Example for building extensions

[djc]

Hey, thanks for all your work on the objc2 crate ecosystem, I've been playing with and it's gotten me pretty excited about being able to build for macOS (and maybe iOS etc) using (hopefully pure) Rust.

As an initial (maybe quite ambitious) project, I've been trying to get an FSKit filesystem going without involving any Swift code (using a lot of assistance from Claude code). I can now see it in Login Items & Extensions / File System Extensions. My next step is to try and mount it, and I have a basic class definition inheriting from FSUnaryFileSystem written up.

However, when I try to mount this thing I get errors like this:

djc-2021 main fs-exp $ mount -F -t FSExp /dev/null ~/tmp/fs-exp
mount: Probing resource: The operation couldn’t be completed. (com.apple.extensionKit.errorDomain error 2.)
mount: Unable to invoke task

A little searching and a bunch of LLM-assisted churn have not led me to a solution so far. Claude seems to have decided that the extension isn't being loaded at all because the right symbols cannot be found in it. I've run experiments with writing a debug log file early in main() and in ctor::ctor() thing (which AIUI should run before main), but these have not resulted in actual debug log files.

I was wondering/hoping if you might have some advice, know of some example code, or a project doing something like this (with an .appex extension).

[madsmtm]

It's been a while since I looked into it, and the lower-level details aren't really documented from what I could find, but looking at my notes I found the following linker¹ flags that you could try passing:

-framework Foundation -e _NSExtensionMain -application_extension

This might help your binary getting recognized as an extension, and might get ctors to load etc.

You will probably still need an Info.plist file with the NSExtension key, and somewhere in there, set NSExtensionPrincipalClass to a class name that you've defined with define_class!.

I'm also pretty sure that there is no fn main() entrypoint in extensions (we override it with -e _NSExtensionMain), I suspect you have to use an extension class (?)

If that is the case, you'll want to make sure the class is actually registered by the time NSExtensionMain is run, this can be done with a ctor as follows:

#[ctor::ctor]
fn register() {
    // Call `class` once, this registers the class with the Objective-C runtime.
    let _ = MyClass::class();
}

define_class!(
    #[unsafe(super(NSObject))] // Subclass whatever is expected for the kind of extension you're creating.
    #[name = "MyClass"] // Name you'll be using in `NSExtensionPrincipalClass`.
    #[ivars = Ivars]
    struct MyClass;

    // ...
);

This is definitely uncharted waters, do let me know what you figure out!

Footnotes

1. Xcode also seems to pass -fapplication-extension to Clang, but that seems to only implement a few restrictions, it doesn't actually change any compilation behaviour. ↩