Trecho para exemplificar o uso incorreto de métodos em protocolos

CurrencyConverterWrong.swift

import Foundation

protocol ExchangeRate {
    var value: Double { get }
}

extension ExchangeRate {
    func businessTaxCalculated() -> Double {
        return value*(1.5/100)
    }
}

class CurrencyConverter {
    
    private var exchangeRate: ExchangeRate
    
    init(exchangeRate: ExchangeRate) {
        self.exchangeRate = exchangeRate
    }
    
    func converter(value: Double) -> Double {
        return (value * exchangeRate.value) - exchangeRate.businessTaxCalculated()
    }
}

/// ...

struct Dollar: ExchangeRate {
    var value: Double
}

let dollar = Dollar(value: 6)
let currencyConverter = CurrencyConverter(exchangeRate: dollar)
let valueConverted = currencyConverter.converter(value: 1)
print(valueConverted) // 5.91

// Testes

import XCTest

struct DollarMock: ExchangeRate {
    var value: Double
}

class CurrencyConverterTest: XCTestCase {
    
    func testConverter_ShouldSuccess() {
        let dollarMock = DollarMock(value: 6)
        
        let currencyConverter = CurrencyConverter(exchangeRate: dollarMock)
        let amountConverter = currencyConverter.converter(value: 1)
        XCTAssertEqual(amountConverter, 5.91)
    }
}

CurrencyConverterTest.defaultTestSuite.run()