phaser-core

# Phaser scene flow (Phaser 4.2; core lifecycle also applies to 3.90)

The Scene Manager runs every scene through a small state machine. Knowing which
transition verb does what prevents the most common Phaser bugs: hidden scenes,
stale state after a restart, and duplicate event listeners.

## Lifecycle order

A scene is **booted once** (status `INIT`) and can be **started any number of
times**. On each start it runs:

```
start → (loading, if preload queued anything) → create → running
```

- `init(data)` — runs first on every start. **Reset run-specific state here.**
- `preload()` — queue asset downloads. Optional.
- `create(data)` — build game objects; assets queued in `preload` are now ready.
- `update(time, delta)` — called every frame while the scene is `running`.

The same `data` object is passed to `init` and `create`, coming from
`scene.start(key, data)` / `scene.launch(key, data)`.

## Transition verbs

| Method | Effect on target | Effect on caller |
|--------|------------------|------------------|
| `start(key, data)` | start (or restart) | **stop** the caller |
| `launch(key, data)` | start in parallel; **restarts** it if already running | caller keeps running |
| `run(key, data)` | wake if sleeping, resume if paused, otherwise (re)start | caller keeps running |
| `switch(key)` | wake if sleeping, otherwise start (a paused target restarts) | **sleep** the caller |
| `pause(key?)` | stop updating, keep rendering | — |
| `resume(key?)` | resume updating | — |
| `sleep(key?)` | stop updating AND rendering, keep state | — |
| `wake(key?)` | resume a sleeping scene | — |
| `stop(key?)` | shutdown (frees objects) | — |

All of these are queued: the Scene Manager applies them at the start of the next game
step, so the target scene has not started yet on the line after `start`/`launch`.

Rules of thumb:

- **HUD/overlay** → `launch` it once; a second `launch` or `run` while it is running
  restarts it, so guard with `this.scene.isActive('hud')`. Scenes draw in scene-list
  order, so it is on top only if it comes after the gameplay scene in the `scene`
  config array; otherwise call `this.scene.bringToTop('hud')`. Toggle with
  `setVisible()` or `sleep`/`wake`, not `start`.
- **Modal pause menu** → `pause` the gameplay scene and `launch` the menu; `resume`
  on close.
- **Title ⇄ gameplay you revisit** → prefer `sleep`/`wake` (state preserved, starts
  only once) over `stop`/`start` (fresh each time) when re-initialisation is costly.

## The restart-state bug

A Scene is a long-lived instance. Fields set in the constructor are set **once**,
so they keep their last value after a restart:

```js
// WRONG — gameOver stays true forever after the first game over.
export default class Play extends Phaser.Scene {
  constructor() {
    super('play');
    this.gameOver = false;   // runs once, never again
  }
}

// RIGHT — init() runs on every (re)start.
export default class Play extends Phaser.Scene {
  constructor() { super('play'); }
  init() { this.gameOver = false; }
}
```

The same applies to module-level arrays that collect game objects: clear them on
`shutdown`, or they will hold destroyed objects from the previous run.

```js
create() {
  this.enemies = [];
  // Clean up when the scene stops so the next run starts empty.
  this.events.once('shutdown', () => { this.enemies.length = 0; });
}
```

## Duplicate listeners

Listeners added in `create` on a **global** emitter (the registry, the input
manager, another scene) survive scene restarts and stack up. Either:

- add them with `.once(...)`, or
- remove them on `shutdown`:

```js
create() {
  const onChange = (p, v) => this.refresh(v);
  this.registry.events.on('changedata-coins', onChange);
  this.events.once('shutdown', () => {
    this.registry.events.off('changedata-coins', onChange);
  });
}
```

## Removing or replacing a scene

You cannot reuse a scene key until the original is destroyed. To swap an instance
under the same key, remove it after it finishes shutting down:

```js
this.scene.remove('level'); // remove by key
// then later
this.scene.add('level', NewLevelScene, true); // true = autostart
```

A handy alternative to juggling keys is to store the active level key in the
registry and add/remove by that value.

