Lesson 5

Prefabs and inheritance

How a prefab gives an entity its whole type in one move, the difference between copied and inherited components, and what overriding does to storage.

A prefab is an entity used as a template. Creating an instance is adding one pair, (IsA, SpaceShip), and the instance gets the prefab’s components. What “gets” means is decided per component by its OnInstantiate trait:

• Override (the default): the component is copied into the instance’s own table.
• Inherit: the component is not copied. The instance reads the prefab’s value, so every instance shares one copy, and queries still match it.
• DontInherit: the instance doesn’t get it at all.

Watch an instance being made

MaxSpeed and Position use the default (copied); Defence is set to Inherit.

Things to notice:

• One move with the full type. Adding (IsA, SpaceShip) moves S1 straight into [MaxSpeed, Position, (IsA, SpaceShip)]. Compare that with creating an entity and adding components one at a time, which walks through a table per component.
• Inherited components aren’t in the table. The instance table has no Defence column, yet the Defended query returns both instances, because it matches Defence on the prefab.
• Changing the prefab changes inherited values only. After ship.set<Defence>({75}) every instance sees 75. After ship.set<MaxSpeed>({200}), the instances keep their copied 100.
• Overriding is a move. s1.set<Defence>({10}) gives S1 its own Defence column, so it moves to a new table, and S2 is swapped into the hole. The query now returns them from two different tables.