silphius/app/ecs-systems/follow-camera.js

import {System} from '@/ecs/index.js';

export default class FollowCamera extends System {

  static queries() {
    return {
      default: ['Camera', 'Position'],
    };
  }

  reindex(entities) {
    super.reindex(entities);
    for (const id of entities) {
      this.updateCamera(1, this.ecs.get(id));
    }
  }

  tick(elapsed) {
    for (const {id} of this.select('default')) {
      this.updateCamera(elapsed * 3, this.ecs.get(id));
    }
  }

  updateCamera(portion, entity) {
    const {Camera, Position} = entity;
    if (Camera && Position) {
      const [px, py] = [
        Math.round(Position.x),
        Math.round(Position.y),
      ];
      if (Camera.x === px && Camera.y === py) {
        return;
      }
      if (Math.abs(Camera.x - px) < 0.1) {
        Camera.x = px;
      }
      if (Math.abs(Camera.y - py) < 0.1) {
        Camera.y = py;
      }
      const [dx, dy] = [
        (px - Camera.x) * portion,
        (py - Camera.y) * portion,
      ];
      [Camera.x, Camera.y] = [
        Camera.x + (Math.sign(dx) * Math.max(0.1, Math.abs(dx))),
        Camera.y + (Math.sign(dy) * Math.max(0.1, Math.abs(dy))),
      ];
    }
  }

}