see https import { IView } from '.' ; export interface ISystem { /**
     * in a similar way to Unity's `Start()`, we can do some initialization works:
     * * create global entities
     * * init event listeners
     */ initialize ? ( ) : Promise < void > ; /**
     * in a similar way to Unity's `Update()`, run once per frame
     */ execute ? ( views ) : Promise < void > ; /**
     * run at the end of each frame
     */ cleanup ? ( ) : void ; /**
     * run once at the end of your program
     */ tearDown ? ( ) : void ; }