d4ffd4ff28
# Objective - Example in error B0003 is not failing anymore after #9822 ## Solution - Update the example code so that is always fail - Also update logs and instructions on how to debug as it's easier now --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
63 lines
2.7 KiB
Markdown
63 lines
2.7 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, (use_0_and_despawn_1, use_1_and_despawn_0))
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.run();
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}
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#[derive(Resource)]
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struct MyEntities(Entity, 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 entity1 = commands.spawn_empty().id();
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let entity2 = commands.spawn_empty().id();
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commands.insert_resource(MyEntities(entity1, entity2));
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}
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fn use_0_and_despawn_1(mut commands: Commands, my_entities: Res<MyEntities>) {
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commands.entity(my_entities.0).insert(Hello);
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commands.entity(my_entities.1).despawn();
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}
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fn use_1_and_despawn_0(mut commands: Commands, my_entities: Res<MyEntities>) {
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commands.entity(my_entities.1).insert(Hello);
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commands.entity(my_entities.0).despawn();
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}
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```
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This will panic, as the system that is executed first will despawn the entity used by the second.
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The default panic message is telling you which entity doesn't exist (`2v0` in the example log just below), the command that failed (adding a component `Hello`) and the system from which it originated (`use_1_and_despawn_0`):
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```text
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thread 'main' panicked at /bevy/crates/bevy_ecs/src/system/commands/mod.rs:1097:13:
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error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 2v0 because it doesn't exist in this World.
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Encountered a panic when applying buffers for system `use_entity_after_despawn::use_1_and_despawn_0`!
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Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
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```
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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` or by configuring the `LogPlugin`. This will log:
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```text
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DEBUG system_commands{name="use_entity_after_despawn::use_0_and_despawn_1"}: bevy_ecs::world::entity_ref: Despawning entity 2v0
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thread 'main' panicked at /bevy/crates/bevy_ecs/src/system/commands/mod.rs:1097:13:
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error[B0003]: Could not insert a bundle (of type `use_entity_after_despawn::Hello`) for entity 2v0 because it doesn't exist in this World.
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Encountered a panic when applying buffers for system `use_entity_after_despawn::use_1_and_despawn_0`!
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Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
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```
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From the first line, you know the entity `2v0` was despawned when executing a command from system `use_0_and_despawn_1`. 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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