Resources as Components

Resources are very similar to Components: they are both data that can be stored in the ECS and queried. The only real difference between them is that querying a resource will return either one or zero resources, whereas querying for a component can return any number of matching entities.

Even so, resources and components have always been separate concepts within the ECS. This leads to some annoying restrictions. While components have ComponentHooks, it's not possible to add lifecycle hooks to resources. The same is true for relations, observers, and a host of other concepts that already exist for components. Moreover, the engine internals contain a lot of duplication because of it.

This first implementation is limited, only enabling observers for resources.

#[derive(Resource)]
struct GlobalSetting;

fn on_add_setting(add: On<Add, GlobalSetting>, query: Query<&LevelSetting>) {
    // ...
}

The main drawbacks are twofold. First it's no longer to derive both Component and Resource for a struct. Secondly ReflectResource has been gutted, use ReflectComponent instead. For more information, see the migration guide.