992681b59b
*This PR description is an edited copy of #5007, written by @alice-i-cecile.* # Objective Follow-up to https://github.com/bevyengine/bevy/pull/2254. The `Resource` trait currently has a blanket implementation for all types that meet its bounds. While ergonomic, this results in several drawbacks: * it is possible to make confusing, silent mistakes such as inserting a function pointer (Foo) rather than a value (Foo::Bar) as a resource * it is challenging to discover if a type is intended to be used as a resource * we cannot later add customization options (see the [RFC](https://github.com/bevyengine/rfcs/blob/main/rfcs/27-derive-component.md) for the equivalent choice for Component). * dependencies can use the same Rust type as a resource in invisibly conflicting ways * raw Rust types used as resources cannot preserve privacy appropriately, as anyone able to access that type can read and write to internal values * we cannot capture a definitive list of possible resources to display to users in an editor ## Notes to reviewers * Review this commit-by-commit; there's effectively no back-tracking and there's a lot of churn in some of these commits. *ira: My commits are not as well organized :')* * I've relaxed the bound on Local to Send + Sync + 'static: I don't think these concerns apply there, so this can keep things simple. Storing e.g. a u32 in a Local is fine, because there's a variable name attached explaining what it does. * I think this is a bad place for the Resource trait to live, but I've left it in place to make reviewing easier. IMO that's best tackled with https://github.com/bevyengine/bevy/issues/4981. ## Changelog `Resource` is no longer automatically implemented for all matching types. Instead, use the new `#[derive(Resource)]` macro. ## Migration Guide Add `#[derive(Resource)]` to all types you are using as a resource. If you are using a third party type as a resource, wrap it in a tuple struct to bypass orphan rules. Consider deriving `Deref` and `DerefMut` to improve ergonomics. `ClearColor` no longer implements `Component`. Using `ClearColor` as a component in 0.8 did nothing. Use the `ClearColorConfig` in the `Camera3d` and `Camera2d` components instead. Co-authored-by: Alice <alice.i.cecile@gmail.com> Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> Co-authored-by: devil-ira <justthecooldude@gmail.com> Co-authored-by: Carter Anderson <mcanders1@gmail.com>
63 lines
2.1 KiB
Rust
63 lines
2.1 KiB
Rust
//! Allows reflection with trait objects.
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use bevy::prelude::*;
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_startup_system(setup)
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.register_type::<MyType>()
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.run();
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}
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#[derive(Reflect)]
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#[reflect(DoThing)]
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struct MyType {
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value: String,
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}
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impl DoThing for MyType {
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fn do_thing(&self) -> String {
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format!("{} World!", self.value)
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}
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}
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#[reflect_trait]
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pub trait DoThing {
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fn do_thing(&self) -> String;
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}
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fn setup(type_registry: Res<AppTypeRegistry>) {
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// First, lets box our type as a Box<dyn Reflect>
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let reflect_value: Box<dyn Reflect> = Box::new(MyType {
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value: "Hello".to_string(),
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});
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// This means we no longer have direct access to MyType or its methods. We can only call Reflect
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// methods on reflect_value. What if we want to call `do_thing` on our type? We could
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// downcast using reflect_value.downcast_ref::<MyType>(), but what if we don't know the type
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// at compile time?
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// Normally in rust we would be out of luck at this point. Lets use our new reflection powers to
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// do something cool!
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let type_registry = type_registry.read();
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// The #[reflect] attribute we put on our DoThing trait generated a new `ReflectDoThing` struct,
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// which implements TypeData. This was added to MyType's TypeRegistration.
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let reflect_do_thing = type_registry
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.get_type_data::<ReflectDoThing>(reflect_value.type_id())
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.unwrap();
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// We can use this generated type to convert our `&dyn Reflect` reference to a `&dyn DoThing`
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// reference
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let my_trait: &dyn DoThing = reflect_do_thing.get(&*reflect_value).unwrap();
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// Which means we can now call do_thing(). Magic!
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info!("{}", my_trait.do_thing());
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// This works because the #[reflect(MyTrait)] we put on MyType informed the Reflect derive to
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// insert a new instance of ReflectDoThing into MyType's registration. The instance knows
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// how to cast &dyn Reflect to &dyn MyType, because it knows that &dyn Reflect should first
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// be downcasted to &MyType, which can then be safely casted to &dyn MyType
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}
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