groups-and-containers

What this skill does

# Groups and Containers
> Logical grouping (Group), visual grouping with transform inheritance (Container), render-layer grouping (Layer), object pooling, and when to use each in Phaser 4.

**Key source paths:** `src/gameobjects/group/`, `src/gameobjects/container/`, `src/gameobjects/layer/`
**Related skills:** ../sprites-and-images/SKILL.md, ../physics-arcade/SKILL.md

## Quick Start

```js
// In a Scene's create() method:

// --- Group: logical collection, no transform, great for pooling ---
const enemies = this.add.group();
enemies.create(100, 200, 'enemy');       // creates Sprite at (100,200)
enemies.create(300, 200, 'enemy');

// --- Container: visual parent with inherited transform ---
const hud = this.add.container(10, 10);
const icon = this.add.image(0, 0, 'heart');
const label = this.add.text(20, 0, 'x3');
hud.add([icon, label]);                  // children move/scale/rotate with hud

// --- Layer: render-ordering bucket, no position/scale ---
const bgLayer = this.add.layer();
const fgLayer = this.add.layer();
bgLayer.add(this.add.image(400, 300, 'sky'));
fgLayer.add(this.add.sprite(400, 300, 'player'));
```

## Core Concepts

### Group vs Container vs Layer

| Feature | Group | Container | Layer |
|---|---|---|---|
| **Purpose** | Logical collection / pool | Visual parent with transform | Render-order bucket |
| **On display list** | No (children are) | Yes (renders children) | Yes (renders children) |
| **Position/rotation/scale** | No | Yes (children inherit) | No |
| **Children storage** | `children` (Set) | `list` (Array) | List (Structs.List) |
| **Physics** | Via physics.add.group() | Limited (offsets if not at 0,0) | No |
| **Input** | No (children can) | Yes (needs hit area shape) | No |
| **Object pooling** | Yes (getFirstDead, kill) | No | No |
| **Masks** | No | Yes (not per-child in Canvas) | Yes |
| **Alpha/blend/visible** | No (batch via setVisible) | Yes | Yes |
| **Nesting** | N/A | Container in Container | Cannot go in Container |
| **Extends** | EventEmitter | GameObject | List |
| **Factory** | `this.add.group()` | `this.add.container(x, y)` | `this.add.layer()` |

### When to Use Each

**Group:** Managing collections of similar objects (enemies, bullets, coins), object pooling with active/inactive lifecycle, physics group collisions. No shared visual transform. Members can belong to multiple Groups simultaneously.

**Container:** Children inherit position, rotation, scale, alpha. Composite UI elements (health bars, inventory slots), moving/rotating clusters as one unit, nested transforms. By default exclusive -- a child can only belong to one Container (use `setExclusive(false)` to override).

**Layer:** Controlling render order of object batches, applying shared alpha/blend/mask. No position/scale/rotation. Lightweight render bucketing.

### Container vs Group at a Glance

- **Container has position, rotation, scale, alpha** -- Group does not. If you need children to move/rotate as a unit, use Container.
- **Container is exclusive by default** -- adding a child removes it from its previous Container. Group is non-exclusive; a game object can be in many Groups.
- **Container is on the display list** -- it renders its children. Group is not on the display list; its children render individually on the Scene.
- **Group supports object pooling** -- getFirstDead, kill, killAndHide. Container does not.
- **Container has performance cost** -- each child requires matrix math per frame. Deeper nesting = more cost. Prefer Group or Layer when transforms are not needed.

