<p>A system is part of the core functionality of the game. After a system is added to the engine, its
update method will be called on every frame of the engine. When the system is removed from the engine,
the update method is no longer called.</p>
<p>The aggregate of all systems in the engine is the functionality of the game, with the update
methods of those systems collectively constituting the engine update loop. Systems generally operate on
node lists - collections of nodes. Each node contains the components from an entity in the engine
that match the node.</p>
The base class for a system.
<p>A system is part of the core functionality of the game. After a system is added to the engine, its update method will be called on every frame of the engine. When the system is removed from the engine, the update method is no longer called.</p>
<p>The aggregate of all systems in the engine is the functionality of the game, with the update methods of those systems collectively constituting the engine update loop. Systems generally operate on node lists - collections of nodes. Each node contains the components from an entity in the engine that match the node.</p>