# Objective
Right now, `TypeInfo` can be accessed directly from a type using either
`Typed::type_info` or `Reflect::get_represented_type_info`.
However, once that `TypeInfo` is accessed, any nested types must be
accessed via the `TypeRegistry`.
```rust
#[derive(Reflect)]
struct Foo {
bar: usize
}
let registry = TypeRegistry::default();
let TypeInfo::Struct(type_info) = Foo::type_info() else {
panic!("expected struct info");
};
let field = type_info.field("bar").unwrap();
let field_info = registry.get_type_info(field.type_id()).unwrap();
assert!(field_info.is::<usize>());;
```
## Solution
Enable nested types within a `TypeInfo` to be retrieved directly.
```rust
#[derive(Reflect)]
struct Foo {
bar: usize
}
let TypeInfo::Struct(type_info) = Foo::type_info() else {
panic!("expected struct info");
};
let field = type_info.field("bar").unwrap();
let field_info = field.type_info().unwrap();
assert!(field_info.is::<usize>());;
```
The particular implementation was chosen for two reasons.
Firstly, we can't just store `TypeInfo` inside another `TypeInfo`
directly. This is because some types are recursive and would result in a
deadlock when trying to create the `TypeInfo` (i.e. it has to create the
`TypeInfo` before it can use it, but it also needs the `TypeInfo` before
it can create it). Therefore, we must instead store the function so it
can be retrieved lazily.
I had considered also using a `OnceLock` or something to lazily cache
the info, but I figured we can look into optimizations later. The API
should remain the same with or without the `OnceLock`.
Secondly, a new wrapper trait had to be introduced: `MaybeTyped`. Like
`RegisterForReflection`, this trait is `#[doc(hidden)]` and only exists
so that we can properly handle dynamic type fields without requiring
them to implement `Typed`. We don't want dynamic types to implement
`Typed` due to the fact that it would make the return type
`Option<&'static TypeInfo>` for all types even though only the dynamic
types ever need to return `None` (see #6971 for details).
Users should never have to interact with this trait as it has a blanket
impl for all `Typed` types. And `Typed` is automatically implemented
when deriving `Reflect` (as it is required).
The one downside is we do need to return `Option<&'static TypeInfo>`
from all these new methods so that we can handle the dynamic cases. If
we didn't have to, we'd be able to get rid of the `Option` entirely. But
I think that's an okay tradeoff for this one part of the API, and keeps
the other APIs intact.
## Testing
This PR contains tests to verify everything works as expected. You can
test locally by running:
```
cargo test --package bevy_reflect
```
---
## Changelog
### Public Changes
- Added `ArrayInfo::item_info` method
- Added `NamedField::type_info` method
- Added `UnnamedField::type_info` method
- Added `ListInfo::item_info` method
- Added `MapInfo::key_info` method
- Added `MapInfo::value_info` method
- All active fields now have a `Typed` bound (remember that this is
automatically satisfied for all types that derive `Reflect`)
### Internal Changes
- Added `MaybeTyped` trait
## Migration Guide
All active fields for reflected types (including lists, maps, tuples,
etc.), must implement `Typed`. For the majority of users this won't have
any visible impact.
However, users implementing `Reflect` manually may need to update their
types to implement `Typed` if they weren't already.
Additionally, custom dynamic types will need to implement the new hidden
`MaybeTyped` trait.
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