mirror of
https://github.com/bevyengine/bevy
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91 lines
2.2 KiB
Markdown
91 lines
2.2 KiB
Markdown
# B0001
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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 component, it is not possible to have two queries on the same component when one request mutable access to it in the same system.
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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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#[derive(Component)]
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struct Player;
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#[derive(Component)]
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struct Enemy;
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fn move_enemies_to_player(
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mut enemies: Query<&mut Transform, With<Enemy>>,
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player: Query<&Transform, With<Player>>,
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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, move_enemies_to_player)
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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 query on `Transform` at the same time.
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You have two solutions:
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Solution #1: use disjoint queries using [`Without`](https://docs.rs/bevy/*/bevy/ecs/query/struct.Without.html)
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As a `Player` entity won't be an `Enemy` at the same time, those two queries will actually never target the same entity. This can be encoded in the query filter with [`Without`](https://docs.rs/bevy/*/bevy/ecs/query/struct.Without.html):
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```rust,no_run
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use bevy::prelude::*;
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#[derive(Component)]
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struct Player;
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#[derive(Component)]
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struct Enemy;
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fn move_enemies_to_player(
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mut enemies: Query<&mut Transform, With<Enemy>>,
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player: Query<&Transform, (With<Player>, Without<Enemy>)>,
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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, move_enemies_to_player)
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.run();
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}
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```
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Solution #2: use a `ParamSet`
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A `ParamSet` will let you have conflicting queries as a parameter, but you will still be responsible of not using them at the same time in your system.
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```rust,no_run
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use bevy::prelude::*;
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#[derive(Component)]
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struct Player;
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#[derive(Component)]
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struct Enemy;
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fn move_enemies_to_player(
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mut transforms: ParamSet<(
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Query<&mut Transform, With<Enemy>>,
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Query<&Transform, With<Player>>,
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)>,
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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, move_enemies_to_player)
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.run();
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}
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```
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