The World and Its Actors

The framework the engine owns is what a check reads. The level that is open, the match phase, the actors in the world, which controller holds which pawn, and what a transition produced are all engine state, reachable from engine.world and engine.instance without the build exporting anything of its own.

The level and the match phase

world.level is the name the world was opened under, and world.state.phase is the match phase. setPhase writes the phase onto the game state, so the two always agree and a check reads either one.

import { LEVELS } from "../constants";
import { createHarness } from "../harness";

it("the title level waits before a match starts", async () => {
  const { engine } = createHarness();
  await engine.initialize();
  const world = engine.world;

  expect(world.level).toBe(LEVELS.title);
  expect(world.state.phase).toBe("waiting");

  engine.debug.startMatch("versus");
  await engine.advance(1);

  expect(world.state.phase).toBe("playing");
});

setPhase also emits match:phase carrying the new phase and the previous one, and setting the phase the mode already holds emits nothing. A check that is about the moment the phase changed subscribes to that event; a check that is about where the match ended up reads world.state.phase.

Finding actors

world.byTag(tag) returns the live actors carrying a tag, world.ofType(type) the live actors that are instances of a class, and world.find(type) the first entry ofType would return, or null. All three answer in spawn order, as a copy the caller owns.

import { Pawn } from "@clockwyrks/structured-2d";
import { FIELD_W, TAGS } from "../constants";

const paddles = world.byTag(TAGS.paddle);
expect(paddles).toHaveLength(2);

const ball = world.byTag(TAGS.ball)[0];
expect(ball.alive).toBe(true);
expect(ball.transform.x).toBeCloseTo(FIELD_W / 2, 3);

expect(world.find(Pawn)).not.toBeNull();

A build names its own actor classes, so a check that named one would be checking a name the case never fixed. The case fixes the tag vocabulary instead, which the project’s own constants.ts states and a build applies through the tags of an ActorSpec or a SpawnSpec. ofType and find stay useful for the framework classes every build shares, Pawn above all, which is how a check reaches the thing a controller possesses without knowing what the build called it.

Player states and scores

world.state.players holds every player state in index order, a bot’s alongside a player’s, and each carries the index, the name, and the score. world.players() returns the player controllers alone, also in index order, and every controller reaches its own state through controller.playerState.

const [p1, p2] = world.state.players;

expect(p1.index).toBe(0);
expect(p1.score).toBe(3);
expect(p2.name).toBe("CPU");
expect(world.players()[0].playerState).toBe(p1);

A game carries its own per-player figures by subclassing PlayerState, so a check that is about a figure the case fixed reads it off the same object once the case has stated that the figure lives there.

Driving a pawn

An actor cannot read the keyboard. Input reaches the simulation through a player controller alone, so a player pawn and an AI pawn are the same class driven by two different controllers, and a check exercises a pawn by writing the controller that drives it.

import { AIController } from "@clockwyrks/structured-2d";
import { PADDLE_SPEED } from "../constants";

class Scripted extends AIController {
  drive = 0;

  override tick(dt: number): void {
    if (this.pawn === null) return;
    this.pawn.transform.y += this.drive * PADDLE_SPEED * dt;
  }
}

world.mode.addBot is how that controller joins the world. Naming pawn: null keeps it from spawning one of its own, and possess then hands it the pawn the check wants driven.

const paddle = world.byTag(TAGS.paddleP2)[0] as Pawn;
const bot = world.mode.addBot(Scripted, {
  name: "check",
  pawn: null,
}) as Scripted;

bot.possess(paddle);
bot.drive = -1;

const before = paddle.transform.y;
await engine.advance(30);

expect(paddle.controller).toBe(bot);
expect(paddle.transform.y).toBeLessThan(before);

possess unpossesses whatever the controller held and whatever held the pawn, notifies the pawn through possessedBy, and emits possession:changed carrying the controller, the new pawn, and the previous one. Controllers tick before any actor, so the pawn’s own tick observes what the scripted controller applied in the same frame.

Observing a transition

A transition emits three events in a fixed order: world:opening before anything is torn down, world:closed once every controller, actor, and the game mode have ended play, and world:opened once the incoming world is built and its game mode has begun play. Subscriptions live on the engine, so one taken before initialize observes the start level being built as well.

const { engine } = createHarness();
const seen: string[] = [];
engine.events.on("world:opening", ({ from, to }) => {
  seen.push(`opening ${from} -> ${to}`);
});
engine.events.on("world:closed", ({ level }) => seen.push(`closed ${level}`));
engine.events.on("world:opened", ({ level }) => seen.push(`opened ${level}`));

await engine.initialize();
engine.world.open(LEVELS.match, { round: 2 });
await engine.advance(1);

expect(seen).toEqual([
  "opening null -> title",
  "opened title",
  "opening title -> match",
  "closed title",
  "opened match",
]);

The first open carries from: null and closes nothing, because there was no outgoing world. A handler on world:opened reads a world whose declared actors and game mode have already begun play, so it asserts on a finished world rather than a half-built one.

What crossed the transition

The game instance is the one framework object that outlives a transition, so a check reads it on both sides and asserts it is the same object. The world, its game mode, its game state, and every actor, component, and controller are rebuilt, and world.time restarts at zero.

const instance = engine.instance;

engine.world.open(LEVELS.match, { round: 2 });
await engine.advance(1);

const world = engine.world;
expect(engine.instance).toBe(instance);
expect(world.level).toBe(LEVELS.match);
expect(world.time).toBe(0);
expect(world.mode.options).toEqual({ round: 2 });

A value the case requires to survive travel is read off the instance by name, because that is where a build must keep it. The frame counter and the accumulated simulated time cross the transition too, so engine.frame().count keeps climbing across it.

const carried = engine.instance as GameInstance & { round: number };
expect(carried.round).toBe(2);

Awaiting a transition

world.open requests a transition rather than performing one. The request is honored once the frame’s ticks and collision are finished, before the frame renders, and one request per frame is honored, so a second call in the same frame replaces the first.

A transition is asynchronous, because the incoming level’s load is awaited before its world is built. engine.advance awaits it before the next frame, so the single advance above resolves with the new world open, its assets loaded, and its actors and game mode having begun play. A check polls nothing and sleeps on nothing; it steps one frame and reads engine.world again.