How to pass data to OOP objects?

Question

I'm new to this whole thing, so please tell me if I'm doing this wrong of if there's a better spot for my question.

I'm making a game with basic systems for projectiles, npcs and items. This game will have multiple variants of each of these types. For the purposes of this example, there will be a Bullet projectile, a Rocket projectile, and a Pistol which shoots Bullets.

The only experience I have with such a system is inside Terraria's source code, where Items are all defined in a massive if-else tree, e.g.

else if (this.type == 467)
{
    this.name = "Fireball";
    this.width = 40;
    this.height = 40;
    this.aiStyle = 1;
    this.hostile = true;
    this.damage = 10;
}

The problem with this approach is (apart from the terrible mess it makes) that adding new items requires modifications to 4-5 different areas of code - especially if you want to add any extra effects, like giving off light, burning enemies etc.

I'm trying to do this in a much better format. I've come up with a simple system that with nested classes, e.g.

public class ProjectileData
{
    public class Bullet : Projectile
    {
        public float speed;
        public Bullet() : base()
        {
            this.type = ProjectileID.Bullet;
            this.damage = 25;
            this.origin = new Vector2(9.5f, 2.5f);
            speed = 10;
        }
        public override void AI()
        {
            this.velocity += new Vector2((float)Math.Cos(rotation), (float)Math.Sin(rotation)) * speed;
        }
        public override void OnHit()
        {
            Game.CreateExplosion(this.position);
        }
    }
    public class Rocket : Projectile
    { ... }
}

With this system, to create a bullet, instead of using something like

Projectile.CreateProjectile(int id, ...)
{
    Projectile projectile = new Projectile(id);
}
...
Projectile.CreateProjectile(ProjectileID.Bullet, ...);

I have

Projectile.CreateProjectile<T>(...) where T : Projectile, new()
{
    T projectile = new T(id);
}
...
Projectile.CreateProjectile<ProjectileData.Bullet>(...);

which looks and works nicely.

However, I've run into a problem: if I want to create an item that shoots a projectile, I can't reference it. For example, with the id-based system I could write something like

item.projectileType = ProjectileID.Bullet

but it isn't possible to assign a Class to a property like that.

I guess this boils down to 2 questions:

• Is this the 'correct' way to pass data to an object or is there a better structure, with or without inheritance??
• Is there a way to reference a class, and use that reference at runtime in a generic method call?

Answer 1

Use the factory-pattern. A factory is an object which creates objects.

So when you have a weapon which shoot projectiles, pass a ProjectileFactory to it and leave the creation of the projectile to that class.

You can then have different classes which extend ProjectileFactory, like BulletProjectileFactory, ExplosiveProjectileFactory, GravityAffectedProjectileFactory, HomingProjectileFactory etc. which create bullets with different game mechanics.

For example, when you want to create a gun which shoots bullets which fly with a speed 8.0 pixels per tick, do 12 damage and use the sprite "bullet.png" you would write something like this:

 Weapon pistol = new Weapon( new BulletProjectileFactory( 8.0f, 12, "bullet.png") );

The fire() method of your weapon-class would then do something like

 projectileFactory.createProjectile();