命令

可参考的官方例子： ecs_guide.

使用 Commands 来创建/销毁实体、现有实体上添加/删除组件、管理资源。

这些命令行为不会立即生效，它们被排在队列中，以便在接下来安全的时候执行。参见：阶段。

(如果你不使用阶段，这意味着你的其他 systems 将在下一帧更新时看到这些操作结果）。

fn spawn_player(
    mut commands: Commands,
) {
    // manage resources
    commands.insert_resource(GoalsReached { main_goal: false, bonus: false });
    commands.remove_resource::<MyResource>();

    // create a new entity using `spawn`
    let entity_id = commands.spawn()
        // add a component
        .insert(ComponentA)
        // add a bundle
        .insert_bundle(MyBundle::default())
        // get the Entity ID
        .id();

    // shorthand for creating an entity with a bundle
    commands.spawn_bundle(PlayerBundle {
        name: PlayerName("Henry".into()),
        xp: PlayerXp(1000),
        health: Health {
            hp: 100.0, extra: 20.0
        },
        _p: Player,
        sprite: Default::default(),
    });

    // spawn another entity
    // NOTE: tuples of arbitrary components are valid bundles
    let other = commands.spawn_bundle((
        ComponentA::default(),
        ComponentB::default(),
        ComponentC::default(),
    )).id();

    // add/remove components of an existing entity
    commands.entity(entity_id)
        .insert(ComponentB)
        .remove::<ComponentA>()
        .remove_bundle::<MyBundle>();

    // despawn an entity
    commands.entity(other).despawn();
}

fn make_all_players_hostile(
    mut commands: Commands,
    query: Query<Entity, With<Player>>,
) {
    for entity in query.iter() {
        // add an `Enemy` component to the entity
        commands.entity(entity).insert(Enemy);
    }
}