How to store functions in UserData?

[Absolucy]

I'm trying to use mlua as a method of my app allowing custom scripts to modify data. However, I cannot figure out how to setup callbacks:

pub struct Route<'a> {
	callback: LuaFunction<'a>,
}

pub struct Router<'a>(Vec<Route<'a>>);

impl<'a> UserData for Router<'a> {
	fn add_methods<'lua, M: UserDataMethods<'lua, Self>>(methods: &mut M) {
		methods.add_method_mut("register", Self::register);
	}
}

impl<'a> Router<'a> {
	fn register(
		_: &Lua,
		router: &mut Router<'a>,
		(_, callback): (LuaValue<'a>, LuaFunction<'a>),
	) -> LuaResult<()> {
		router.0.push(Route {
			callback,
		});
		Ok(())
	}
}

This obviously fails, due to lifetime stuff. How would I properly do this?

[khvzak]

Could you please provide more context/full example?

Your code compiles fine:

let mut router = Router(Vec::new());
let f = lua.create_function(|_, ()| Ok(()))?;
Router::register(&lua, &mut router, (LuaNil, f))?;

[Absolucy]

Whoops, I forgot the actual part that was causing me trouble, the UserData implementation.

I updated the original post:

impl<'a> UserData for Router<'a> {
	fn add_methods<'lua, M: UserDataMethods<'lua, Self>>(methods: &mut M) {
		methods.add_method_mut("register", Self::register);
	}
}

[khvzak]

Unfortunately you cannot refer to a LuaFunction in that way :(
A possible solution could be storing Vec<Route> inside Lua (in a UserData) and refer to it using get_user_value() / set_user_value() or use Registry (via named_registry_value).

An example using get/set_user_value

pub struct Router;

impl UserData for Router {
    fn add_methods<'lua, M: UserDataMethods<'lua, Self>>(methods: &mut M) {
        methods.add_function(
            "register",
            |lua: &Lua, (router, _, callback): (AnyUserData, LuaValue, LuaFunction)| {
                // To get `router` as a Router instance:
                // let router = router.borrow::<Router>()?;
                let routes = match router.get_user_value::<Option<LuaTable>>()? {
                    Some(t) => t,
                    None => {
                        let table = lua.create_table()?;
                        router.set_user_value(table.clone())?;
                        table
                    }
                };
                let len = routes.raw_len();
                routes.raw_insert(len + 1, callback)?;
                Ok(())
            },
        );
    }
}

[Absolucy]

Wait, how do I pull the Router out of the &Lua from elsewhere? Trying to get_raw it just complains that it needs Clone, although I can't find where Clone is required.

[khvzak]

If router is a Lua global object, you can get it via lua.globals().get("router")?; (as your original type or AnyUserData)

[Absolucy]

error[E0277]: the trait bound `Router: Clone` is not satisfied
  --> src/server/worker.rs:26:38
   |
26 |         let router: Router = lua.globals().get("router").unwrap();
   |                                            ^^^ the trait `Clone` is not implemented for `Router`
   |
   = note: required because of the requirements on the impl of `FromLua<'_>` for `Router`

Which doesn't make sense, because FromLua<'lua>: Sized and I don't think Sized needs Clone (it's the other way around)?

[khvzak]

Clone requirement is defined there:
https://github.com/khvzak/mlua/blob/master/src/conversion.rs#L131
In this case you need to wrap Router to Arc (because there is no way to move value out from Lua in a Rust way).

Alternative way:

let router_ud = lua.globals().get::<_, AnyUserData>("router")?;
let router = router_ud.borrow::<Router>()?;

You can also use router_ud to get stored callbacks. router will have type Ref<Router> which is similar to &Router.
https://doc.rust-lang.org/std/cell/struct.Ref.html

[Absolucy]

@khvzak thanks, I was just using "Go to Definition" which took me to value.rs