Systems

A system is simply a piece of logic that runs.

Logic

The description above is pretty vague, but this is because a system is not constrained in what it can and can't do, it can operate on entities and components (for example the SpriteSystem, which generates renderables from Sprites and Transforms), or it can operate without any entities and components, sending and receiving messages (for example a system that stops the game when the player is destroyed). These systems are Stateless Systems (does not track entities or components) and Stateful Systems (tracks entities and components).

Messages

All systems are Listeners, meaning that they can receive messages, alongside this all systems store a reference to a Message Bus which allows them to send messages.

Systems can override the OnMessage method to handle messages, the base system includes basic handling already which listens for an update message, and calls the protected update which can be overridden to add logic that runs on every update.

Be aware that if a system does override the OnMessage method, the base OnMessage method should be called if the system still needs to handle updates.

Stateless Systems

Stateless Systems (just referred to as Systems as they are the base class) do not track any entities or components, they simply send and receive messages.

Stateful Systems

See the Stateful Systems page for details.