---
name: phaser-core
description: >
  Set up and debug a Phaser 4 game: the Game config, the Scene lifecycle
  (init/preload/create/update), the asset loader, cameras, and cross-scene
  communication. Use when building or debugging a Phaser game — when the user
  mentions Phaser, Phaser.Game, Phaser.Scene, preload/create/update, this.load,
  this.add, or scene transitions. For Arcade Physics movement/collisions use
  phaser-arcade-physics.
---

# Phaser 4 Core

Set up the foundation of a Phaser game: the `Game` config, the `Scene`
lifecycle, asset loading, cameras, and passing data between scenes. Targets
**Phaser 4.2** for new projects; keep an existing Phaser 3.90 project on its
pinned major unless the user explicitly asks for a migration.

## When to use

- Use when starting a Phaser game, wiring the `Phaser.Game` config, structuring
  `Scene`s, loading assets in `preload`, or fixing scene transitions and shared
  state.
- Use when the project has `phaser` in `package.json` or `import Phaser from 'phaser'`,
  and code uses `preload()`/`create()`/`update()`.

**When *not* to use:** movement, velocity, colliders, gravity, or overlap → use
`phaser-arcade-physics`. Complex rigid-body simulation uses Matter physics (a
separate concern). For cross-engine save/load patterns use `save-systems`.

## Core workflow

1. **Detect the installed major first.** Read `package.json` and the lockfile. Use
   Phaser 4.2 for new work; do not silently rewrite a Phaser 3 project as Phaser 4.
2. **Create the game from a config.** `new Phaser.Game(config)` with `type:
   Phaser.AUTO` (WebGL with Canvas fallback), a `width`/`height`, and a `scene`
   array. The first scene (and any with `active: true`) starts automatically.
3. **Model each screen as a `Scene`.** Subclass `Phaser.Scene`, pass a unique
   `key` to `super`, and implement the lifecycle: `init(data)` → `preload()` →
   `create(data)` → `update(time, delta)`.
4. **Load assets in `preload`, use them in `create`.** Queued assets are not
   available until `create`. The loader is per-scene; the cache it fills is global.
5. **Reset per-run state in `init()`, not the constructor.** A scene instance is
   reused across restarts, so constructor-set fields keep stale values.
6. **Move between screens** with `this.scene.start/launch/switch/sleep/wake`.
   Share data through `this.registry` (global) or a sibling scene's event emitter.
7. **Run and observe.** Serve the page, open it, and confirm assets load (watch the
   Network tab and console) and scenes switch as expected before assuming success.

## Patterns

### 1. Game config + boot (ES module)

```js
// main.js — one Game owns the renderer, loop, cache, and Scene Manager.
import Phaser from 'phaser';
import BootScene from './scenes/BootScene.js';
import PlayScene from './scenes/PlayScene.js';

const config = {
  type: Phaser.AUTO,            // WebGL if available, else Canvas
  width: 800,
  height: 600,
  backgroundColor: '#1d1d28',
  scale: { mode: Phaser.Scale.FIT, autoCenter: Phaser.Scale.CENTER_BOTH },
  scene: [BootScene, PlayScene] // BootScene starts first
};

new Phaser.Game(config);
```

### 2. A Scene with the full lifecycle

```js
// scenes/PlayScene.js
import Phaser from 'phaser';

export default class PlayScene extends Phaser.Scene {
  constructor() {
    super('play');                  // unique scene key
  }

  init(data) {
    // Reset run-specific state HERE so restarts start clean.
    this.score = 0;
    this.level = data.level ?? 1;
  }

  preload() {
    // Queue downloads. Not usable until create().
    this.load.image('player', 'assets/player.png');
    this.load.spritesheet('coin', 'assets/coin.png', { frameWidth: 16, frameHeight: 16 });
  }

  create() {
    this.player = this.add.sprite(400, 300, 'player');
    this.scoreText = this.add.text(10, 10, 'Score: 0', { fontSize: '20px', color: '#fff' });
    this.cursors = this.input.keyboard.createCursorKeys();
  }

  update(time, delta) {
    // delta is milliseconds since last frame; divide by 1000 for seconds.
    const speed = 200 * (delta / 1000);
    if (this.cursors.left.isDown)  this.player.x -= speed;
    if (this.cursors.right.isDown) this.player.x += speed;
  }
}
```

### 3. Cross-scene data + events

```js
// The registry is a global DataManager shared by every scene.
this.registry.set('coins', 0);                 // in any scene
const coins = this.registry.get('coins');      // read anywhere

// React to registry changes (e.g. a HUD scene listening to gameplay):
this.registry.events.on('changedata-coins', (parent, value) => {
  this.coinText.setText(`Coins: ${value}`);
});

// Talk directly to another running scene via its event emitter:
const ui = this.scene.get('hud');
ui.events.emit('show-message', 'Level cleared!');
```

