http://www.linuxgraphics.cn/android/graphics_system.html
Java level is Surface/View concept. But in c++ level, the flow is as follows:
1. JNI call "new SurfaceComposerClient" in SurfaceSession_init
2. SurfaceComposerClient will get surfaceflinger service by asking servicemanager
3. call createConnection and get SurfaceFlingerClient, SufaceFlinger keeps its own "Client" map
4. call (BnSurfaceFlingerClient/BClient)SurfaceFlinger::createSurface, to create layer, and get LayerBaseClient::Surface, and casting to Surface
5. The following operation in JNI is to triggered SufaceComposerClient/Surface function
When app get the surface, it can start drawing. The SurfaceFlinger is a thread(SurfaceFlinger::threadLoop) that wait for drawing event. When event comes, it drawing all surface in Z-order and send it to framebuffer(hw.flip).
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