45 lines
1.5 KiB
Markdown
45 lines
1.5 KiB
Markdown
# B0002
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To keep [Rust rules on references](https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html#the-rules-of-references) (either one mutable reference or any number of immutable references) on a resource, it is not possible to have more than one resource of a kind if one is mutable in the same system. This can happen between [`Res<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.Res.html) and [`ResMut<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.ResMut.html) for the same `T`, or between [`NonSend<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.NonSend.html) and [`NonSendMut<T>`](https://docs.rs/bevy/*/bevy/ecs/system/struct.NonSendMut.html) for the same `T`.
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Erroneous code example:
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```rust,should_panic
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use bevy::prelude::*;
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fn update_materials(
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mut material_updater: ResMut<Assets<StandardMaterial>>,
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current_materials: Res<Assets<StandardMaterial>>,
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) {
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// ...
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}
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_systems(Update, update_materials)
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.run();
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}
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```
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This will panic, as it's not possible to have both a mutable and an immutable resource on `State` at the same time.
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As a mutable resource already provide access to the current resource value, you can remove the immutable resource.
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```rust,no_run
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use bevy::prelude::*;
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fn update_materials(
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mut material_updater: ResMut<Assets<StandardMaterial>>,
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) {
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// ...
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}
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fn main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_systems(Update, update_materials)
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.run();
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}
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```
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