Open source of App Inventor Extensions By PeterZhong
You can compile the extension with the extension-builder by ColinTree. Visit the website and input the information(including the path of the folder[Note: not the path of the Java file]),then click build with Github
and wait. You'll get an extension file easily in less than 5 minutes.
Of course you can compile the extension locally. See more about this at App Inventor's website.
It's welcome for you to be a contributor for my extensions, and of course you can use my source code in different ways, but you can not use my source code to earn money. Thanks!