Dynamic types (`DynamicStruct`, `DynamicTupleStruct`, `DynamicTuple`, `DynamicList` and `DynamicMap`) are used when deserializing scenes, but currently they can only be applied to existing concrete types. This leads to issues when trying to spawn non trivial deserialized scene.
For components, the issue is avoided by requiring that reflected components implement ~~`FromResources`~~ `FromWorld` (or `Default`). When spawning, a new concrete type is created that way, and the dynamic type is applied to it. Unfortunately, some components don't have any valid implementation of these traits.
In addition, any `Vec` or `HashMap` inside a component will panic when a dynamic type is pushed into it (for instance, `Text` panics when adding a text section).
To solve this issue, this PR adds the `FromReflect` trait that creates a concrete type from a dynamic type that represent it, derives the trait alongside the `Reflect` trait, drops the ~~`FromResources`~~ `FromWorld` requirement on reflected components, ~~and enables reflection for UI and Text bundles~~. It also adds the requirement that fields ignored with `#[reflect(ignore)]` implement `Default`, since we need to initialize them somehow.
Co-authored-by: Carter Anderson <mcanders1@gmail.com>
After #1697 I looked at all other Iterators from Bevy and added overrides for `size_hint` where it wasn't done.
Also implemented `ExactSizeIterator` where applicable.
Fixes#1100
Implementors must make sure that `Reflect::any` and `Reflect::any_mut` both return the `self` reference passed in (both for logical correctness and downcast safety).
