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Entity propagation

· 2 min read

This post explains entity propagation behavior in umg.

in UMG, deletion and cloning are propagated to other entities through components.

For example: = otherEnt
-- `ent` references `otherEnt` through component foo

-- otherEnt is cloned too...

-- otherEnt is deleted too!

This concept is known as "propagation".
But... why are the clone and delete operations propagated?

Consider the following scenario:

-- Save and delete the ent:
local data = serialize(ent)

-- Now load the saved ent:
local ent = load(data)

This operation could happen when we are saving a world.

The big question is: what should happen to otherEnt?
It obviously needs to be saved alongside ent... Or else ent will lose component data.

What about deletion though?
The simple solution is to keep otherEnt alive.
Let's consider this.

When ent is saved, a copy of otherEnt is serialized and saved to data. This is because it is referenced inside the foo component.

Therefore when the data is loaded, a copy of otherEnt is also loaded.

Aha! And that's the big problem.
If we didn't delete otherEnt, then we would have two copies of it in the ECS.
In order to have entities within components, we need propagation.

Nested propagation

Propagation is applied through entities, as well as through data structures. For example: = {
hi = {ent_1, ent_2}
-- These are propagated.
-- (So is anything that these entities reference)