Cannot mutate userdata from a field on "root" userdata

[SeanOMik]

I have a struct passed to Lua as userdata. It has a field that is also returned to lua as userdata. That field has a method that mutates it's self. When the method on it is used, it is not modify the field in the "root" userdata like I expected. Here's some code that would show what I mean:

-- c variable is provided by Rust host to Lua.
-- type is `Camera` userdata.

-- c.transform is `Transform` userdata
c.transform:translate(0, 0.15, 0)
print("Moving camera to: " .. tostring(c.transform))

The above code prints the original value, the transform is not modified. In the below code, the transform is modified:

local t = c.transform 
t:translate(0, 0.15 * dt, 0)
c.transform = t

I believe that its because I'm using add_field_method_get in the UserData trait impl of Camera to add a field to get transform. In the method, I copy Transform before returning it to Lua. How could I return a reference?

[CrashAndSideburns]

I'm trying to embed Lua into an emulator that I'm working on, and I'm running into the exact same issue. Is there any chance that you've resolved this? I can't seem to find any documentation discussing how this should be done, or find anything similar being done in examples.

[khvzak]

There are couple of options:

1. Wrap nested struct to Rc<RefCell<_>> then it can be mutated when cloned
2. Keep nested struct in AnyUserData from the beginning (cloning it only creates a new references to existing object)

[CrashAndSideburns]

Thanks! I just got it working by wrapping the inner struct in Rc<RefCell<_>>. I don't suppose that there would be a way to do this without wrapping every single field of the struct in this? I would need to make some pretty big changes to the code base of my emulator to do that. It feels like it should be possible to just wrap the outer struct, but maybe I'm wrong? For instance, I can do

struct Foo {
    inner: BarWrapper,
}

struct BarWrapper(Rc<RefCell<Bar>>);

struct Bar {
    value: String
}

but would it be possible to wrap Foo instead of Bar? Sorry if this isn't explained well, I can try to put together a sample from my code if this isn't helpful.

[khvzak]

You can wrap only Foo, but then you need to create few other structs, like BarWrapper(Rc<RefCell<Foo>>) to attach different methods to them.
Eg. when you call bar:update() the code should know that inner field of Foo needs to be updated.

[CrashAndSideburns]

Ah, thanks! That works. That's going to end up being a lot simpler than overhauling all of the types in the core of my emulator, and it should help decouple the Lua interfaces from the actual code. Thanks so much!