Entity.IsZero or world.IsAlive

[KudaCoder]

Wondering if someone could tell me the individual use case of these entity checking methods? I'm currently using IsZero to check an entity returned from a Map.GetRelation call to ensure that the entity wasn't previously deleted by another system and I'm wondering if there is a scenario that I'm missing where IsAlive would also be applicable in this use case? Or maybe IsAlive is there only check I should be doing?

Any feedback would be much appreciated

[mlange-42]

Hi @KudaCoder,
thank you for reaching out!

These two functions are quite different and serve different purposes. In your case, both would work, but IsZero should be preferred. When an entity that is the target of a relation is deleted, all entities that have a relation "pointing" to it are moved into another table, with the "zero entity" as the new target. As the zero entity is never alive, IsAlive would also work here, but IsZero is more clear.

What you should note is that the zero entity is the zero value of the Entity type and is reserved for denoting "no entity", or an uninitialized Entity variable. The world will never return the zero entity on entity creation, nor will any entity that was obtained from the world ever turn into a zero entity. You can create a zero entity in these two ways:

var entity Entity   // Uninitialized variable
entity := Entity{}  // Explicit zero entity

The IsAlive check is for cases where you store an entity (e.g. in a grid, or in a component of another entity, or in any persistent variable) and want to check whether it was removed/deleted from the world meanwhile. If an entity gets deleted, the state (i.e. ID and generation) of your stored entity does not change (sure, you store by value, not by pointer, so it can't). But due to the generation counter of the entity, the world can tell that it is no longer alive, even if the ID gets re-used in a newly created entity. Further, any operations on a dead entity will panic with a message like "can't get components of a dead entity" or similar.

Hope this somewhat lengthy explanation clarifies you question.

[KudaCoder]

Hi Martin,

Thanks so much for the clarification. This is exactly what I needed to know.

Really loving using Ark ECS by the way. Keep up the good work dude

Cheers