class properties

gistfile1.swift

class Tank {
    class var bonusDamage: Double {
        return Double(Upgrade.level) * 2.5
    }
    
    let baseDamage = 10.0
    
    var damage: Double {
        return self.baseDamage + Tank.bonusDamage
    }
    
    class func upgrade() {
        Upgrade.level += 1
    }
    
    struct Upgrade {
        static var level = 0
    }
}

var tank = Tank()

println(tank.damage)
// 10.0

Tank.upgrade()

println(tank.damage)
// 12.5

Tank.upgrade()

println(tank.damage)
// 15.0