Svelto.ECS is an awesome project, however I found understanding underlying entity structure can be pretty confusing to new users like myself. It has powerful tools like groups and filters, but lacks of wrapper to make it intutive. I thought it will be much easier to understand group with structured Schema, and it is worth to make your code flexible, design change proof. That is the initial motivation I wrote this Svelto.ECS.Schema extension.
Now if you wonder why this extensions named Schema, think of a RDBMS schema, there is tables, rows, columns, indexes. ECS is basically in-memory database but faster. In RDBMS, tables can hold records having specific combination of columns. In Svelto.ECS, groups can hold entities having specific combination of components. That is why I chose to take friendly terms from RDBMS and define Schema of ECS.
In Schema extensions, Rows are abstracted layer of traits that a Entity can have. It can be extended, layered, mxied and matched very flexiblely. Engines should query entities as Result Sets, while actual Entity Descriptor is defined with Descriptor Rows.
Table is a wrapper for list of Group in Svelto ECS, mimicking RDBMS Tables. It is also a good replacement for GroupCompound if used with Primary Key. A Table is tied up to specific Descriptor Row. You can either define single Table, or ranged, or combined Table from defined onces. Using IEntityTable<out TRow> interface is a way to treat Tables with same common interface same way.
Primary Key splits Table into multiple internal Group, so your iteration can be optimized. Primary Key can used with Index, in flavor of performance. Generally, if you have few and commonly used keys you should prefer to use Primary Key.
Index is a wrapper for Filter in Svelto ECS, mimicking RDMBS Indexes. It saves list of entities per key, and you can query entities having speicific key.
Foreign Key is a wrapper for EntityReference, and you can use it to make a 1-N or N-N relationship. You can query them with Join as well!
Schema is extendible definition set of above. A Schema can contain Tables, PKs, FKs, Indexes and State Machines. While Groups must be rigid and static in Svelto, Schema is effective way to hide the limitation and make it easier to modify when game design changed.
Schema extensions also functionality to define Finite State Machine in Svelto. You can define States, Conditions, Callbacks of FSM, and query Entities with speicific State.
We looked into features of Schema extensions. Next document will teach you how to use Schema extensions, starting from defining Rows.