Decompile XAPK file
I have worked pretty a lot analyzing Android projects with different reverse engineering tools, generally for secure purposes, to see what can be seen by a hacker when he/she gets my app. I've met a problem that I couldn't find any of decompilers capable to decompile files of "XAPK" format (not simple APK). I know, asking for a way to find a tool is not kind of questions good for stack overflow, but I am pretty sure there are many people with the same problem and I find it highly relevant. I tried apk-tool, decompilers provided with androguard tool, online-decompilers, none of them are capable of working with XAPKs.
Answer
The .xapk is just a ZIP file which contains the original .apk along with some other files. You can simply unzip the .xapk and decompile the .apk within.
For more details, there's Decompiling XAPK Files. Basically, if you want to decompile an .xapk, all you have to do is unzip it, look for the original .apk, and decompile it normally.