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# Objective
There's a lot of code duplication in `access.rs`. The same logic is
duplicated between components and resources. This also takes up
unnecessary memory in `Access`, as it relies on bitsets spanning the
entire `ComponentId` range.
## Solution
Since resources are now a special kind of component, this can be
removed.
## Limitations
Since `!Send` data queries used `Access` resources, `!Send` data queries
now conflict with broad queries.
```rust
// 0.18
fn system(q1_: Query<EntityMut>, q2_: NonSend<R>) {} // valid, does not conflict
// 0.19
fn system(q1_: Query<EntityMut>, q2_: NonSend<R>) {} // invalid, does conflict
```
Given how rarely non-send data is used, I recommend using
```
// 0.19
fn system(q1_: Query<EntityMut, Without<R>>, q2_: NonSend<R>) {} // works again
```
If this is also unacceptable, this PR is blocked on the `!Send` data
removal from the ECS (or some hacky workaround).
## Extra Attention
@chescock brought `AssetChanged` to my attention. It has a weird access
pattern. See the following example:
```rust
fn system(c: Query<&mut AssetChanges<Mesh>>, r: Query<(), AssetChanged<Mesh>>) {}
```
System `c` registers access with `add_write` for `AssetChanges<Mesh>`,
while `r` registers access with `add_read` for both `Mesh` and
`AssetChanges<Mesh>`. This system is invalid, and I've added a test to
reflect that. However, since this stuff is tricky, I would like some
extra eyes on it. Currently, it looks *fine*.
96 lines
2.4 KiB
Markdown
96 lines
2.4 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 requests 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 for 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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## Note
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If you run this error while using resources, check out [B0001](https://bevy.org/learn/errors/b0002) for additional information.
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