What is the recommended way to create new entities from a system ?

[Greu]

Hello, I recently started to play with Flecs and I have a question regarding the creation of entities.

I have a "Task" system that is responsible for spawning some enemies.
This system is creating new entities and trying to set the values of their Position component.

void SpawnEnemies(ecs_iter_t *it)
{
    ...
    ecs_entity_t enemy = ecs_new_w_pair(it->world, EcsIsA, enemyPrefab);
    ecs_set(it->world, enemy, Position, {x, y});
    ....
}

When executing, I'm getting the following error
fatal: entity.c: 3624: assert: ecs_id_is_valid(world, id) INVALID_PARAMETER

From what I understand, it's because the entity is not created directly (because the world is in readonly) and when I'm trying to set the component values, the entity id not considered valid.

Is it a correct pattern to create entity and set component values from inside a system ?
How can I do that properly ? Should I make my system "Immediate" ? Should I pre create entity ids before ?

Thank you.

[SanderMertens]

This should work, as long as the system is not multi threaded.

[Greu]

Thanks for you answer.

I tried to switch the system to immediate and it didn't work.
I tried with ecs_defer_suspend/resume and it didn't work.

After checking the component id in my system i realized the problem was somewhere else. It was coming from the way I defined the Position component. I declared it with ECS_COMPONENT_DECLARE(Position) before my system function. Then I defined it later in my main function using ECS_COMPONENT(world, Position);

Replacing ECS_COMPONENT(world, Position); by ECS_COMPONENT_DEFINE(world, Position); fixed the issue.

Here was my test

#include <stdio.h>
#include "flecs.h"

typedef struct
{
    double x, y;
} Position;

ECS_COMPONENT_DECLARE(Position);

static ecs_entity_t enemyPrefab;

static void EntitySpawner(ecs_iter_t *it)
{
    ecs_world_t *world = it->world;

    ecs_entity_t enemy1 = ecs_new_w_pair(world, EcsIsA, enemyPrefab);
    ecs_set(world, enemy1, Position, {100, 100});
}

int main(int argc, char *argv[])
{
    ecs_world_t *world = ecs_init_w_args(argc, argv);

    ECS_COMPONENT(world, Position); // Issue fixed by replacing with ECS_COMPONENT_DEFINE(world, Position);

    enemyPrefab = ecs_entity(world, {.add = ecs_ids(EcsPrefab)});
    ecs_set(world, enemyPrefab, Position, {});

    ecs_system(world, {
                          .entity = ecs_entity(world, {.add = ecs_ids(ecs_dependson(EcsOnUpdate))}),
                          .callback = EntitySpawner,
                      });

    ecs_progress(world, 0);

    ecs_fini(world);
}

[SanderMertens]

Ah! That's right, you have to use ECS_COMPONENT_DEFINE when using ECS_COMPONENT_DECLARE. I'll mark this as answered.