For energy efficient game loop, it still runs in a fixed frequency, e.g. in 60fps or 30fps. For each frame, the time is spent on event processing, state update

For energy efficient game loop, it still runs in a fixed frequency, e.g. in 60fps or 30fps. For each frame, the time is spent on event processing, state update, rendering, and sleep. Some of the steps can be skipped. The goal is to lower the times spent in the non-sleeping steps, and sleeps more for each frame. Event queues are used to track things in long duration across multiple frames, like user input, animation sequences, sound sequences, pending state update, pending physics update, pending AI update, etc. In each frame, check the event queues. If they are empty, sleep. Otherwise, process the events in the queues. For user input, do the state update corresponding to the input, like updating the camera position. For ongoing animations, advance the steps in the sequences (the drawing is done in the rendering step). When an animation sequence completes, remove it from the queue. Same for sound. For pending state update, run the update. E.g. the ship is moving, do the state update on its velocity and position. For ECS, you might end up calling or not calling the systems for the item type for batch update. E.g. when the bullet queue is not empty, you might just run the bullet related systems over all bullets for simplicity. When all the bullets end and the queue is empty, the bullet related systems can be skipped. The physics queue has the items being simulated. Run those. Physics could run at a slower rate like 5fps, so the queued items would be skipped most of the times. Same with the AI items in the AI queue. Whenever there's a state update, mark the screen as outdated. At the rendering step, run rendering if the screen is outdated. It's simpler to just run the whole rendering for any change. Ideally, when there's no event in the queues or the state updates are not triggered, there's no need for rendering. This applies to using ECS or not. ECS is for tracking states of entities and do batch update with systems.