bevy_reflect: Reflect arrays (#4701)

# Objective

> ℹ️ **Note**: This is a rebased version of #2383. A large portion of it has not been touched (only a few minor changes) so that any additional discussion may happen here. All credit should go to @NathanSWard for their work on the original PR.

- Currently reflection is not supported for arrays.
- Fixes #1213

## Solution

* Implement reflection for arrays via the `Array` trait.
* Note, `Array` is different from `List` in the way that you cannot push elements onto an array as they are statically sized.
* Now `List` is defined as a sub-trait of `Array`.

---

## Changelog

* Added the `Array` reflection trait
* Allows arrays up to length 32 to be reflected via the `Array` trait

## Migration Guide

* The `List` trait now has the `Array` supertrait. This means that `clone_dynamic` will need to specify which version to use:
  ```rust
  // Before
  let cloned = my_list.clone_dynamic();
  // After
  let cloned = List::clone_dynamic(&my_list);
  ```
* All implementers of `List` will now need to implement `Array` (this mostly involves moving the existing methods to the `Array` impl)

Co-authored-by: NathanW <nathansward@comcast.net>
Co-authored-by: MrGVSV <49806985+MrGVSV@users.noreply.github.com>

crates/bevy_reflect/src/array.rs

@@ -0,0 +1,300 @@
+use crate::{serde::Serializable, Reflect, ReflectMut, ReflectRef};
+use serde::ser::SerializeSeq;
+use std::{
+    any::Any,
+    hash::{Hash, Hasher},
+};
+
+/// A static-sized array of [`Reflect`] items.
+///
+/// This corresponds to types like `[T; N]` (arrays).
+///
+/// Currently, this only supports arrays of up to 32 items. It can technically
+/// contain more than 32, but the blanket [`GetTypeRegistration`] is only
+/// implemented up to the 32 item limit due to a [limitation] on `Deserialize`.
+///
+/// [`GetTypeRegistration`]: crate::GetTypeRegistration
+/// [limitation]: https://github.com/serde-rs/serde/issues/1937
+pub trait Array: Reflect {
+    /// Returns a reference to the element at `index`, or `None` if out of bounds.
+    fn get(&self, index: usize) -> Option<&dyn Reflect>;
+    /// Returns a mutable reference to the element at `index`, or `None` if out of bounds.
+    fn get_mut(&mut self, index: usize) -> Option<&mut dyn Reflect>;
+    /// Returns the number of elements in the collection.
+    fn len(&self) -> usize;
+    /// Returns `true` if the collection contains no elements.
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+    /// Returns an iterator over the collection.
+    fn iter(&self) -> ArrayIter;
+
+    fn clone_dynamic(&self) -> DynamicArray {
+        DynamicArray {
+            name: self.type_name().to_string(),
+            values: self.iter().map(|value| value.clone_value()).collect(),
+        }
+    }
+}
+
+/// A fixed-size list of reflected values.
+///
+/// This differs from [`DynamicList`] in that the size of the [`DynamicArray`]
+/// is constant, whereas a [`DynamicList`] can have items added and removed.
+///
+/// This isn't to say that a [`DynamicArray`] is immutable— its items
+/// can be mutated— just that the _number_ of items cannot change.
+///
+/// [`DynamicList`]: crate::DynamicList
+pub struct DynamicArray {
+    pub(crate) name: String,
+    pub(crate) values: Box<[Box<dyn Reflect>]>,
+}
+
+impl DynamicArray {
+    #[inline]
+    pub fn new(values: Box<[Box<dyn Reflect>]>) -> Self {
+        Self {
+            name: String::default(),
+            values,
+        }
+    }
+
+    pub fn from_vec<T: Reflect>(values: Vec<T>) -> Self {
+        Self {
+            name: String::default(),
+            values: values
+                .into_iter()
+                .map(|field| Box::new(field) as Box<dyn Reflect>)
+                .collect::<Vec<_>>()
+                .into_boxed_slice(),
+        }
+    }
+
+    #[inline]
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+
+    #[inline]
+    pub fn set_name(&mut self, name: String) {
+        self.name = name;
+    }
+}
+
+// SAFE: any and any_mut both return self
+unsafe impl Reflect for DynamicArray {
+    #[inline]
+    fn type_name(&self) -> &str {
+        self.name.as_str()
+    }
+
+    #[inline]
+    fn any(&self) -> &dyn Any {
+        self
+    }
+
+    #[inline]
+    fn any_mut(&mut self) -> &mut dyn Any {
+        self
+    }
+
+    #[inline]
+    fn as_reflect(&self) -> &dyn Reflect {
+        self
+    }
+
+    #[inline]
+    fn as_reflect_mut(&mut self) -> &mut dyn Reflect {
+        self
+    }
+
+    fn apply(&mut self, value: &dyn Reflect) {
+        array_apply(self, value);
+    }
+
+    #[inline]
+    fn set(&mut self, value: Box<dyn Reflect>) -> Result<(), Box<dyn Reflect>> {
+        *self = value.take()?;