## Common Patterns

### Creating and Populating Groups

```js
// Empty group, add existing objects
const gems = this.add.group();
gems.add(existingSprite);
gems.addMultiple([sprite1, sprite2, sprite3]);

// Group with config -- creates children automatically
const coins = this.add.group({
    classType: Phaser.GameObjects.Sprite,
    key: 'coin',
    quantity: 10,           // overrides frameQuantity
    setXY: { x: 50, y: 300, stepX: 60 },
    setScale: { x: 0.5, y: 0.5 }
});

// Custom class type with pool limit
const bullets = this.add.group({
    classType: Bullet,       // must accept (scene, x, y, key, frame)
    maxSize: 30,
    defaultKey: 'bullet',
    runChildUpdate: true     // calls child.update() each frame
});
```

### Object Pooling with getFirstDead

The core pooling pattern: deactivate objects instead of destroying them, then reuse inactive ones.

```js
// Setup pool
const bullets = this.add.group({
    classType: Phaser.GameObjects.Sprite,
    defaultKey: 'bullet',
    maxSize: 30
});

// Fire a bullet -- get() finds first inactive member or creates one
function fireBullet(x, y) {
    const bullet = bullets.get(x, y);

    if (bullet) {
        bullet.setActive(true);
        bullet.setVisible(true);
        bullet.body.velocity.y = -300;   // if physics enabled
    }
}

// Deactivate when off-screen or on hit
function killBullet(bullet) {
    bullets.killAndHide(bullet);         // sets active=false, visible=false
    // If using physics, also reset the body:
    // bullet.body.stop();
}

// Alternative: manual getFirst
const inactive = bullets.getFirst(false);     // first where active===false
const active = bullets.getFirstAlive();       // first where active===true
const dead = bullets.getFirstDead(true, x, y); // first inactive, create if null
```

**Pool helper methods on Group:**

| Method | Description |
|---|---|
| `get(x, y, key, frame)` | Shortcut: `getFirst(false, true, ...)` -- finds inactive or creates |
| `getFirst(state, createIfNull, x, y, key, frame)` | First member matching active `state` |
| `getFirstAlive(createIfNull, x, y, key, frame)` | First member where `active===true` |
| `getFirstDead(createIfNull, x, y, key, frame)` | First member where `active===false` |
| `getLast(state, createIfNull, x, y, key, frame)` | Like getFirst but searches back-to-front |
| `kill(gameObject)` | Sets `active=false` on a member |
| `killAndHide(gameObject)` | Sets `active=false` and `visible=false` |
| `countActive(value)` | Count members where `active===value` (default true) |
| `getTotalUsed()` | Count of active members |
| `getTotalFree()` | `maxSize - active count` (remaining pool capacity) |
| `isFull()` | True if `children.size >= maxSize` |

### Physics Groups

Physics groups extend Group with automatic body assignment. See ../physics-arcade/SKILL.md for full details.

```js
// Arcade Physics group -- every member gets a dynamic body
const enemies = this.physics.add.group({
    key: 'enemy',
    quantity: 5,
    setXY: { x: 100, y: 100, stepX: 80 }
});

// Static physics group -- immovable bodies
const platforms = this.physics.add.staticGroup();
platforms.create(400, 568, 'ground');

// Collide physics groups with each other
this.physics.add.collider(player, platforms);
this.physics.add.overlap(bullets, enemies, onHit);
```

### Containers with Nested Transforms

```js
// HUD that follows camera
const hud = this.add.container(10, 10);
hud.setScrollFactor(0);     // pinned to camera

const healthBar = this.add.rectangle(0, 0, 200, 20, 0x00ff00);
const healthText = this.add.text(210, -5, '100 HP');
hud.add([healthBar, healthText]);

// Move everything at once
hud.setPosition(50, 50);

// Scale and rotate propagate to children
hud.setScale(1.5);
hud.setRotation(0.1);

// Alpha affects all children
hud.setAlpha(0.8);

// Nested containers
const inventory = this.add.container(300, 500);
for (let i = 0; i < 5; i++) {
    const slot = this.add.container(i * 55, 0);
    slot.add([
        this.add.rectangle(0, 0, 48, 48, 0x333333),
        this.add.image(0, 0, `item-${i}`)
    ]);
    inventory.add(slot);    // Container inside Container
}
```

**Key Container methods:**

| Method | Description |
|---|---|
| `add(child)` / `addAt(child, index)` | Add Game Object(s); removes from display list |
| `remove(child, destroyChild)` | Remove; optionally destroy |
| `getAt(index)` / `getIndex(child)` | Access by index |
| `getByName(name)` / `getFir