### 4. Scene transitions (pick the right verb)

```js
this.scene.start('gameover', { score: this.score }); // stop this scene, start target
this.scene.launch('hud');        // run a second scene in parallel (restarts it if already running)
this.scene.run('hud');           // wake if sleeping, resume if paused, else (re)start
this.scene.bringToTop('hud');    // draw order follows the scene list, not launch order
this.scene.switch('menu');       // sleep this scene, start/wake target
this.scene.pause();              // freeze updates but keep rendering (modal)
this.scene.sleep();              // stop updating AND rendering, keep state for wake
```

### 5. A camera that follows the player

```js
this.cameras.main.setBounds(0, 0, 1600, 1200);  // world size
this.cameras.main.startFollow(this.player, true, 0.1, 0.1); // smooth lerp follow
this.cameras.main.setZoom(1.5);
```

## Pitfalls

- **Assets are `undefined` in `create`/`update`** → you forgot to queue them in
  `preload`, or used the wrong key. The loader runs between `preload` and `create`.
- **State leaks across a restart** → you set fields in the constructor. The Scene
  instance is reused; reset run state in `init()` and clear arrays on `shutdown`.
- **`this.scene.start` vs `this.scene.launch`** → `start` stops the calling scene;
  `launch` runs the target alongside it. Using `start` for a HUD hides the game.
- **HUD loses its state or draws behind the game** → `launch` (and `run`) on a scene
  that is already running shuts it down and starts it again; launch the HUD once, or
  guard with `if (!this.scene.isActive('hud'))`. Scenes draw in scene-list order, so a
  HUD listed before the gameplay scene renders underneath it until you call
  `this.scene.bringToTop('hud')`.
- **Scene state read right after `start`/`launch` is stale** → scene operations are
  queued and run at the start of the next game step, not synchronously.
- **`this` is wrong in a callback** → arrow functions keep the Scene's `this`; plain
  `function` callbacks need a context argument or `.bind(this)`.
- **Phaser 2 tutorials don't work** → "States" were renamed to "Scenes" in Phaser 3,
  and each Scene owns its own systems (input, cameras, tweens) rather than a global
  Game World.
- **Phaser 3 custom pipelines fail in Phaser 4** → Phaser 4 rebuilt the renderer and
  replaced the old FX/pipeline extension points. Migrate custom shaders and renderer
  plugins against the Phaser 4 guide; do not mechanically copy internal renderer code.
- **A Phaser 3 snippet runs but looks wrong in Phaser 4** → common renames: `preFX`/
  `postFX` and `BitmapMask` became filters (`obj.enableFilters()` then
  `obj.filters.internal`/`.external`); `setPipeline('Light2D')` became
  `setLighting(true)`; `setTintFill(c)` only logs an error, so use
  `setTint(c).setTintMode(Phaser.TintModes.FILL)`; `Phaser.Math.TAU` is now 2π (use
  `Phaser.Math.PI_OVER_2` where v3 code meant π/2); `Geom.Point` is gone (use
  `Phaser.Math.Vector2`). Canvas rendering still works but is deprecated.
- **Pixel art is blurry or shimmers while moving** → set `pixelArt: true` in the game
  config. It turns off antialiasing and turns on `roundPixels`, which defaults to
  `false` in Phaser 4.
- **Nothing renders / black screen** → confirm the canvas mounted, `width`/`height`
  are set, and a scene actually started (check `game.scene.dump()` output).
- **Two canvases (two games running) after a hot reload or a React/Vue remount** →
  a second `new Phaser.Game` ran while the first was still alive. Keep one instance
  and call `game.destroy(true)` when the host component unmounts; the teardown
  finishes on the next frame. Leave `noReturn` false, or no new game can start on
  the page.

## References

- For the full scene state machine (pause/resume vs sleep/wake vs stop/start, the
  restart-state bug, and removing/replacing scenes), read
  `references/scene-flow.md`.

## Related skills

- `phaser-arcade-physics` — velocity, gravity, colliders, overlap, and groups.
- `input-systems` — rebindable, multi-device input architecture (engine-agnostic).
- `pixijs-rendering` / `threejs-scene-setup` — other browser rendering stacks.
- `platformer` / `puzzle` — genre templates that compose Phaser skills.