bevy/crates/bevy_reflect
Gino Valente 53910e07ae
bevy_reflect: Improve reflection serialization error messages (#13867)
# Objective

The error messages that appear when a value cannot be serialized or
deserialized via reflection could be slightly improved.

When one of these operations fails, some users are confused about how to
resolve the issue. I've spoken with a few who didn't know they could
register `ReflectSerialize` themselves. We should try to clarify this to
some degree in the error messages.

## Solution

Add some more detail to the error messages.

For example, replacing this:

```
Type 'core::ops::RangeInclusive<f32>' did not register ReflectSerialize
```

with this:

```
Type `core::ops::RangeInclusive<f32>` did not register the `ReflectSerialize` type data. For certain types, this may need to be registered manually using `register_type_data`
```

I also added a separate error message if the type was not registered in
the type registry at all:

```
Type `core::ops::RangeInclusive<f32>` is not registered in the type registry
```

## Testing

You can test locally by running:

```
cargo test --package bevy_reflect
```

---

## Changelog

- Added error message for missing type registration when serializing
reflect data
- Changed error message for missing `ReflectSerialize` registration when
serializing reflect data
- Changed error message for missing `ReflectDeserialize` registration
when deserializing reflect data
2024-06-17 18:13:46 +00:00
..
compile_fail Add on_unimplemented Diagnostics to Most Public Traits (#13347) (#13662) 2024-06-04 00:31:34 +00:00
derive bevy_reflect: enum_utility cleanup (#13424) 2024-05-22 21:18:57 +00:00
examples fix nightly clippy warnings (#6395) 2022-10-28 21:03:01 +00:00
src bevy_reflect: Improve reflection serialization error messages (#13867) 2024-06-17 18:13:46 +00:00
Cargo.toml Move bevy_math Reflect impls (#13520) 2024-05-27 14:15:22 +00:00
README.md add and fix shields in Readmes (#9993) 2023-10-15 00:52:31 +00:00

Bevy Reflect

License Crates.io Downloads Docs Discord

This crate enables you to dynamically interact with Rust types:

  • Derive the Reflect traits
  • Interact with fields using their names (for named structs) or indices (for tuple structs)
  • "Patch" your types with new values
  • Look up nested fields using "path strings"
  • Iterate over struct fields
  • Automatically serialize and deserialize via Serde (without explicit serde impls)
  • Trait "reflection"

Features

Derive the Reflect traits

// this will automatically implement the Reflect trait and the Struct trait (because the type is a struct)
#[derive(Reflect)]
struct Foo {
    a: u32,
    b: Bar,
    c: Vec<i32>,
    d: Vec<Baz>,
}

// this will automatically implement the Reflect trait and the TupleStruct trait (because the type is a tuple struct)
#[derive(Reflect)]
struct Bar(String);

#[derive(Reflect)]
struct Baz {
    value: f32,
}

// We will use this value to illustrate `bevy_reflect` features
let mut foo = Foo {
    a: 1,
    b: Bar("hello".to_string()),
    c: vec![1, 2],
    d: vec![Baz { value: 3.14 }],
};

Interact with fields using their names

assert_eq!(*foo.get_field::<u32>("a").unwrap(), 1);

*foo.get_field_mut::<u32>("a").unwrap() = 2;

assert_eq!(foo.a, 2);

"Patch" your types with new values

let mut dynamic_struct = DynamicStruct::default();
dynamic_struct.insert("a", 42u32);
dynamic_struct.insert("c", vec![3, 4, 5]);

foo.apply(&dynamic_struct);

assert_eq!(foo.a, 42);
assert_eq!(foo.c, vec![3, 4, 5]);

Look up nested fields using "path strings"

let value = *foo.get_path::<f32>("d[0].value").unwrap();
assert_eq!(value, 3.14);

Iterate over struct fields

for (i, value: &Reflect) in foo.iter_fields().enumerate() {
    let field_name = foo.name_at(i).unwrap();
    if let Some(value) = value.downcast_ref::<u32>() {
        println!("{} is a u32 with the value: {}", field_name, *value);
    }
}

Automatically serialize and deserialize via Serde (without explicit serde impls)

let mut registry = TypeRegistry::default();
registry.register::<u32>();
registry.register::<i32>();
registry.register::<f32>();
registry.register::<String>();
registry.register::<Bar>();
registry.register::<Baz>();

let serializer = ReflectSerializer::new(&foo, &registry);
let serialized = ron::ser::to_string_pretty(&serializer, ron::ser::PrettyConfig::default()).unwrap();

let mut deserializer = ron::de::Deserializer::from_str(&serialized).unwrap();
let reflect_deserializer = ReflectDeserializer::new(&registry);
let value = reflect_deserializer.deserialize(&mut deserializer).unwrap();
let dynamic_struct = value.take::<DynamicStruct>().unwrap();

assert!(foo.reflect_partial_eq(&dynamic_struct).unwrap());

Trait "reflection"

Call a trait on a given &dyn Reflect reference without knowing the underlying type!

#[derive(Reflect)]
#[reflect(DoThing)]
struct MyType {
    value: String,
}

impl DoThing for MyType {
    fn do_thing(&self) -> String {
        format!("{} World!", self.value)
    }
}

#[reflect_trait]
pub trait DoThing {
    fn do_thing(&self) -> String;
}

// First, lets box our type as a Box<dyn Reflect>
let reflect_value: Box<dyn Reflect> = Box::new(MyType {
    value: "Hello".to_string(),
});

// This means we no longer have direct access to MyType or its methods. We can only call Reflect methods on reflect_value.
// What if we want to call `do_thing` on our type? We could downcast using reflect_value.downcast_ref::<MyType>(), but what if we
// don't know the type at compile time?

// Normally in rust we would be out of luck at this point. Lets use our new reflection powers to do something cool!
let mut type_registry = TypeRegistry::default();
type_registry.register::<MyType>();

// The #[reflect] attribute we put on our DoThing trait generated a new `ReflectDoThing` struct, which implements TypeData.
// This was added to MyType's TypeRegistration.
let reflect_do_thing = type_registry
    .get_type_data::<ReflectDoThing>(reflect_value.type_id())
    .unwrap();

// We can use this generated type to convert our `&dyn Reflect` reference to a `&dyn DoThing` reference
let my_trait: &dyn DoThing = reflect_do_thing.get(&*reflect_value).unwrap();

// Which means we can now call do_thing(). Magic!
println!("{}", my_trait.do_thing());

// This works because the #[reflect(MyTrait)] we put on MyType informed the Reflect derive to insert a new instance
// of ReflectDoThing into MyType's registration. The instance knows how to cast &dyn Reflect to &dyn DoThing, because it
// knows that &dyn Reflect should first be downcasted to &MyType, which can then be safely casted to &dyn DoThing

Why make this?

The whole point of Rust is static safety! Why build something that makes it easy to throw it all away?

  • Some problems are inherently dynamic (scripting, some types of serialization / deserialization)
  • Sometimes the dynamic way is easier
  • Sometimes the dynamic way puts less burden on your users to derive a bunch of traits (this was a big motivator for the Bevy project)