How can I pass in a global table, allow user to modify it via script and then read the updated values from Rust?

[jamesmcl113]

Hi, I'm trying to use mlua to allow users of my app to configure it with lua scripts. I'd like to pass in a global akin to something like vim.opts that users can modify and the app can read after executing the script. I think the serialize example is kinda what I'm looking for.

My first attempt is something like this:

let lua = Lua::new();
let globals = lua.globals();

let config = Config {
    a: 0,
    b: String::new(),
};
globals.set("config", config).unwrap();

lua.load(
    r#"
    config.a = 4
    config.b = "hi from lua"
"#,
)
.exec()
.unwrap();

let config_from_lua: Config = globals.get("config").unwrap();

println!("{}", config_from_lua.b);

And config looks like this:

#[derive(Serialize, Deserialize, Clone)]
pub struct Config {
    a: i32,
    b: String,
}

impl UserData for Config {}

impl<'lua> FromLua<'lua> for Config {
    fn from_lua(
        value: mlua::prelude::LuaValue<'lua>,
        lua: &'lua Lua,
    ) -> mlua::prelude::LuaResult<Self> {
        match value {
            mlua::Value::UserData(ud) => Ok(ud.take::<Self>()?),
            _ => unreachable!(),
        }
    }
}

When I try to run this code I get a runtime error at the .exec() call with attempt to index a Config value (global 'config'). I think I might be misunderstanding how UserData should be used.

[khvzak]

To get access to the Config fields, you need to implement accessors (yet; future mlua versions will have a derive macro for this)
For your case the code example would be:

#[derive(Serialize, Deserialize, Clone, mlua::FromLua)]
pub struct Config {
    a: i32,
    b: String,
}

impl mlua::UserData for Config {
    fn add_fields<'lua, F: mlua::UserDataFields<'lua, Self>>(fields: &mut F) {
        fields.add_field_method_get("a", |_, this| Ok(this.a));
        fields.add_field_method_get("b", |_, this| Ok(this.b.clone()));
        fields.add_field_method_set("a", |_, this, a| {
            this.a = a;
            Ok(())
        });
        fields.add_field_method_set("b", |_, this, b| {
            this.b = b;
            Ok(())
        });
    }
}

Also you don't need to manually implement FromLua as with macros feature flag you can derive it with #[derive(mlua::FromLua)]