B0003

由于命令是异步执行的，因此有可能在命令执行时，对不再存在的实体发出命令。

错误代码示例

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, (use_0_and_despawn_1, use_1_and_despawn_0))
        .run();
}

#[derive(Resource)]
struct MyEntities(Entity, Entity);

#[derive(Component)]
struct Hello;

fn setup(mut commands: Commands) {
    let entity1 = commands.spawn_empty().id();
    let entity2 = commands.spawn_empty().id();
    commands.insert_resource(MyEntities(entity1, entity2));
}

fn use_0_and_despawn_1(mut commands: Commands, my_entities: Res<MyEntities>) {
    commands.entity(my_entities.0).insert(Hello);
    commands.entity(my_entities.1).despawn();
}

fn use_1_and_despawn_0(mut commands: Commands, my_entities: Res<MyEntities>) {
    commands.entity(my_entities.1).insert(Hello);
    commands.entity(my_entities.0).despawn();
}

这会导致恐慌，因为第一个执行的系统将取消分配第二个系统使用的实体。

默认的恐慌消息会告诉你哪个实体不存在（示例日志中 2v0 ），哪个命令失败了（添加 Hello 组件），以及它来自哪个系统（use_1_and_despawn_0）。

thread 'main' panicked at /bevy/crates/bevy_ecs/src/system/commands/mod.rs:1097:13:
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.
Encountered a panic when applying buffers for system `use_entity_after_despawn::use_1_and_despawn_0`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!

要获取创建取消分配命令的系统，你可以为 bevy_ecs 板条箱启用 DEBUG 日志，例如通过设置环境变量 RUST_LOG=bevy_ecs=debug 或配置 LogPlugin。这将记录

DEBUG system_commands{name="use_entity_after_despawn::use_0_and_despawn_1"}: bevy_ecs::world::entity_ref: Despawning entity 2v0
thread 'main' panicked at /bevy/crates/bevy_ecs/src/system/commands/mod.rs:1097:13:
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.
Encountered a panic when applying buffers for system `use_entity_after_despawn::use_1_and_despawn_0`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!

从第一行，你可以知道实体 2v0 在从系统 use_0_and_despawn_1 执行命令时被取消分配了。在实际情况下，你可能会有很多日志行，你需要从恐慌消息中搜索确切的实体。