Tech7 min read

Object pooling in ActionScript 3, step by step

Creating and discarding many short-lived objects makes the garbage collector work in bursts, and those bursts show up as stutter on phones. A pool keeps finished objects around so they can be reused.

A minimal pool

package pool {
    public class Pool {
        private var _items:Vector.<Object> = new Vector.<Object>();
        private var _factory:Function;

        public function Pool(factory:Function, prewarm:int = 0) {
            _factory = factory;
            for (var i:int = 0; i < prewarm; i++) {
                _items.push(_factory());
            }
        }

        public function take():Object {
            return _items.length > 0 ? _items.pop() : _factory();
        }

        public function give(item:Object):void {
            _items.push(item);
        }
    }
}

Using it

var bullets:Pool = new Pool(function():Object {
    return new Bullet();
}, 40);

// firing
var b:Bullet = bullets.take() as Bullet;
b.reset(x, y, angle);
layer.addChild(b);

// when it leaves the screen
layer.removeChild(b);
bullets.give(b);

Rules that keep it safe

• Give every pooled class a reset() method that puts it back into a clean state. Do not rely on the constructor.
• Remove event listeners before returning an object, or it will keep reacting while it sits in the pool.
• Pre-warm the pool during a loading screen so the first burst of use does not allocate.
• Measure first. A pool adds bookkeeping, and it only pays off for objects created many times per second.

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