+        Ok(())
+    }
+
+    #[inline]
+    fn reflect_ref(&self) -> ReflectRef {
+        ReflectRef::Array(self)
+    }
+
+    #[inline]
+    fn reflect_mut(&mut self) -> ReflectMut {
+        ReflectMut::Array(self)
+    }
+
+    #[inline]
+    fn clone_value(&self) -> Box<dyn Reflect> {
+        Box::new(self.clone_dynamic())
+    }
+
+    #[inline]
+    fn reflect_hash(&self) -> Option<u64> {
+        array_hash(self)
+    }
+
+    fn reflect_partial_eq(&self, value: &dyn Reflect) -> Option<bool> {
+        array_partial_eq(self, value)
+    }
+
+    fn serializable(&self) -> Option<Serializable> {
+        Some(Serializable::Borrowed(self))
+    }
+}
+
+impl Array for DynamicArray {
+    #[inline]
+    fn get(&self, index: usize) -> Option<&dyn Reflect> {
+        self.values.get(index).map(|value| &**value)
+    }
+
+    #[inline]
+    fn get_mut(&mut self, index: usize) -> Option<&mut dyn Reflect> {
+        self.values.get_mut(index).map(|value| &mut **value)
+    }
+
+    #[inline]
+    fn len(&self) -> usize {
+        self.values.len()
+    }
+
+    #[inline]
+    fn iter(&self) -> ArrayIter {
+        ArrayIter {
+            array: self,
+            index: 0,
+        }
+    }
+
+    #[inline]
+    fn clone_dynamic(&self) -> DynamicArray {
+        DynamicArray {
+            name: self.name.clone(),
+            values: self
+                .values
+                .iter()
+                .map(|value| value.clone_value())
+                .collect(),
+        }
+    }
+}
+
+/// An iterator over an [`Array`].
+pub struct ArrayIter<'a> {
+    pub(crate) array: &'a dyn Array,
+    pub(crate) index: usize,
+}
+
+impl<'a> Iterator for ArrayIter<'a> {
+    type Item = &'a dyn Reflect;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        let value = self.array.get(self.index);
+        self.index += 1;
+        value
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.array.len();
+        (size, Some(size))
+    }
+}
+
+impl<'a> ExactSizeIterator for ArrayIter<'a> {}
+
+impl<'a> serde::Serialize for dyn Array {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        array_serialize(self, serializer)
+    }
+}
+
+impl serde::Serialize for DynamicArray {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        array_serialize(self, serializer)
+    }
+}
+
+/// Serializes the given [array](Array).
+#[inline]
+pub fn array_serialize<A: Array + ?Sized, S>(array: &A, serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: serde::Serializer,
+{
+    let mut seq = serializer.serialize_seq(Some(array.len()))?;
+    for element in array.iter() {
+        let serializable = element.serializable().ok_or_else(|| {
+            serde::ser::Error::custom(format!(
+                "Type '{}' does not support `Reflect` serialization",
+                element.type_name()
+            ))
+        })?;
+        seq.serialize_element(serializable.borrow())?;
+    }
+    seq.end()
+}
+
+/// Returns the `u64` hash of the given [array](Array).
+#[inline]
+pub fn array_hash<A: Array>(array: &A) -> Option<u64> {
+    let mut hasher = crate::ReflectHasher::default();
+    std::any::Any::type_id(array).hash(&mut hasher);
+    array.len().hash(&mut hasher);
+    for value in array.iter() {
+        hasher.write_u64(value.reflect_hash()?)
+    }
+    Some(hasher.finish())
+}
+
+/// Applies the reflected [array](Array) data to the given [array](Array).
+///
+/// # Panics
+///
+/// * Panics if the two arrays have differing lengths.
+/// * Panics if the reflected value is not a [valid array](ReflectRef::Array).
+///
+#[inline]
+pub fn array_apply<A: Array>(array: &mut A, reflect: &dyn Reflect) {
+    if let ReflectRef::Array(reflect_array) = reflect.reflect_ref() {
+        if array.len() != reflect_array.len() {
+            panic!("Attempted to apply different sized `Array` types.");
+        }
+        for (i, value) in reflect_array.iter().enumerate() {
+            let v = array.get_mut(i).unwrap();
+            v.apply(value);
+        }
+    } else {
+        panic!("Attempted to apply a non-`Array` type to an `Array` type.");
+    }
+}
+
+/// Compares two [arrays](Array) (one concrete and one reflected) to see if they
+/// are equal.
+#[inline]
+pub fn array_partial_eq<A: Array>(array: &A, reflect: &dyn Reflect) -> Option<bool> {
+    match reflect.reflect_ref() {
+        ReflectRef::Array(reflect_array) if reflect_array.len() == array.len() => {
+            for (a, b) in array.iter().zip(reflect_array.iter()) {
+                if let Some(false) | None = a.reflect_partial_eq(b) {
+                    return Some(false);
+                }
+            }
+        }
+        _ => return Some(false),
+    }
+
+    Some(true)
+}

