Factory Pattern

The Factory pattern moves the choice of which class to create into one place. The rest of the program asks the factory for "a goblin" and gets an object back, without naming the class Goblin.

The products share a parent class, so the code that uses them works with any of them:

# enemy.gd
class_name Enemy
extends RefCounted

var hp := 10

func attack() -> String:
	return "The enemy waits"

# goblin.gd
class_name Goblin
extends Enemy

func attack() -> String:
	return "The goblin stabs"

# orc.gd
class_name Orc
extends Enemy

func attack() -> String:
	return "The orc smashes"

The factory is a class with a static func that turns a name into an object. Its return type is the parent class, so it can return any kind of enemy, or null for a name it does not know:

# enemy_factory.gd
class_name EnemyFactory
extends RefCounted

static func create(kind: String) -> Enemy:
	match kind:
		"goblin":
			return Goblin.new()
		"orc":
			return Orc.new()
	return null

# main.gd
for kind in ["orc", "goblin", "slime"]:
	var enemy := EnemyFactory.create(kind)
	if enemy == null:
		print("Unknown enemy: ", kind)
	else:
		print(enemy.attack())

The orc smashes
The goblin stabs
Unknown enemy: slime

Names like these usually come from data: a spawn table, a level file, a command the player typed. The factory is the only place that maps them to classes, so adding a Dragon takes one new file and one new branch in the match.

A factory can also set the object up before returning it, so every caller gets an enemy that is ready to use:

static func create_boss(kind: String, level: int) -> Enemy:
	var boss := create(kind)
	if boss != null:
		boss.hp = 100 * level
	return boss

EnemyFactory.create_boss("orc", 3) returns an Orc with 300 HP.