Android GUI更新过程

 1.ViewRoot
在private void draw(boolean fullRedrawNeeded)中，会调用lockCanvas，从而获取一个Canvas对象，然后调用递归调用子窗口(View)的draw函数去绘制自己，最后调用unlockCanvasAndPost让Surface把自己更新到屏幕上。

canvas = surface.lockCanvas(dirty);
mView.draw(canvas);
surface.unlockCanvasAndPost(canvas);

2.android/view/Surface
它主要对一些native函数的包装。

    private native Canvas lockCanvasNative(Rect dirty);
    public native   void unlockCanvasAndPost(Canvas canvas);

3.android_view_Surface.cpp
这是native函数的实现。
Surface_lockCanvas它先锁住Surface，从而获取Surface相关的信息，如果格式、宽度、高度和像素缓冲区。然后创建一个bitmap，这个bitmap和共享一个像素缓冲区，这样View就能直接把自己绘制到Surface上了。

status_t err = surface->lock(&info, &dirtyRegion);
bitmap.setPixels(info.bits);
nativeCanvas->setBitmapDevice(bitmap);

Surface_unlockCanvasAndPost它断开SkCanvas与Surface之间的关系，然后调用Surface的unlockAndPost。

nativeCanvas->setBitmapDevice(SkBitmap());
status_t err = surface->unlockAndPost();

4.surfaceflinger_client/Surface.cpp
Surface::lock调用dequeueBuffer获取一个可用的Buffer，dequeueBuffer会调用SharedBufferClient::dequeue等到backbuffer可用，然后锁住这个Buffer并填充相关信息。

status_t err = dequeueBuffer(&backBuffer);
err = lockBuffer(backBuffer.get());

Surface::unlockAndPost先unlock Buffer，然后调queueBuffer显示Buffer。

status_t err = mLockedBuffer->unlock();
err = queueBuffer(mLockedBuffer.get());

Surface::queueBuffer是比较有意思的，它设置更新的区域，然后通知服务端(SurfaceFlinger)。

mSharedBufferClient->setDirtyRegion(bufIdx, mDirtyRegion);
err = mSharedBufferClient->queue(bufIdx);
client->signalServer();

通过binder发送请求给服务：

    virtual void signal() const
    {
       Parcel data, reply;
       data.writeInterfaceToken(ISurfaceComposer::getInterfaceDescriptor());
       remote()->transact(BnSurfaceComposer::SIGNAL, data, &reply, IBinder::FLAG_ONEWAY);
    }

5.SurfaceFlinger

BnSurfaceComposer::onTransact
       case SIGNAL: {
           CHECK_INTERFACE(ISurfaceComposer, data, reply);
           signal();
       } break;

这是SurfaceFlinger中处理SIGNAL的代码，它调用SurfaceFlinger::signal，再调用 SurfaceFlinger::signalEvent，最后调用MessageQueue::invalidate唤醒是 SurfaceFlinger的主循环。

在主循环中waitForEvent返回，再handleRepaint去合成个Layer/Surface。

bool SurfaceFlinger::threadLoop()
{
    waitForEvent();
       handleRepaint();
       postFramebuffer();
}

SurfaceFlinger::handleRepaint会调用composeSurfaces把需要绘制各个Surface合并起来，绘制到FrameBuffer的BackBuffer上。

    for (size_t i=0 ; i<count ; ++i) {
       const sp<LayerBase>& layer = layers[i];
       const Region& visibleRegion(layer->visibleRegionScreen);
       if (!visibleRegion.isEmpty())  {
           /* broncho cy add */
#ifdef USEOVERLAY
           mToppest[1] = mToppest[0];
           mToppest[0] = layer->getIdentity();
#endif
           /* broncho end */
           const Region clip(dirty.intersect(visibleRegion));
           if (!clip.isEmpty()) {
               layer->draw(clip);
           }
       }
    }

SurfaceFlinger::postFramebuffer最后把FrameBuffer的BackBuffer显示到屏幕上：

hw.flip(mInvalidRegion); -->eglSwapBuffers(dpy, surface);