# Objective
There's currently no way to iterate through all the type data in a
`TypeRegistration`. While these are all type-erased, it can still be
useful to see what types (by `TypeId`) are registered for a given type.
Additionally, it might be good to have ways of dynamically working with
`TypeRegistration`.
## Solution
Added a way to iterate through all type data on a given
`TypeRegistration`. This PR also adds methods for working with type data
dynamically as well as methods for conveniently checking if a given type
data exists on the registration.
I also took this opportunity to reorganize the methods on
`TypeRegistration` as it has always bothered me haha (i.e. the
constructor not being at the top, etc.).
## Testing
You can test locally by running:
```
cargo test --package bevy_reflect
```
---
## Showcase
The type-erased type data on a `TypeRegistration` can now be iterated!
```rust
#[derive(Reflect)]
struct Foo;
#[derive(Clone)]
struct DataA(i32);
#[derive(Clone)]
struct DataB(i32);
let mut registration = TypeRegistration::of::<Foo>();
registration.insert(DataA(123));
registration.insert(DataB(456));
let mut iter = registration.iter();
let (id, data) = iter.next().unwrap();
assert_eq!(id, TypeId::of::<DataA>());
assert_eq!(data.downcast_ref::<DataA>().unwrap().0, 123);
let (id, data) = iter.next().unwrap();
assert_eq!(id, TypeId::of::<DataB>());
assert_eq!(data.downcast_ref::<DataB>().unwrap().0, 456);
assert!(iter.next().is_none());
```
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