I would just like to ask if SurfaceFlinger is always called for any type of drawing into the screen?
Example, displaying of JPG file to the screen.
SurfaceFlinger is not what draws your window. It allocates a frame buffer for your window, which the framework running in your application draws directly to without interacting with SurfaceFlinger. The only interaction SurfaceFlinger is involved with when you draw your window is to composite the final new frame buffer to the screen once you are done drawing a frame.
Yes, SurfaceFlinger is Android’s compositor so it takes everything that will get displayed, figures out what the resulting frame will look like and then sends it off to be displayed on the screen via the graphics card’s EGL interface.
You can get the idea that it controls the result of everything you see in a post by Android developer Jeff Sharkey where he tints the whole screen for nightmode. I also found a beamer presentation that looks good about this topic.
SurfaceFlinger is an Android system service, responsible for
compositing all the application and system surfaces into a single
buffer that is finally to be displayed by display controller.
Let’s zoom in above statement.
SurfaceFlinger is a system wide service but it is not directly
available to application developer as Sensor or other services can
be. Every time you want to update your UI, SurfaceFlinger will kick
in. This explains why SurfaceFlinger is a battery drainer.
Besides your application surfaces, there are system surfaces,
including status bar, navigation bar and, when rotation happens,
surfaces created by the system for rotation animation. Most
applications have only one active surface – the one of current
foreground activity, others have more than one when SurfaceView is
used in the view hierarchy or Presentation mode is used.
SurfaceFlinger is responsible for COMPOSITING all those surfaces. A
common misunderstanding is that SurfaceFinger is for DRAWING. It is
not correct. Drawing is the job of OpenGL. The interesting thing is
SurfaceFlinger used openGL for compositing as well.
The composition result will be put in a system buffer, or native
window, which is the source for display controller to fetch data from.
This is what you see in the screen.