How to Use

ObjectPool

Created by ChevyRay on September 25th, 2013 at UnityPatterns

ObjectPool Updated to 2.0

Posted by ChevyRay on August 2nd, 2014

It’s been awhile since my last update, but this is one I’ve been wanting to do for awhile. The ObjectPool tool has been updated to version 2.0, and has some nice new features!

• You can now reference prefabs by GameObject, which was not possible before
• GameObject now has extension functions in addition to the component ones
• The initialPoolSize parameter has been added to CreatePool(), and tells it how many objects to pre-instantiate, these will all be initially disabled and waiting to be spawned
• You can also pre-instantiate prefabs via the inspector by attaching ObjectPool to a GameObject
• Additional functions have been added for searching/counting instances

Features

• Selectively pool objects based on prefab type
• Simple and expressive syntax for instantiation and recycling
• Easily pre-instantiate objects to prevent runtime instantiation
• Search and track all spawned/pooled instances in the scene

How to Use

Normally, when you instantiate and destroy instances of prefabs, you are constantly creating new objects and destroying them at runtime, which can cause runtime garbage collection and occasional framerate drops. ObjectPool can prevent this by pre-instantiating objects for you, which are then re-used instead of being destroyed!

Spawning Pooled Objects

For example, if I have a Turret that shoots Bullet objects, I can just create 10 bullets and re-use those same 10 objects. The bullets will never be destroyed, just de-activated and re-activated when you respawn them.

To do this with ObjectPool, you just have to call CreatePool() on the prefab that you want to be pooled.

public class Turret : MonoBehaviour
{
    public Bullet bulletPrefab;

    void Start()
    {
        //Create a pool with 10 pre-instantiated bullets in it
        bulletPrefab.CreatePool(10);
 
        //Or you could also pre-instantiate none, and the system will instantiate them as it needs them
        bulletPrefab.CreatePool();
    }
}

Now all you have to do is replace all your calls to Instantiate() and Destroy() with calls to ObjectPool’s Spawn() and Recycle(). So for example, when the Turret shoots, I spawn a bullet instance.

public class Turret : MonoBehaviour
{
    public Bullet bulletPrefab;
 
    public void ShootBullet()
    {
        //Spawn a bullet at my position with my rotation
        bulletPrefab.Spawn(transform.position, transform.rotation);
    }
}

When you want to recycle the instance, you can just call Recycle() on the component or game object you want to de-spawn. Here we will Recycle() our Bullet instance when it collides with something.

public class Bullet : MonoBehaviour
{
    void OnCollisionEnter(Collider other)
    {
        //De-activate the object and return it to the spawn pool
        gameObject.Recycle();
 
        //You can also use this:
        //this.Recycle();
    }
}

The Spawn() function returns a reference to the created instance, so if you want to store it or call any additional methods on it, you can do so. Unlike Unity’s Instantiate(), you do not have to cast the return value to a GameObject or Component.

Be Careful With Recycled Objects!

Now that your objects are being recycled and re-used, you have to be careful, because if your instances have any variables that change, you’ll have to manually reset them every time a new instance gets spawned. You can do this by using Unity’s OnEnable() and OnDisable() functions, which will be called whenever your instance is spawned or recycled.

For example, this is incorrect:

public class Bullet : MonoBehaviour
{
    public float travelDuration;
    float timer = 0; //Only gets set to zero once!
 
    void Update()
    {
        timer += Time.deltaTime;
 
        if (timer >= travelDuration)
        {
            gameObject.Recycle();
        }
    }
}

Why? Because our timer variable counts up, but never returns to zero! So the second time this Bullet spawns, it will immediately recycle itself. We can easily fix this:

public class Bullet : MonoBehaviour
{
    public float travelDuration;
    float timer;
 
    void OnEnable()
    {
        //Correct! Now timer resets every single time:
        timer = 0;
    }
 
    void Update()
    {
        timer += Time.deltaTime;
 
        if (timer >= travelDuration)
        {
            gameObject.Recycle();
        }
    }
}

That’s better, now our Bullet will correctly reset his timer variable every time he respawns. It’s a little bit annoying having to do this yourself, but the gained speed from using pooled objects is worth it!