I am trying to implement a very basic physics system, specifically gravity, in C. I have structured the individual components as follows:

typedef struct Gravity {
  float force[2];
} Gravity;

typedef struct Rigidbody {
  float velocity2];
  float acceleration[2];
} Rigidbody;

typedef struct Transform {
  float position[2];
  float rotation[2];
  float scale[2];
} Transform;

In my update function I iterate through all living entities that have those three components then directly update the array values for that specific entity:

void updatePhysics(ECS* ecs, float delta) {
  if (!ecs)
    return;

  for (Entity entity = 0; entity < (ecs -> entities -> count); entity  ) {
    if (isLiving(ecs, entity)) {
      Gravity* gravity = getComponent(ecs, entity, GRAVITY);
      Rigidbody* rigidbody = getComponent(ecs, entity, RIGIDBODY);
      Transform* transform = getComponent(ecs, entity, RIGIDBODY);

      if ((gravity == NULL) || (rigidbody == NULL) || (transform == NULL))
        continue;

      printf("Update Method Check, Position: (%.1f, %.1f)\n", (transform -> position)[0], (transform -> position)[1]);
      for (int itr = 0; itr < 2; itr  ) {
        (gravity -> force)[itr] *= delta;
        (rigidbody -> velocity)[itr] *= delta;

        (transform -> position)[itr]  = (gravity -> force)[itr];
        (transform -> position)[itr]  = (rigidbody -> velocity)[itr];
      }
    }
  }
}

I created a test entity with the required components and check it's current position in my main game loop:

static void frame(void) {
  if (keypress(KEY_ESCAPE))
    app.quit = true;

  Transform* transform = getComponent(app.ecs, player, TRANSFORM);
  printf("Main Game Loop, Position: (%.1f, %.1f)\n\n", (transform -> position)[0], (transform -> position)[1]);
}

Everything should work, but the position of the entity doesn't update globally, only locally in the physics update function. The output of the print statements:

Update Method Check, Position: (0.0, 0.0)
Main Game Loop, Position: (0.0, 0.0)

Update Method Check, Position: (0.0, -10.0)
Main Game Loop, Position: (0.0, 0.0)

Update Method Check, Position: (0.0, -30.0)
Main Game Loop, Position: (0.0, 0.0)

Update Method Check, Position: (0.0, -70.0)
Main Game Loop, Position: (0.0, 0.0)

Update Method Check, Position: (0.0, -150.0)
Main Game Loop, Position: (0.0, 0.0)

...

What am I missing that's preventing the entity's position from being updated globally?

Solution: There was an error in my getComponent() method where I was getting a Rigidbody component instead of a Transform component. Made the change to the proper component type and the bug is fixed.

Transform* transform = getComponent(ecs, entity, TRANSFORM);

CodePudding user response：

getComponent() doesn't return the same pointer in updatePhysics() and your test runner. Either because of defect in getComponent() (not shared with us), or it is being called with different arguments.