I want to see the code of an iOS app.
The app I downloaded is .deb.
First question: Is it possible to install .deb file on iOS or I downloaded a wrong file?
After unpacking the .deb file, I got some files, including .nib, .storyboard and some other files.
My main question is, How to decompile these .nib files?
I tried to decompile these files using NibToXibConverter, but I didn't succeed.
Answer
Answer to your first question:
I think you downloaded the wrong file. .deb files for iOS are common for jailbreak community and not for apps, I think all of the Cydia stuffs are packed into .deb's. If your app is especially made for the jailbreak scene then you may try to install it using Cydia's auto install feature on a jailbroken device.
Answer to your main question:
This is general info about decompiling and modifying iOS apps (not specific for nibs, but you may still find it useful).
iOS apps are distibuted in .ipa archives, which are ordinary zip archives. They contain, usually one executable in the form of Mach-O file, and resources like .plist, sounds, images - every resource you include in your xcode project. Decompiling the executable file is only possible on jailbroken iDevice, because it is encrypted especially for the current device and the magic key to decrypt the binary is burned inside the device CPU and as far as I know there is no software way to read that key. The encryption system here is called FairPlay.
Briefly described, one way to decrypt that binary is to have the app installed on a jailbroken device, and launch it from command line using a debugger (gdb) which you can download from Cydia. You have to set a breakpoint somewhere, and when your code execution pauses at the breakpoint, you are doing a memory dump of the device ram - and that is your decrypted bin.
So when you have the decrypted binary, you can run one tool called "class-dump", and get the information for declared classes and their methods if the app is written in Objective C. Once you know this information you can alter the implementation of given method from some class used in your app using "Cydia Subtstrate" a.k.a "Mobile Substrate". In fact that is the way all of the jailbroken iOS tweaks are made. Basically you are hooking a method call, and when that method get invoked, it uses your implementation. Furthermore your implementation can call the original implementation, and that is really useful if you want to make some small code addition.
I am not sure whether those things possible in Swift, but if you are dealing with a Hybrid app like the ones done with cordova, phonegap, etc., then you can see all of its javascript source because it is persisted as a resource inside the "ipa" file. In addition you can alter that javascript code directly if you have a jailbroken device and find where the app is installed on file system (usually /var/containers/Bundle/Application/uuid_for_your_app/). However, I think there is no way to get that patched version and redistribute it (correct me if I'm wrong).