How we get the source code of the app?
Save your project to your computer as an aia file. This is simply a zip file.
Open this with your unzip program, and explore the files, most will open with a notepad type app.
If you are looking for java source code, then you will be out of luck.
To be fair, that's because App Inventor doesn't generate Java code, it generates YAIL (Young Android Intermediate Language). YAIL is a language based on Scheme. You can see what it looks like if you open your browser's developer tools and connect the companion. The YAIL is logged to the console for debugging purposes, but it's not really formatted for human consumption. You can also see the sources for App Inventor itself, a portion of which is linked into your app when it is compiled, on GitHub:
There you are then 
Dave Wolber at USF did have a project called the App Inventor Java Bridge that could take an App Inventor project and generate an Eclipse project with corresponding Java code. I don't know that it has been kept up with the more recent development of App Inventor though, but it may work for simple apps.
Most folks have reported that as dead / not working now
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I have converted the aia to zip. But I could n't find any text files.
Just try opening the files you can find: .scm, .bky, .properties
Btw, no need to convert / rename the aia to zip, because the aia is already a zip file.
Thank you very much. It is new information to me.
I found that .scm and .bky files, But those codes don't contains the logic. I felt like that after reading.
But I don't have anything to open aia file. I could see the .scm .bky files by converting to zip file
E.g.: 7-Zip
Thank you 