object-pooling

Object Pooling

Every Instantiate() allocates memory. Every Destroy() triggers GC. Pool objects you create and destroy frequently: projectiles, particles, enemies, pickups, audio sources.

Unity Built-In ObjectPool<T> (2021+)

using UnityEngine.Pool;

public sealed class ProjectilePool : MonoBehaviour
{
    [SerializeField] private Projectile _prefab;
    [SerializeField] private int _defaultCapacity = 20;
    [SerializeField] private int _maxSize = 100;

    private ObjectPool<Projectile> _pool;

    private void Awake()
    {
        _pool = new ObjectPool<Projectile>(
            createFunc: CreateProjectile,
            actionOnGet: OnGetProjectile,
            actionOnRelease: OnReleaseProjectile,
            actionOnDestroy: OnDestroyProjectile,
            collectionCheck: false,
            defaultCapacity: _defaultCapacity,
            maxSize: _maxSize
        );
    }

    public Projectile Get() => _pool.Get();

    public void Release(Projectile projectile) => _pool.Release(projectile);

    private Projectile CreateProjectile()
    {
        Projectile projectile = Instantiate(_prefab);
        projectile.SetPool(this);
        return projectile;
    }

    private void OnGetProjectile(Projectile projectile)
    {
        projectile.gameObject.SetActive(true);
    }

    private void OnReleaseProjectile(Projectile projectile)
    {
        projectile.gameObject.SetActive(false);
    }

    private void OnDestroyProjectile(Projectile projectile)
    {
        Destroy(projectile.gameObject);
    }
}

// Projectile returns itself to pool
public sealed class Projectile : MonoBehaviour
{
    private ProjectilePool _pool;

    public void SetPool(ProjectilePool pool) => _pool = pool;

    public void ReturnToPool()
    {
        _pool.Release(this);
    }
}

Warm-Up (Pre-Spawn)

Pre-instantiate objects during loading to avoid runtime hitches:

private void Start()
{
    // Pre-warm the pool
    List<Projectile> temp = new List<Projectile>();
    for (int i = 0; i < _defaultCapacity; i++)
    {
        temp.Add(_pool.Get());
    }
    for (int i = 0; i < temp.Count; i++)
    {
        _pool.Release(temp[i]);
    }
    temp.Clear();
}

Return-to-Pool Lifecycle

The key contract: objects must reset their state when returned to pool.

private void OnReleaseProjectile(Projectile projectile)
{
    // Reset state
    projectile.transform.position = Vector3.zero;
    projectile.transform.rotation = Quaternion.identity;
    projectile.ResetState(); // Clear velocity, damage flags, timers

    // Deactivate
    projectile.gameObject.SetActive(false);
}

When to Pool

Pool these:

• Projectiles (bullets, arrows, spells)
• Particle effects
• Audio sources (one-shot sounds)
• Enemies in wave-based games
• Pickup items
• Damage numbers / floating text
• Trail renderers

Don't pool these:

• One-time objects (boss, unique NPCs)
• Tiny objects created once (data containers)
• Objects that live the entire scene

Pool Sizing

• Start small — 10-20 instances for most pools
• Monitor — if you see Instantiate in Profiler during gameplay, increase pool size
• Max cap — set maxSize to prevent unbounded growth (e.g., 100-200)
• Per-level tuning — different levels may need different pool sizes

Generic Pool Manager

public sealed class PoolManager : MonoBehaviour
{
    private readonly Dictionary<GameObject, ObjectPool<GameObject>> _pools = new();

    public GameObject Get(GameObject prefab, Vector3 position, Quaternion rotation)
    {
        if (!_pools.ContainsKey(prefab))
        {
            _pools[prefab] = new ObjectPool<GameObject>(
                () => Instantiate(prefab),
                obj => obj.SetActive(true),
                obj => obj.SetActive(false),
                obj => Destroy(obj),
                false, 10, 100
            );
        }

        GameObject obj = _pools[prefab].Get();
        obj.transform.SetPositionAndRotation(position, rotation);
        return obj;
    }

    public void Release(GameObject prefab, GameObject instance)
    {
        _pools[prefab].Release(instance);
    }
}

Cached WaitForSeconds

Don't forget to pool WaitForSeconds:

// BAD — allocates every time
yield return new WaitForSeconds(0.5f);

// GOOD — cache and reuse
private readonly WaitForSeconds _halfSecond = new WaitForSeconds(0.5f);
yield return _halfSecond;