How to Write Unit Tests in Swift Using Xcode

Unit Testing default template in Xcode

Writing Unit Tests are as important as your application code. As a developer, we often ignore to write Unit Tests, because of a tight project deadline.

In this article, we will learn,

• What is Unit Testing?
• How to Include Unit Test Target to the Project?
• How to Run Unit Tests?
• How to Write Unit Tests in Swift Using Xcode?

What is Unit Testing?

• In computer programming, unit testing is a software testing method by which individual units of source code are tested to determine whether they are fit for use.
• Using unit testing, we verify our core functionality, common workflow, boundary conditions, etc.
• Unit tests are usually written by developers and can be written before, during and after development.

How to Include Unit Test Target to the Project?

A unit test target can be added to the project at 2 different times.

• During project creation, by selecting Include Unit Tests checkbox

Add Unit Test Target During Project Creation

• If the project is already created then, navigate to File ➞ New ➞ Target (or click the “+” sign from the project window) and search for Unit Testing Bundle

Add Unit Test Target After Project Creation

How to Run Unit Tests?

At this stage, we have a unit test target with the Swift file and plist. If you wish, you can add multiple unit test targets to the project.

There are three ways to run unit tests.

1. To run all the unit tests, Go to the menu bar and select Product ➞ Test or use the keyboard shortcut Command(⌘) + U
2. Click the little arrow button in the Test Navigator (⌘6)
3. Click the diamond button (♦) in the gutter.

Steps to Run Unit Tests

To run an individual test, click the diamond button in front of the test method name in the gutter or in the Test navigator

When all the test executes successfully, you should see green checkmarks in the Test Navigator and Test Succeeded message on the Xcode window.

Tip: Run your last test again using keyboard shortcut: ⌃ Control + ⌥ Option + ⌘ Command + G

How to Write Unit Tests in Swift Using Xcode?

• Before writing any unit test, you must know what functionality or logic you need to test. Otherwise, you end up with too many test cases that are not really useful and wasting precious development time.
• The default unit test target template imports the testing framework, XCTest, and defines a UnitTestExampleTests subclass of XCTestCase, with setUp(), tearDown(), testExample() and testPerformanceExample() methods.
• The setUp() & tearDown() instance method will call before and after each test method respectively.

Note: To know more about setUp() and tearDown() method read Apple’s official article on Understanding Setup and Teardown for Test Methods.

At this stage, we are ready to write our first test.

Unit Test #1

Add the following code to the end of UnitTestExampleTests:

//  UnitTestEampleTests.swift
import XCTest
func testSumOfTwoNumbers() {
   let num1 = 10
   let num2 = 20
   let sum = num1 + num2
   XCTAssertEqual(sum, 30, "sum should be 30")
}

• A test method’s name always starts with the test, followed by a description of what it tests.
• Here, we are verifying the value of sum variable using XCTAssertEqual — Asserts that two values are equal.
• Run your test using the keyboard shortcut ⌘+U to verify the sum value. If the test case pass, then it will show a green checkmark in the Test Navigator.

Unit Test #2 — Accessing methods written in code

Suppose we have written the method in our project which will return a product of the two integer numbers. Now, we want to verify the output of that method thru the unit test.

Add the following code to the end of UnitTestExampleTests:

//  UnitTestEampleTests.swift
import XCTest
@testable import UnitTestEample
func testProductOfTwoNumbers() {
   let vc = ViewController()
   let product = vc.productOfTwoNumbers(num1: 2, num2: 3)
   XCTAssertEqual(product, 6, "product should be 6")
}

• To access your code into the UnitTest module, you have to use the keyword @testable import
• Here, we are verifying the output of the method productOfTwoNumbers which is written into the ViewController class.

Unit Test #3 — Async method

So far we have learned how to write unit tests and how to access methods written in our main project. Now we’ll learn how to write a unit test for an async method (ex: network call)

Add the following code to the end of UnitTestExampleTests:

//  UnitTestEampleTests.swift
func testFetchTODOList() {
   let exp = expectation(description:"fetching to-do list from server")
   
   let session: URLSession = URLSession(configuration: .default)
   let url = URL(string: "https://jsonplaceholder.typicode.com/todos")
   session.dataTask(with: url!) { data, response, error in
      XCTAssertNil(error)
      exp.fulfill()
   }.resume()
   waitForExpectations(timeout: 10.0) { (error) in
      print(error?.localizedDescription ?? "error")
   }
}

This test checks that fetching to-do lists from the server returns expected output or not. To check the async method, we used XCTestExpectation an object. Where we first define the expectation and wait for the desired time until expectation fulfills.

• First of all, we create an expectation(description:) which will return an XCTestExpectation object, stored in exp. The description parameter describes what you expect to happen.
• Next, we call waitForExpectation(timeout:handler:) keeps the test running until all expectations are fulfilled, or the timeout interval ends, whichever happens first.
• Finally, we call exp.fulfill() method when the result meets our expectations.

You can check out the full list of Apple’s XCTestAssertions

milanpanchal/UnitTestEample

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Thanks for reading! 👨🏼‍💻