bevy/crates/bevy_reflect/src/utility.rs

//! Helpers for working with Bevy reflection.

use crate::TypeInfo;
use bevy_utils::{FixedState, HashMap};
use once_cell::race::OnceBox;
use parking_lot::RwLock;
use std::{
    any::{Any, TypeId},
    hash::BuildHasher,
};

/// A container for [`TypeInfo`] over non-generic types, allowing instances to be stored statically.
///
/// This is specifically meant for use with _non_-generic types. If your type _is_ generic,
/// then use [`GenericTypeInfoCell`] instead. Otherwise, it will not take into account all
/// monomorphizations of your type.
///
/// ## Example
///
/// ```
/// # use std::any::Any;
/// # use bevy_reflect::{NamedField, Reflect, ReflectMut, ReflectOwned, ReflectRef, StructInfo, Typed, TypeInfo};
/// use bevy_reflect::utility::NonGenericTypeInfoCell;
///
/// struct Foo {
///   bar: i32
/// }
///
/// impl Typed for Foo {
///   fn type_info() -> &'static TypeInfo {
///     static CELL: NonGenericTypeInfoCell = NonGenericTypeInfoCell::new();
///     CELL.get_or_set(|| {
///       let fields = [NamedField::new::<i32>("bar")];
///       let info = StructInfo::new::<Self>("Foo", &fields);
///       TypeInfo::Struct(info)
///     })
///   }
/// }
/// #
/// # impl Reflect for Foo {
/// #   fn type_name(&self) -> &str { todo!() }
/// #   fn get_type_info(&self) -> &'static TypeInfo { todo!() }
/// #   fn into_any(self: Box<Self>) -> Box<dyn Any> { todo!() }
/// #   fn as_any(&self) -> &dyn Any { todo!() }
/// #   fn as_any_mut(&mut self) -> &mut dyn Any { todo!() }
/// #   fn into_reflect(self: Box<Self>) -> Box<dyn Reflect> { todo!() }
/// #   fn as_reflect(&self) -> &dyn Reflect { todo!() }
/// #   fn as_reflect_mut(&mut self) -> &mut dyn Reflect { todo!() }
/// #   fn apply(&mut self, value: &dyn Reflect) { todo!() }
/// #   fn set(&mut self, value: Box<dyn Reflect>) -> Result<(), Box<dyn Reflect>> { todo!() }
/// #   fn reflect_ref(&self) -> ReflectRef { todo!() }
/// #   fn reflect_mut(&mut self) -> ReflectMut { todo!() }
/// #   fn reflect_owned(self: Box<Self>) -> ReflectOwned { todo!() }
/// #   fn clone_value(&self) -> Box<dyn Reflect> { todo!() }
/// # }
/// ```
pub struct NonGenericTypeInfoCell(OnceBox<TypeInfo>);

impl NonGenericTypeInfoCell {
    /// Initialize a [`NonGenericTypeInfoCell`] for non-generic types.
    pub const fn new() -> Self {
        Self(OnceBox::new())
    }

    /// Returns a reference to the [`TypeInfo`] stored in the cell.
    ///
    /// If there is no [`TypeInfo`] found, a new one will be generated from the given function.
    ///
    /// [`TypeInfos`]: TypeInfo
    pub fn get_or_set<F>(&self, f: F) -> &TypeInfo
    where
        F: FnOnce() -> TypeInfo,
    {
        self.0.get_or_init(|| Box::new(f()))
    }
}

/// A container for [`TypeInfo`] over generic types, allowing instances to be stored statically.
///
/// This is specifically meant for use with generic types. If your type isn't generic,
/// then use [`NonGenericTypeInfoCell`] instead as it should be much more performant.
///
/// ## Example
///
/// ```
/// # use std::any::Any;
/// # use bevy_reflect::{Reflect, ReflectMut, ReflectOwned, ReflectRef, TupleStructInfo, Typed, TypeInfo, UnnamedField};
/// use bevy_reflect::utility::GenericTypeInfoCell;
///
/// struct Foo<T: Reflect>(T);
///
/// impl<T: Reflect> Typed for Foo<T> {
///   fn type_info() -> &'static TypeInfo {
///     static CELL: GenericTypeInfoCell = GenericTypeInfoCell::new();
///     CELL.get_or_insert::<Self, _>(|| {
///       let fields = [UnnamedField::new::<T>(0)];
///       let info = TupleStructInfo::new::<Self>("Foo", &fields);
///       TypeInfo::TupleStruct(info)
///     })
///   }
/// }
/// #
/// # impl<T: Reflect> Reflect for Foo<T> {
/// #   fn type_name(&self) -> &str { todo!() }
/// #   fn get_type_info(&self) -> &'static TypeInfo { todo!() }
/// #   fn into_any(self: Box<Self>) -> Box<dyn Any> { todo!() }
/// #   fn as_any(&self) -> &dyn Any { todo!() }
/// #   fn as_any_mut(&mut self) -> &mut dyn Any { todo!() }
/// #   fn into_reflect(self: Box<Self>) -> Box<dyn Reflect> { todo!() }
/// #   fn as_reflect(&self) -> &dyn Reflect { todo!() }
/// #   fn as_reflect_mut(&mut self) -> &mut dyn Reflect { todo!() }
/// #   fn apply(&mut self, value: &dyn Reflect) { todo!() }
/// #   fn set(&mut self, value: Box<dyn Reflect>) -> Result<(), Box<dyn Reflect>> { todo!() }
/// #   fn reflect_ref(&self) -> ReflectRef { todo!() }
/// #   fn reflect_mut(&mut self) -> ReflectMut { todo!() }
/// #   fn reflect_owned(self: Box<Self>) -> ReflectOwned { todo!() }
/// #   fn clone_value(&self) -> Box<dyn Reflect> { todo!() }
/// # }
/// ```
pub struct GenericTypeInfoCell(OnceBox<RwLock<HashMap<TypeId, &'static TypeInfo>>>);

impl GenericTypeInfoCell {
    /// Initialize a [`GenericTypeInfoCell`] for generic types.
    pub const fn new() -> Self {
        Self(OnceBox::new())
    }

    /// Returns a reference to the [`TypeInfo`] stored in the cell.
    ///
    /// This method will then return the correct [`TypeInfo`] reference for the given type `T`.
    /// If there is no [`TypeInfo`] found, a new one will be generated from the given function.
    pub fn get_or_insert<T, F>(&self, f: F) -> &TypeInfo
    where
        T: Any + ?Sized,
        F: FnOnce() -> TypeInfo,
    {
        let type_id = TypeId::of::<T>();
        // let mapping = self.0.get_or_init(|| Box::new(RwLock::default()));
        let mapping = self.0.get_or_init(Box::default);
        if let Some(info) = mapping.read().get(&type_id) {
            return info;
        }

        mapping.write().entry(type_id).or_insert_with(|| {
            // We leak here in order to obtain a `&'static` reference.
            // Otherwise, we won't be able to return a reference due to the `RwLock`.
            // This should be okay, though, since we expect it to remain statically
            // available over the course of the application.
            Box::leak(Box::new(f()))
        })
    }
}

/// Deterministic fixed state hasher to be used by implementors of [`Reflect::reflect_hash`].
///
/// Hashes should be deterministic across processes so hashes can be used as
/// checksums for saved scenes, rollback snapshots etc. This function returns
/// such a hasher.
///
/// [`Reflect::reflect_hash`]: crate::Reflect
#[inline]
pub fn reflect_hasher() -> bevy_utils::AHasher {
    FixedState.build_hasher()
}