Reading component of another source in a relationship

[Greu]

I have entities that can "follow" (move towards) other entities. To implement this behavior i was thinking about using the relationship features of flecs which looks awesome. I read the doc about Relationship and Flecs Query Language.

To query the components I'm using something similar to

SpaceShip($this), DockedTo($this, $planet), Planet($planet)

Here is my test

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

typedef struct
{
    double x, y;
} Position, Destination;

ECS_COMPONENT_DECLARE(Position);
ECS_COMPONENT_DECLARE(Destination);

ECS_TAG_DECLARE(Follow);

static void UpdateDestination(ecs_iter_t *it)
{
    Destination *destination = ecs_field(it, Destination, 0);
    const Position *position = ecs_field(it, Position, 2);

    for (int i = 0; i < it->count; i++)
    {
        destination[i] = position[i];
        assert(destination[i].x == 123);
        assert(destination[i].y == 456);
    }
}

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

    // World configuration
    ECS_COMPONENT_DEFINE(world, Position);
    ECS_COMPONENT_DEFINE(world, Destination);
    ECS_TAG_DEFINE(world, Follow);
    ECS_SYSTEM(world, UpdateDestination, EcsOnUpdate, [out] Destination($this), Follow($this, $followee), [in] Position($followee));

    // Entity prefab
    ecs_entity_t entityPrefab = ecs_entity(world, {.add = ecs_ids(EcsPrefab)});
    ecs_set(world, entityPrefab, Position, {0});
    ecs_set(world, entityPrefab, Destination, {0});

    // Followee
    ecs_entity_t followee = ecs_new_w_pair(world, EcsIsA, entityPrefab);
    ecs_set(world, followee, Position, {123, 456});

    // Follower
    ecs_entity_t follower1 = ecs_new_w_pair(world, EcsIsA, entityPrefab);
    ecs_add_pair(world, follower1, Follow, followee);
    ecs_entity_t follower2 = ecs_new_w_pair(world, EcsIsA, entityPrefab);
    ecs_add_pair(world, follower2, Follow, followee);

    ecs_progress(world, 0);
    ecs_fini(world);
}

I'm creating three entities, an entity followee that will be followed by follower1 and follower2.
I'm expecting to always have x=123 in the Destination component since the two followers are following the same followee with Position{123,456}.

In the system loop, for the first entity (i = 0) it is working, however the assert is failing for the second entity (i = 1, and above if i'm adding more followers).

What am i missing/doing wrong ?
Thank you.

[Greu]

After doing more tests I think I understand the behavior now.

What I was assuming:

• UpdateDestination is called once per frame
• count contains the amount of entities that match the query
• position[i] contains the Position component of the followee for the follower i

How it seems to work:

• UpdateDestination can be call multiple time per frame, once for each different followee (once for each different relationship subject)
• count contains the amount of entities that have the same followee (same relationship subject)
• position contains only one value

Understanding that allowed me to adjust my test and this is now working as intended:

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

typedef struct
{
    double x, y;
} Position, Destination;

ECS_COMPONENT_DECLARE(Position);
ECS_COMPONENT_DECLARE(Destination);

ECS_TAG_DECLARE(Follow);

static void UpdateDestination(ecs_iter_t *it)
{
    Destination *destination = ecs_field(it, Destination, 0);
    const Position followeePosition = *ecs_field(it, Position, 2);

    for (int i = 0; i < it->count; i++)
    {
        destination[i] = followeePosition ;
        assert(destination[i].x == 123);
        assert(destination[i].y == 456);
    }
}

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

    // World configuration
    ECS_COMPONENT_DEFINE(world, Position);
    ECS_COMPONENT_DEFINE(world, Destination);
    ECS_TAG_DEFINE(world, Follow);
    ECS_SYSTEM(world, UpdateDestination, EcsOnUpdate, [out] Destination($this), Follow($this, $followee), [in] Position($followee));

    // Entity prefab
    ecs_entity_t entityPrefab = ecs_entity(world, {.add = ecs_ids(EcsPrefab)});
    ecs_set(world, entityPrefab, Position, {0});
    ecs_set(world, entityPrefab, Destination, {0});

    // Followee
    ecs_entity_t followee = ecs_new_w_pair(world, EcsIsA, entityPrefab);
    ecs_set(world, followee, Position, {123, 456});

    // Follower
    ecs_entity_t follower1 = ecs_new_w_pair(world, EcsIsA, entityPrefab);
    ecs_add_pair(world, follower1, Follow, followee);
    ecs_entity_t follower2 = ecs_new_w_pair(world, EcsIsA, entityPrefab);
    ecs_add_pair(world, follower2, Follow, followee);

    ecs_progress(world, 0);
    ecs_fini(world);
}

[SanderMertens]

Yup that's how it works!