Controlling Script Bindings

In this book we reffer to anything accessible by a script, which allows it to communicate with your Rust code a binding (which in previous versions was more generically referred to as a script API).

The "binding" here being used as in: binding script code to rust code.

Dynamic Functions

Everything callable by scripts must first be registered in the dynamic function registry. Notably we do not make use of the normal bevy function registry to improve performance and usability. This means you cannot call just any function.

In order for a function to be callable by a script it must adhere to a few requirements:

Each argument must implement FromScript.
Each return type must implement IntoScript.
Each argument must also implement GetInnerTypeDependencies
Each return type must also implement GetInnerTypeDependencies

The into/from requirements allow us to convert these types to ScriptValue's, and each supported scripting language can then marshall these into the script.

Note these types are implemented for primitives, but if you want to interact with one of your Reflect implementing types, you will need to use one of Ref<T>, Mut<T> or Val<T> wrappers in place of &T, &mut T and T respectively.

These wrappers enable us to safely interact with bevy, and claim any necessary mutex'es on Resources, Components or Allocations.

The GetInnerTypeDependencies, trait is simply a local trait alias for GetTypeRegistration with less strict type requirements. It allows us to register all the types necessary for the function calls, so that you don't have to register anything manually. If your type implements GetTypeRegistration you should not face any issues on this front.

Registering Script Functions

Registering functions can be done via the NamespaceBuilder like below:

    NamespaceBuilder::<ReflectReference>::new(&mut world)
        .register(
            "hello_world",
            |s: String| {
                println!(s)
            },
        );

This will allow you to call this function within lua like so:

hello_world("hi from lua!")

Context Arguments

Each script function call always receives 2 context arguments, namely:

CallerContext
WorldCallbackAccess

The first one is configured by the caller, and contains requests from the caller to your function, such as "I am calling you from a 1-indexed array system, please convert the index first", This argument is only relevant if you're targeting multiple languages.

The second argument gives you access to the world from your function.

You can opt-in to receive these arguments by adding them to your closure arguments in the above order (either both or just one)

Generic Arguments

Sometimes you might want to be generic over the type of argument you're accepting, you can do so by accepting ScriptValue arguments like so:

    NamespaceBuilder::<ReflectReference>::new(&mut world)
        .register(
            "is_integer",
            |s: ScriptValue| {
                match s {
                    ScriptValue::Integer(i) => true,
                    _ => false
                }
            },
        );

You can treat return values similarly.

Fallible functions

Your script functions can return errors either by: