iOS에서는 테스트 코드를 어떻게 작성할까요?
궁금증을 해결하기 위해 Unit Test에 대해 알아볼게요!

다음의 코드를 참고하시면 좋아요.

GitHub - ChangJin-Lee/Swift-UnitTesting: Swift-UnitTesting

Unit Test

Arrange

• Arrange inputs and targets. Arrange steps should set up the test case. Does the test require any objects or special settings? Does it need to prep a database? Does it need to log into a web app? Handle all of these operations at the start of the test.
• 요약 : 앱에 들어오는 input 값

Act

• Act on the target behavior. Act steps should cover the main thing to be tested. This could be calling a function or method, calling a REST API, or interacting with a web page. Keep actions focused on the target behavior.
• 요약 : 실제로 행위를 하는 것. function call.

Assert

• Assert expected outcomes. Act steps should elicit some sort of response. Assert steps verify the goodness or badness of that response. Sometimes, assertions are as simple as checking numeric or string values. Other times, they may require checking multiple facets of a system. Assertions will ultimately determine if the test passes or fails.
• 요약 : Act 단계에서 받은 반응들이 올바른지 판단하는 단계

target struct, function

struct Calculation {
    func calculateTip(of enteredAmount: Double, with tip: Double) -> Double? {
        guard enteredAmount >= 0 && tip >= 0 else { return nil }
        let tipPercentage = tip / 100
        return enteredAmount * tipPercentage
        
    }
}

calculationTip 안에 enteredAmount와 tip이 파라미터로 들어와요.
우리는 이 두 값이 변할 때 문제가 될만한 경우를 파악해서 test code를 작성할거에요.

TestCode

//
//  CalculationTests.swift
//  unitTestingTests
//
//  Created by CHANG JIN LEE on 2023/07/07.
//

import XCTest
@testable import unitTesting

final class CalculationTests: XCTestCase {

    func testSuccessfulTipCalculation(){
        // Given (Arrange)

        let enteredAmount = 100.00
        let tipSlider = 25.0
        let calculation = Calculation()

        // When (Act)
        let tip = calculation.calculateTip(of: enteredAmount, with: tipSlider)

        // Then (Assert)
        XCTAssertEqual(tip, 25)
    }

    func testNegativeEnteredAmountTipCalculation(){
        // Given (Arrange)

        let enteredAmount = -100.00
        let tipSlider = 25.0
        let calculation = Calculation()

        // When (Act)
        let tip = calculation.calculateTip(of: enteredAmount, with: tipSlider)

        // Then (Assert)
        XCTAssertNil(tip)
    }
}

func testSuccessfulTipCalculation() 에서는 계산된 팁이 25인지 아닌지 판단해요.
만약 25라면 테스트를 통활 것이고, 아니라면 통과하지 못해요.

그런데 코딩을 하다가 누군가 Calculation struct 안에 있는 calculationTip 함수 내부에 guard enteredAmount >= 0 && tip >= 0 else { return nil } 부분을 주석처리 했다고 생각해보아요.

struct Calculation {
    func calculateTip(of enteredAmount: Double, with tip: Double) -> Double? {
        //guard enteredAmount >= 0 && tip >= 0 else { return nil }
        let tipPercentage = tip / 100
        return enteredAmount * tipPercentage
        
    }
}

그렇다면 func testNegativeEnteredAmountTipCalculation() 에서 테스트를 통과하지 못해요.
왜냐하면 tip은 nil이 아닌 값을 반환하기 때문이에요.

참고자료

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https://www.youtube.com/watch?v=opkU2UuPk0A