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//! 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::{
ArgList, DynamicFunction, DynamicFunctionMut, FunctionResult, IntoFunction,
IntoFunctionMut, Return, SignatureInfo,
},
PartialReflect, 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 function: DynamicFunction<'static> = 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().with_owned(2_i32).with_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 PartialReflect> = return_value.unwrap_owned();
assert_eq!(value.try_take::<i32>().unwrap(), 4);
// The same can also be done for closures that capture references to their environment.
// Closures that capture their environment immutably can be converted into a `DynamicFunction`
// using the `IntoFunction` trait.
let minimum = 5;
let clamp = |value: i32| value.max(minimum);
let function: DynamicFunction = dbg!(clamp.into_function());
let args = dbg!(ArgList::new().with_owned(2_i32));
let return_value = dbg!(function.call(args).unwrap());
let value: Box<dyn PartialReflect> = return_value.unwrap_owned();
assert_eq!(value.try_take::<i32>().unwrap(), 5);
// We can also handle closures that capture their environment mutably
// using the `IntoFunctionMut` trait.
let mut count = 0;
let increment = |amount: i32| count += amount;
let closure: DynamicFunctionMut = dbg!(increment.into_function_mut());
let args = dbg!(ArgList::new().with_owned(5_i32));
// Because `DynamicFunctionMut` mutably borrows `total`,
// it will need to be dropped before `total` can be accessed again.
// This can be done manually with `drop(closure)` or by using the `DynamicFunctionMut::call_once` method.
dbg!(closure.call_once(args).unwrap());
assert_eq!(count, 5);
// Generic functions can also be converted into a `DynamicFunction`,
// however, they will need to be manually monomorphized first.
fn stringify<T: ToString>(value: T) -> String {
value.to_string()
}
// We have to manually specify the concrete generic type we want to use.
let function = stringify::<i32>.into_function();
let args = ArgList::new().with_owned(123_i32);
let return_value = function.call(args).unwrap();
let value: Box<dyn PartialReflect> = return_value.unwrap_owned();
assert_eq!(value.try_take::<String>().unwrap(), "123");
// To make things a little easier, we can also "overload" functions.
// This makes it so that a single `DynamicFunction` can represent multiple functions,
// and the correct one is chosen based on the types of the arguments.
// Each function overload must have a unique argument signature.
let function = stringify::<i32>
.into_function()
.with_overload(stringify::<f32>);
// Now our `function` accepts both `i32` and `f32` arguments.
let args = ArgList::new().with_owned(1.23_f32);
let return_value = function.call(args).unwrap();
let value: Box<dyn PartialReflect> = return_value.unwrap_owned();
assert_eq!(value.try_take::<String>().unwrap(), "1.23");
// Function overloading even allows us to have a variable number of arguments.
let function = (|| 0)
.into_function()
.with_overload(|a: i32| a)
.with_overload(|a: i32, b: i32| a + b)
.with_overload(|a: i32, b: i32, c: i32| a + b + c);
let args = ArgList::new()
.with_owned(1_i32)
.with_owned(2_i32)
.with_owned(3_i32);
let return_value = function.call(args).unwrap();
let value: Box<dyn PartialReflect> = return_value.unwrap_owned();
assert_eq!(value.try_take::<i32>().unwrap(), 6);
// As stated earlier, `IntoFunction` 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 (unless manually monomorphized like `foo::<i32>.into_function()`).
// 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 set_value = dbg!(Data::set_value.into_function());
let args = dbg!(ArgList::new().with_mut(&mut data)).with_owned(String::from("Hello, world!"));
dbg!(set_value.call(args).unwrap());
assert_eq!(data.value, "Hello, world!");
let get_value = dbg!(Data::get_value.into_function());
let args = dbg!(ArgList::new().with_ref(&data));
let return_value = dbg!(get_value.call(args).unwrap());
let value: &dyn PartialReflect = return_value.unwrap_ref();
assert_eq!(value.try_downcast_ref::<String>().unwrap(), "Hello, world!");
// 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 get_or_insert_function = dbg!(DynamicFunction::new(
|mut args: ArgList| -> FunctionResult {
// The `ArgList` contains the arguments in the order they were pushed.
// The `DynamicFunction` will validate that the list contains
// exactly the number of arguments we expect.
// We can retrieve them out in order (note that this modifies the `ArgList`):
let value = args.take::<i32>()?;
let container = args.take::<&mut Option<i32>>()?;
// We could have also done the following to make use of type inference:
// let value = args.take_owned()?;
// let container = args.take_mut()?;
Ok(Return::Ref(get_or_insert(value, container)))
},
// Functions can be either anonymous or named.
// It's good practice, though, to try and name your functions whenever possible.
// This makes it easier to debug and is also required for function registration.
// We can either give it a custom name or use the function's type name as
// derived from `std::any::type_name_of_val`.
SignatureInfo::named(std::any::type_name_of_val(&get_or_insert))
// We can always change the name if needed.
// It's a good idea to also ensure that the name is unique,
// such as by using its type name or by prefixing it with your crate name.
.with_name("my_crate::get_or_insert")
// Since our function takes arguments, we should provide that argument information.
// This is used to validate arguments when calling the function.
// And it aids consumers of the function with their own validation and debugging.
// Arguments should be provided in the order they are defined in the function.
.with_arg::<i32>("value")
.with_arg::<&mut Option<i32>>("container")
// We can provide return information as well.
.with_return::<&i32>(),
));
let mut container: Option<i32> = None;
let args = dbg!(ArgList::new().with_owned(5_i32).with_mut(&mut container));
let value = dbg!(get_or_insert_function.call(args).unwrap()).unwrap_ref();
assert_eq!(value.try_downcast_ref::<i32>(), Some(&5));
let args = dbg!(ArgList::new().with_owned(500_i32).with_mut(&mut container));
let value = dbg!(get_or_insert_function.call(args).unwrap()).unwrap_ref();
assert_eq!(value.try_downcast_ref::<i32>(), Some(&5));
}