crates/bevy_reflect/src/impls/smallvec.rs

@@ -1,9 +1,11 @@
-use smallvec::{Array, SmallVec};
+use smallvec::SmallVec;
 use std::any::Any;
 
-use crate::{serde::Serializable, FromReflect, List, ListIter, Reflect, ReflectMut, ReflectRef};
+use crate::{
+    serde::Serializable, Array, ArrayIter, FromReflect, List, Reflect, ReflectMut, ReflectRef,
+};
 
-impl<T: Array + Send + Sync + 'static> List for SmallVec<T>
+impl<T: smallvec::Array + Send + Sync + 'static> Array for SmallVec<T>
 where
     T::Item: FromReflect + Clone,
 {
@@ -27,9 +29,21 @@ where
         <SmallVec<T>>::len(self)
     }
 
+    fn iter(&self) -> ArrayIter {
+        ArrayIter {
+            array: self,
+            index: 0,
+        }
+    }
+}
+
+impl<T: smallvec::Array + Send + Sync + 'static> List for SmallVec<T>
+where
+    T::Item: FromReflect + Clone,
+{
     fn push(&mut self, value: Box<dyn Reflect>) {
         let value = value.take::<T::Item>().unwrap_or_else(|value| {
-            <T as Array>::Item::from_reflect(&*value).unwrap_or_else(|| {
+            <T as smallvec::Array>::Item::from_reflect(&*value).unwrap_or_else(|| {
                 panic!(
                     "Attempted to push invalid value of type {}.",
                     value.type_name()
@@ -38,17 +52,10 @@ where
         });
         SmallVec::push(self, value);
     }
-
-    fn iter(&self) -> ListIter {
-        ListIter {
-            list: self,
-            index: 0,
-        }
-    }
 }
 
 // SAFE: any and any_mut both return self
-unsafe impl<T: Array + Send + Sync + 'static> Reflect for SmallVec<T>
+unsafe impl<T: smallvec::Array + Send + Sync + 'static> Reflect for SmallVec<T>
 where
     T::Item: FromReflect + Clone,
 {
@@ -90,7 +97,7 @@ where
     }
 
     fn clone_value(&self) -> Box<dyn Reflect> {
-        Box::new(self.clone_dynamic())
+        Box::new(List::clone_dynamic(self))
     }
 
     fn reflect_hash(&self) -> Option<u64> {
@@ -106,15 +113,15 @@ where
     }
 }
 
-impl<T: Array + Send + Sync + 'static> FromReflect for SmallVec<T>
+impl<T: smallvec::Array + Send + Sync + 'static> FromReflect for SmallVec<T>
 where
     T::Item: FromReflect + Clone,
 {
     fn from_reflect(reflect: &dyn Reflect) -> Option<Self> {
         if let ReflectRef::List(ref_list) = reflect.reflect_ref() {
             let mut new_list = Self::with_capacity(ref_list.len());
             for field in ref_list.iter() {
-                new_list.push(<T as Array>::Item::from_reflect(field)?);
+                new_list.push(<T as smallvec::Array>::Item::from_reflect(field)?);
             }
             Some(new_list)
         } else {