Thank you so much for your instruction.
I was told that I don’t need to know too much details. For each pair of .cpp and .h files, certain functionality are played by them.
I just need to modify such files and let other files untouched, the new functionality could be realized.
But when I found the flow goes in and out of some .cpp files like I mentioned before, I start to worry about what really happened in there.
I will look into the documents and source files to find more.
Thanks a lot