d1158288d5
# Objective - Fixes https://github.com/bevyengine/bevy/issues/8641 ## Solution - Update the B0003 docs to reflect the changes brought to Bevy with the release of ECS Schedule v3
109 lines
5.0 KiB
Markdown
109 lines
5.0 KiB
Markdown
# B0003
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As commands are executed asynchronously, it is possible to issue a command on an entity that will no longer exist at the time of the command execution.
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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 main() {
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App::new()
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.add_plugins(DefaultPlugins)
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.add_systems(Startup, setup)
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.add_systems(Update, despawning)
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.add_systems(Update, use_entity.after(despawning))
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.run();
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}
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#[derive(Resource)]
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struct MyEntity(Entity);
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#[derive(Component)]
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struct Hello;
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fn setup(mut commands: Commands) {
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let entity = commands.spawn_empty().id();
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commands.insert_resource(MyEntity(entity));
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}
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fn despawning(mut commands: Commands, entity: Option<Res<MyEntity>>) {
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if let Some(my_entity) = entity {
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commands.entity(my_entity.0).despawn();
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commands.remove_resource::<MyEntity>();
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}
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}
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fn use_entity(mut commands: Commands, entity: Option<Res<MyEntity>>) {
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if let Some(my_entity) = entity {
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commands.entity(my_entity.0).insert(Hello);
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}
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}
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```
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This will panic, as system `use_entity` is executed after system `despawning`. Without the system ordering specified here, the ordering would be random and this code would panic half the time.
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The default panic message is telling you the entity id (`1v0`) and the command that failed (adding a component `Hello`):
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```text
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thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
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```
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But you don't know which system tried to add a component, and which system despawned the entity.
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To get the system that created the command that panics, you can enable the `trace` feature of Bevy. This will add a panic handler that will print more information:
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```text
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0: bevy_ecs::system::commands::system_commands
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with name="use_entity_after_despawn::use_entity"
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at crates/bevy_ecs/src/system/commands/mod.rs:117
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1: bevy_ecs::world::schedule
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with name=Update
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at crates/bevy_ecs/src/world/mod.rs:1755
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2: bevy_ecs::schedule::executor::single_threaded::system
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with name="bevy_app::main_schedule::Main::run_main"
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at crates/bevy_ecs/src/schedule/executor/single_threaded.rs:98
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3: bevy_ecs::world::schedule
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with name=Main
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at crates/bevy_ecs/src/world/mod.rs:1755
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4: bevy_app::app::main app
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at crates/bevy_app/src/app.rs:249
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5: bevy_app::app::update
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at crates/bevy_app/src/app.rs:246
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6: bevy_winit::winit event_handler
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at crates/bevy_winit/src/lib.rs:338
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7: bevy_app::app::bevy_app
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at crates/bevy_app/src/app.rs:290
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thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
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```
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From the first two lines, you now know that it panics while executing a command from the system `use_entity`.
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To get the system that created the despawn command, you can enable DEBUG logs for crate `bevy_ecs`, for example by setting the environment variable `RUST_LOG=bevy_ecs=debug`. This will log:
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```text
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DEBUG schedule{name=Main}:system{name="bevy_app::main_schedule::Main::run_main"}:schedule{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world::entity_ref: Despawning entity 1v0
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thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
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```
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From the first line, you know the entity `1v0` was despawned when executing a command from system `despawning`. In a real case, you could have many log lines, you will need to search for the exact entity from the panic message.
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Combining those two, you should get enough information to understand why this panic is happening and how to fix it:
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```text
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DEBUG schedule{name=Main}:system{name="bevy_app::main_schedule::Main::run_main"}:schedule{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world::entity_ref: Despawning entity 1v0
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0: bevy_ecs::system::commands::system_commands
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with name="use_entity_after_despawn::use_entity"
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at crates/bevy_ecs/src/system/commands/mod.rs:117
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1: bevy_ecs::world::schedule
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with name=Update
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at crates/bevy_ecs/src/world/mod.rs:1755
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2: bevy_ecs::schedule::executor::single_threaded::system
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with name="bevy_app::main_schedule::Main::run_main"
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at crates/bevy_ecs/src/schedule/executor/single_threaded.rs:98
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3: bevy_ecs::world::schedule
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with name=Main
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at crates/bevy_ecs/src/world/mod.rs:1755
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thread 'main' panicked at 'error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 1v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:934:13
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```
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