对个应用程序，每个程序都有有一个Layer，这些Layer从上到下，一层层的排放，上面的层会挡住下面的。那么handleMessageRefresh会去算出每个层的可视区域，最终在他他们合并显示出来。如下图：
主要工作如下： 1.计算各个Layer的显示区域 2.合成显示：a（软件）.使用openGL把各个Layer的可视区域在一个内存上描绘出来。b（硬件）.使用HWcomposer合成 下面我们查看handleMessageRefresh：
void SurfaceFlinger::handleMessageRefresh() {
/*做一些预先的处理*/
preComposition();
/*计算各个层的可视区域。对每个屏幕构建可视的Layer*/
rebuildLayerStacks();
setUpHWComposer();
doDebugFlashRegions();
doComposition();
postComposition(refreshStartTime);
首先我们来看看preComposition
preCompositionvoid SurfaceFlinger::preComposition()
/*在Layer.cpp中实现*/
onPreComposition()
/*如果有多个buffer需要处理返回true*/
return mQueuedFrames > 0 || mSidebandStreamChanged || mAutoRefresh;
needExtraInvalidate = true;
signalLayerUpdate();
void SurfaceFlinger::rebuildLayerStacks() {
// rebuild the visible layer list per screen
/*对每个显示器质性for中代码*/
for (size_t dpy=0 ; dpyisDisplayOn()) {
/*计算可视区域*/
SurfaceFlinger::computeVisibleRegions(layers,hw->getLayerStack(), dirtyRegion, opaqueRegion);
假设有多个APP，如下体所示： 
在最上面的APP可以完全显示，下面的APP会被遮挡一部分。怎么计算各个Layer的可视区域了？ 1.从Z轴最大的层开始计算， 2.Z轴小的Layer会被他上面的Layer遮盖 下面我们使用一个例子来讲解这些代码： 假设有3个层：
void SurfaceFlinger::computeVisibleRegions(
const LayerVector& currentLayers, uint32_t layerStack,
Region& outDirtyRegion, Region& outOpaqueRegion)
{
ATRACE_CALL();
/*在当前层之上的不透明区域*/
Region aboveOpaqueLayers;
/*当前层之前的被遮盖的区域:.opaque+transucent，当前层之上各层课时区域*/
Region aboveCoveredLayers;
/**/
Region dirty;
outDirtyRegion.clear();
/*当前需要合成的Layer层数*/
size_t i = currentLayers.size();
while (i--) {
/*从Z轴最大的那一层开始处理*/
const sp& layer = currentLayers[i];
// start with the whole surface at its current location
const Layer::State& s(layer->getDrawingState());
// only consider the layers on the given layer stack
if (s.layerStack != layerStack)
continue;
/*
* opaqueRegion: area of a surface that is fully opaque.
*/
Region opaqueRegion;
/*
* visibleRegion: area of a surface that is visible on screen
* and not fully transparent. This is essentially the layer's
* footprint minus the opaque regions above it.
* Areas covered by a translucent surface are considered visible.
*/
Region visibleRegion;
/*
* coveredRegion: area of a surface that is covered by all
* visible regions above it (which includes the translucent areas).
*/
Region coveredRegion;
/*
* transparentRegion: area of a surface that is hinted to be completely
* transparent. This is only used to tell when the layer has no visible
* non-transparent regions and can be removed from the layer list. It
* does not affect the visibleRegion of this layer or any layers
* beneath it. The hint may not be correct if apps don't respect the
* SurfaceView restrictions (which, sadly, some don't).
*/
Region transparentRegion;
// handle hidden surfaces by setting the visible region to empty
if (CC_LIKELY(layer->isVisible())) {
const bool translucent = !layer->isOpaque(s);
Rect bounds(s.active.transform.transform(layer->computeBounds()));
visibleRegion.set(bounds);
if (!visibleRegion.isEmpty()) {
// Remove the transparent area from the visible region
if (translucent) {
const Transform tr(s.active.transform);
if (tr.preserveRects()) {
// transform the transparent region
transparentRegion = tr.transform(s.activeTransparentRegion);
} else {
// transformation too complex, can't do the
// transparent region optimization.
transparentRegion.clear();
}
}
// compute the opaque region
const int32_t layerOrientation = s.active.transform.getOrientation();
if (s.alpha==255 && !translucent &&
((layerOrientation & Transform::ROT_INVALID) == false)) {
// the opaque region is the layer's footprint
opaqueRegion = visibleRegion;
}
}
}
// Clip the covered region to the visible region
coveredRegion = aboveCoveredLayers.intersect(visibleRegion);
// Update aboveCoveredLayers for next (lower) layer
aboveCoveredLayers.orSelf(visibleRegion);
// subtract the opaque region covered by the layers above us
visibleRegion.subtractSelf(aboveOpaqueLayers);
// compute this layer's dirty region
if (layer->contentDirty) {
// we need to invalidate the whole region
dirty = visibleRegion;
// as well, as the old visible region
dirty.orSelf(layer->visibleRegion);
layer->contentDirty = false;
} else {
/* compute the exposed region:
* the exposed region consists of two components:
* 1) what's VISIBLE now and was COVERED before
* 2) what's EXPOSED now less what was EXPOSED before
*
* note that (1) is conservative, we start with the whole
* visible region but only keep what used to be covered by
* something -- which mean it may have been exposed.
*
* (2) handles areas that were not covered by anything but got
* exposed because of a resize.
*/
const Region newExposed = visibleRegion - coveredRegion;
const Region oldVisibleRegion = layer->visibleRegion;
const Region oldCoveredRegion = layer->coveredRegion;
const Region oldExposed = oldVisibleRegion - oldCoveredRegion;
dirty = (visibleRegion&oldCoveredRegion) | (newExposed-oldExposed);
}
dirty.subtractSelf(aboveOpaqueLayers);
// accumulate to the screen dirty region
outDirtyRegion.orSelf(dirty);
// Update aboveOpaqueLayers for next (lower) layer
aboveOpaqueLayers.orSelf(opaqueRegion);
// Store the visible region in screen space
layer->setVisibleRegion(visibleRegion);
layer->setCoveredRegion(coveredRegion);
layer->setVisibleNonTransparentRegion(
visibleRegion.subtract(transparentRegion));
}
outOpaqueRegion = aboveOpaqueLayers;
}
代码注释比较详细，有兴趣的同学可以分析一下。
