Add FromReflect::from_reflect_owned

[Shatur]

What problem does this solve or what need does it fill?

Currently to apply a reflect value that was deserialized from bytes it copies all its fields. It would be nice to have an ability avoid this extra copy. For example, when doing networking and sending world state in a form of reflect - it could be a lot of extra copies.

What solution would you like?

Add from_reflect_owned to FromReflect trait.

Implementation strategy.

1. Add ReflectOwned enum with boxes like ReflectRef to get owned version.
2. Add reflect_owned method to Reflect like reflect_ref that returns ReflectOwned introduced above.
3. Use drain method from [Merged by Bors] - bevy_reflect: Get owned fields #5728 for arrays / maps and add owned versions of things like Struct::field.
4. Use all of the above to implement from_reflect_owned similar to non-owned version:
   

   bevy/crates/bevy_reflect/bevy_reflect_derive/src/from_reflect.rs

   Lines 132 to 137 in aa8c745

   	fn from_reflect(reflect: &dyn #bevy_reflect_path::Reflect) -> Option<Self> {
   	if let #bevy_reflect_path::ReflectRef::#ref_struct_type(#ref_struct) = reflect.reflect_ref() {
   	#constructor
   	} else {
   	None
   	}

[alice-i-cecile]

Great writeup! This is a bit more advanced than most good-first-issues, but the detailed plan and existing methods to learn from should help a ton (assuming you're not planning to tackle this yourself).

[Shatur]

I think I could try to tackle if people like this suggestion.

[alice-i-cecile]

I like it a lot! The basic motivation is solid and the approach seems good.

[Shatur]

Having an issue with implementing from_reflect_owned for structs. DynamicStruct stores its elements inside Vec:

bevy/crates/bevy_reflect/src/struct_trait.rs

Line 261 in aa8c745

fields: Vec<Box<dyn Reflect>>,

Non-owned version uses acess by index or field name:

bevy/crates/bevy_reflect/src/struct_trait.rs

Lines 262 to 263 in aa8c745

	field_names: Vec<Cow<'static, str>>,
	field_indices: HashMap<Cow<'static, str>, usize>,

But in owned version I need to move out values somehow. Can't figure out how to do it, so help is appreciated (just take a look at todo!() in #6421).

[Shatur]

No longer interested in it, closing.