Systems & Schedules

Systems are functions operating on entities with specific components. Run each frame.

Game loop:

System order matters:

• Physics before render: render the new positions.
• Input before physics: act on this frame's input.
• AI before physics: AI sets velocities, physics moves.

Schedules:

• Group systems by phase (Input, Update, PostUpdate, Render).
• Bevy: stages, schedules, system sets.
• Unity: Update, FixedUpdate, LateUpdate.

Parallelism:

• Systems can run in parallel if they access different components.
• Conflict: two systems writing same component → must serialize.
• Bevy/ECS frameworks compute the dependency graph + parallelize.

Events:

• Systems communicate via events (mailbox).
• Producer system writes to event queue.
• Consumer system reads queue next frame.

Common events:

• Collision: a hit b.
• Damage: entity took N damage.
• Spawn: create new entity.
• Death: entity destroyed.

Decoupling: damage system doesn't care WHO damaged; just processes events.

Resources (singletons):

• Time, input, asset registry.
• Not per-entity; globally accessible.

Systems request resources by type. Framework provides.