bankAccount.swift

class BankAccount {
    var balance = 0.00
    
    func deposit(amount: Double) {
        balance += amount
    }
    
    func withdrawal(amount: Double) {
        balance -= amount
    }
}

BankAccountSpec.swift

import Quick
import Nimble

class BankAccountSpec: QuickSpec {
    override func spec() {
        it("starts with a 0.00 balance") {
            let account = BankAccount()
            expect(account.balance).to(equal(0.00))
        }
        
        describe("#deposit") {
            it("increases balance") {
                let account = BankAccount()
                account.deposit(133.70)
                expect(account.balance).to(equal(133.70))
            }
        }
        
        describe("#withdrawal") {
            it("decreases balance") {
                let account = BankAccount()
                account.deposit(133.70)
                account.withdrawal(100.00)
                expect(account.balance).to(equal(33.70))
            }
        }
    }
}