function_reflection.rs

//! This example demonstrates how functions can be called dynamically using reflection.
//!
//! Function reflection is useful for calling regular Rust functions in a dynamic context,
//! where the types of arguments, return values, and even the function itself aren't known at compile time.
//!
//! This can be used for things like adding scripting support to your application,
//! processing deserialized reflection data, or even just storing type-erased versions of your functions.

use bevy::reflect::func::args::ArgInfo;
use bevy::reflect::func::{
    ArgList, DynamicFunction, FunctionInfo, IntoFunction, Return, ReturnInfo,
};
use bevy::reflect::Reflect;

// Note that the `dbg!` invocations are used purely for demonstration purposes
// and are not strictly necessary for the example to work.
fn main() {
    // There are times when it may be helpful to store a function away for later.
    // In Rust, we can do this by storing either a function pointer or a function trait object.
    // For example, say we wanted to store the following function:
    fn add(left: i32, right: i32) -> i32 {
        left + right
    }

    // We could store it as either of the following:
    let fn_pointer: fn(i32, i32) -> i32 = add;
    let fn_trait_object: Box<dyn Fn(i32, i32) -> i32> = Box::new(add);

    // And we can call them like so:
    let result = fn_pointer(2, 2);
    assert_eq!(result, 4);
    let result = fn_trait_object(2, 2);
    assert_eq!(result, 4);

    // However, you'll notice that we have to know the types of the arguments and return value at compile time.
    // This means there's not really a way to store or call these functions dynamically at runtime.
    // Luckily, Bevy's reflection crate comes with a set of tools for doing just that!
    // We do this by first converting our function into the reflection-based `DynamicFunction` type
    // using the `IntoFunction` trait.
    let mut function: DynamicFunction = dbg!(add.into_function());

    // This time, you'll notice that `DynamicFunction` doesn't take any information about the function's arguments or return value.
    // This is because `DynamicFunction` checks the types of the arguments and return value at runtime.
    // Now we can generate a list of arguments:
    let args: ArgList = dbg!(ArgList::new().push_owned(2_i32).push_owned(2_i32));

    // And finally, we can call the function.
    // This returns a `Result` indicating whether the function was called successfully.
    // For now, we'll just unwrap it to get our `Return` value,
    // which is an enum containing the function's return value.
    let return_value: Return = dbg!(function.call(args).unwrap());

    // The `Return` value can be pattern matched or unwrapped to get the underlying reflection data.
    // For the sake of brevity, we'll just unwrap it here and downcast it to the expected type of `i32`.
    let value: Box<dyn Reflect> = return_value.unwrap_owned();
    assert_eq!(value.take::<i32>().unwrap(), 4);

    // The same can also be done for closures.
    let mut count = 0;
    let increment = |amount: i32| {
        count += amount;
    };
    let increment_function: DynamicFunction = dbg!(increment.into_function());
    let args = dbg!(ArgList::new().push_owned(5_i32));
    // `DynamicFunction`s containing closures that capture their environment like this one
    // may need to be dropped before those captured variables may be used again.
    // This can be done manually with `drop` or by using the `Function::call_once` method.
    dbg!(increment_function.call_once(args).unwrap());
    assert_eq!(count, 5);

    // As stated before, this works for many kinds of simple functions.
    // Functions with non-reflectable arguments or return values may not be able to be converted.
    // Generic functions are also not supported.
    // Additionally, the lifetime of the return value is tied to the lifetime of the first argument.
    // However, this means that many methods (i.e. functions with a `self` parameter) are also supported:
    #[derive(Reflect, Default)]
    struct Data {
        value: String,
    }

    impl Data {
        fn set_value(&mut self, value: String) {
            self.value = value;
        }

        // Note that only `&'static str` implements `Reflect`.
        // To get around this limitation we can use `&String` instead.
        fn get_value(&self) -> &String {
            &self.value
        }
    }

    let mut data = Data::default();

    let mut set_value = dbg!(Data::set_value.into_function());
    let args = dbg!(ArgList::new().push_mut(&mut data)).push_owned(String::from("Hello, world!"));
    dbg!(set_value.call(args).unwrap());
    assert_eq!(data.value, "Hello, world!");

    let mut get_value = dbg!(Data::get_value.into_function());
    let args = dbg!(ArgList::new().push_ref(&data));
    let return_value = dbg!(get_value.call(args).unwrap());
    let value: &dyn Reflect = return_value.unwrap_ref();
    assert_eq!(value.downcast_ref::<String>().unwrap(), "Hello, world!");

    // Lastly, for more complex use cases, you can always create a custom `DynamicFunction` manually.
    // This is useful for functions that can't be converted via the `IntoFunction` trait.
    // For example, this function doesn't implement `IntoFunction` due to the fact that
    // the lifetime of the return value is not tied to the lifetime of the first argument.
    fn get_or_insert(value: i32, container: &mut Option<i32>) -> &i32 {
        if container.is_none() {
            *container = Some(value);
        }

        container.as_ref().unwrap()
    }

    let mut get_or_insert_function = dbg!(DynamicFunction::new(
        |mut args, info| {
            let container_info = &info.args()[1];
            let value_info = &info.args()[0];

            // The `ArgList` contains the arguments in the order they were pushed.
            // Therefore, we need to pop them in reverse order.
            let container = args
                .pop()
                .unwrap()
                .take_mut::<Option<i32>>(container_info)
                .unwrap();
            let value = args.pop().unwrap().take_owned::<i32>(value_info).unwrap();

            Ok(Return::Ref(get_or_insert(value, container)))
        },
        FunctionInfo::new()
            // We can optionally provide a name for the function
            .with_name("get_or_insert")
            // Since our function takes arguments, we MUST provide that argument information.
            // The arguments should be provided in the order they are defined in the function.
            // This is used to validate any arguments given at runtime.
            .with_args(vec![
                ArgInfo::new::<i32>(0).with_name("value"),
                ArgInfo::new::<&mut Option<i32>>(1).with_name("container"),
            ])
            // We can optionally provide return information as well.
            .with_return_info(ReturnInfo::new::<&i32>()),
    ));

    let mut container: Option<i32> = None;

    let args = dbg!(ArgList::new().push_owned(5_i32).push_mut(&mut container));
    let value = dbg!(get_or_insert_function.call(args).unwrap()).unwrap_ref();
    assert_eq!(value.downcast_ref::<i32>(), Some(&5));

    let args = dbg!(ArgList::new().push_owned(500_i32).push_mut(&mut container));
    let value = dbg!(get_or_insert_function.call(args).unwrap()).unwrap_ref();
    assert_eq!(value.downcast_ref::<i32>(), Some(&5));
}