Unit testing code with DispatchQueue

One way of testing code with DispatchQueue is by using expectations (https://developer.apple.com/documentation/xctest/asynchronous_tests_and_expectations/testing_asynchronous_operations_with_expectations) but in most of the cases it’s more clean to treat the DispatchQueue as a dependency. In those cases as almost always passing the dependency through the constructor is the ideal way. This gives us the option to make clear to a reader that this object is doing some work on a queue and also enable us to use the object with a different queue if needed.

Let’s see a typical example with a ViewController:

import UIKit

class DispatchQueueOnViewController: UIViewController {

    @IBOutlet weak var someDataLabel: UILabel!
    
    init() {
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    @IBAction func didTapButton(_ sender: Any) {
        DispatchQueue.main.async {
            self.someDataLabel.text = "something loaded"
        }
    }
}

import UIKit

class DispatchQueueOnViewController: UIViewController {

@IBOutlet weak var someDataLabel: UILabel!

init() {

super.init(nibName: nil, bundle: nil)

}

required init?(coder: NSCoder) {

fatalError("init(coder:) has not been implemented")

}

@IBAction func didTapButton(_ sender: Any) {

DispatchQueue.main.async {

self.someDataLabel.text = "something loaded"

}

Using dependency injection we can write the above code as follows:

import UIKit

class DispatchQueueOnViewController: UIViewController {

    @IBOutlet weak var someDataLabel: UILabel!
    private let dispatchQueue: DispatchQueueType

    init(dispatchQueue: DispatchQueueType) {
        self.dispatchQueue = dispatchQueue
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    @IBAction func didTapButton(_ sender: Any) {
        dispatchQueue.async {
            self.someDataLabel.text = "something loaded"
        }
    }
}

protocol DispatchQueueType {
    func async(execute work: @escaping @convention(block) () -> Void)
}

extension DispatchQueue: DispatchQueueType {
    func async(execute work: @escaping @convention(block) () -> Void) {
        async(group: nil, qos: .unspecified, flags: [], execute: work)
    }
}

import UIKit

class DispatchQueueOnViewController: UIViewController {

@IBOutlet weak var someDataLabel: UILabel!

private let dispatchQueue: DispatchQueueType

init(dispatchQueue: DispatchQueueType) {

self.dispatchQueue = dispatchQueue

super.init(nibName: nil, bundle: nil)

}

required init?(coder: NSCoder) {

fatalError("init(coder:) has not been implemented")

}

@IBAction func didTapButton(_ sender: Any) {

dispatchQueue.async {

self.someDataLabel.text = "something loaded"

}

protocol DispatchQueueType {

func async(execute work: @escaping @convention(block) () -> Void)

}

extension DispatchQueue: DispatchQueueType {

func async(execute work: @escaping @convention(block) () -> Void) {

async(group: nil, qos: .unspecified, flags: [], execute: work)

}

Now in order to unit testing now the didTapButton action all we have to do is to initialize the ViewController with a test double for the DispatchQueue.

import XCTest
@testable import UnitTestDispatchQueue

class DispatchQueueOnViewControllerTests: XCTestCase {
    
    func test_DidTapButton_setsValueOnLabel() {
        let fakeQueue = DispatchQueueFake()
        let sut = DispatchQueueOnViewController(dispatchQueue: fakeQueue)
        _ = sut.view
        
        sut.didTapButton(UIButton())
        
        XCTAssertEqual("something loaded" , sut.someDataLabel.text)
    }
}

final class DispatchQueueFake: DispatchQueueType {
    func async(execute work: @escaping @convention(block) () -> Void) {
        work()
    }
}