SimplePool.cs

///
/// Simple pooling for Unity.
///   Author: Martin "quill18" Glaude (quill18@quill18.com)
///   Latest Version: https://gist.github.com/quill18/5a7cfffae68892621267
///   License: CC0 (http://creativecommons.org/publicdomain/zero/1.0/)
///   UPDATES:
/// 	2015-04-16: Changed Pool to use a Stack generic.
/// 
/// Usage:
/// 
///   There's no need to do any special setup of any kind.
/// 
///   Instead of calling Instantiate(), use this:
///       SimplePool.Spawn(somePrefab, somePosition, someRotation);
/// 
///   Instead of destroying an object, use this:
///       SimplePool.Despawn(myGameObject);
/// 
///   If desired, you can preload the pool with a number of instances:
///       SimplePool.Preload(somePrefab, 20);
/// 
/// Remember that Awake and Start will only ever be called on the first instantiation
/// and that member variables won't be reset automatically.  You should reset your
/// object yourself after calling Spawn().  (i.e. You'll have to do things like set
/// the object's HPs to max, reset animation states, etc...)
/// 
/// 
/// 


using UnityEngine;
using System.Collections.Generic;

public static class SimplePool {

	// You can avoid resizing of the Stack's internal data by
	// setting this to a number equal to or greater to what you
	// expect most of your pool sizes to be.
	// Note, you can also use Preload() to set the initial size
	// of a pool -- this can be handy if only some of your pools
	// are going to be exceptionally large (for example, your bullets.)
	const int DEFAULT_POOL_SIZE = 3;

	/// <summary>
	/// The Pool class represents the pool for a particular prefab.
	/// </summary>
	class Pool {
		// We append an id to the name of anything we instantiate.
		// This is purely cosmetic.
		int nextId=1;

		// The structure containing our inactive objects.
		// Why a Stack and not a List? Because we'll never need to
		// pluck an object from the start or middle of the array.
		// We'll always just grab the last one, which eliminates
		// any need to shuffle the objects around in memory.
		Stack<GameObject> inactive;

		// The prefab that we are pooling
		GameObject prefab;

		// Constructor
		public Pool(GameObject prefab, int initialQty) {
			this.prefab = prefab;

			// If Stack uses a linked list internally, then this
			// whole initialQty thing is a placebo that we could
			// strip out for more minimal code. But it can't *hurt*.
			inactive = new Stack<GameObject>(initialQty);
		}

		// Spawn an object from our pool
		public GameObject Spawn(Vector3 pos, Quaternion rot) {
			GameObject obj;
			if(inactive.Count==0) {
				// We don't have an object in our pool, so we
				// instantiate a whole new object.
				obj = (GameObject)GameObject.Instantiate(prefab, pos, rot);
				obj.name = prefab.name + " ("+(nextId++)+")";

				// Add a PoolMember component so we know what pool
				// we belong to.
				obj.AddComponent<PoolMember>().myPool = this;
			}
			else {
				// Grab the last object in the inactive array
				obj = inactive.Pop();

				if(obj == null) {
					// The inactive object we expected to find no longer exists.
					// The most likely causes are:
					//   - Someone calling Destroy() on our object
					//   - A scene change (which will destroy all our objects).
					//     NOTE: This could be prevented with a DontDestroyOnLoad
					//	   if you really don't want this.
					// No worries -- we'll just try the next one in our sequence.

					return Spawn(pos, rot);
				}
			}

			obj.transform.position = pos;
			obj.transform.rotation = rot;
			obj.SetActive(true);
			return obj;

		}

		// Return an object to the inactive pool.
		public void Despawn(GameObject obj) {
			obj.SetActive(false);

			// Since Stack doesn't have a Capacity member, we can't control
			// the growth factor if it does have to expand an internal array.
			// On the other hand, it might simply be using a linked list 
			// internally.  But then, why does it allow us to specify a size
			// in the constructor? Maybe it's a placebo? Stack is weird.
			inactive.Push(obj);
		}

	}


	/// <summary>
	/// Added to freshly instantiated objects, so we can link back
	/// to the correct pool on despawn.
	/// </summary>
	class PoolMember : MonoBehaviour {
		public Pool myPool;
	}

	// All of our pools
	static Dictionary< GameObject, Pool > pools;

	/// <summary>
	/// Initialize our dictionary.
	/// </summary>
	static void Init (GameObject prefab=null, int qty = DEFAULT_POOL_SIZE) {
		if(pools == null) {
			pools = new Dictionary<GameObject, Pool>();
		}
		if(prefab!=null && pools.ContainsKey(prefab) == false) {
			pools[prefab] = new Pool(prefab, qty);
		}
	}

	/// <summary>
	/// If you want to preload a few copies of an object at the start
	/// of a scene, you can use this. Really not needed unless you're
	/// going to go from zero instances to 100+ very quickly.
	/// Could technically be optimized more, but in practice the
	/// Spawn/Despawn sequence is going to be pretty darn quick and
	/// this avoids code duplication.
	/// </summary>
	static public void Preload(GameObject prefab, int qty = 1) {
		Init(prefab, qty);

		// Make an array to grab the objects we're about to pre-spawn.
		GameObject[] obs = new GameObject[qty];
		for (int i = 0; i < qty; i++) {
			obs[i] = Spawn (prefab, Vector3.zero, Quaternion.identity);
		}

		// Now despawn them all.
		for (int i = 0; i < qty; i++) {
			Despawn( obs[i] );
		}
	}

	/// <summary>
	/// Spawns a copy of the specified prefab (instantiating one if required).
	/// NOTE: Remember that Awake() or Start() will only run on the very first
	/// spawn and that member variables won't get reset.  OnEnable will run
	/// after spawning -- but remember that toggling IsActive will also
	/// call that function.
	/// </summary>
	static public GameObject Spawn(GameObject prefab, Vector3 pos, Quaternion rot) {
		Init(prefab);

		return pools[prefab].Spawn(pos, rot);
	}

	/// <summary>
	/// Despawn the specified gameobject back into its pool.
	/// </summary>
	static public void Despawn(GameObject obj) {
		PoolMember pm = obj.GetComponent<PoolMember>();
		if(pm == null) {
			Debug.Log ("Object '"+obj.name+"' wasn't spawned from a pool. Destroying it instead.");
			GameObject.Destroy(obj);
		}
		else {
			pm.myPool.Despawn(obj);
		}
	}
	
}

How would one call Spawn within a Coroutine? I ask, as I'm looking to spawn an object after X seconds.

From what I can tell Coroutines do not support being called on an object that is not activated, which is what this script expects.

Great job!

5 Years on and I am learning from your videos because you don't skip straight to the 'best' methods you explain every step you make.

And if you need the despawn after a delay without using dirty coroutines

/// <summary>
/// Despawn the specified gameobject back into its pool after a delay
/// </summary>
public static void Despawn(GameObject obj, int after)
{
    Task.Delay(after).ContinueWith(t=> UnityMainThreadDispatcher.Instance().Enqueue(() => Despawn(obj)));
}

Using UnityMainThreadDispatcher. Why ? Because you can't interact with UI outside main thread ;)