Example on how to elegantly test a delegate protocol.

ScreenOverlayViewModel.swift

//
//  ScreenOverlayViewModel.swift
//
//  Created by Felix Mau on 12.10.19.
//  Copyright © 2019 Felix Mau. All rights reserved.
//

import Foundation

protocol ScreenOverlayViewModelDelegate: AnyObject {
  func screenOverlayViewModelDidUpdate(alpha: CGFloat)
}

final class ScreenOverlayViewModel {

  // MARK: - Public Properties

  weak var delegate: ScreenOverlayViewModelDelegate?

  // MARK: - Public Methods

  func showOverlay() {
    delegate?.screenOverlayViewModelDidUpdate(alpha: 1.0)
  }

  func hideOverlay() {
    delegate?.screenOverlayViewModelDidUpdate(alpha: 0.0)
  }
}

ScreenOverlayViewModelTestCase.swift

//
//  ScreenOverlayViewModelTestCase.swift
//
//  Created by Felix Mau on 12.10.19.
//  Copyright © 2019 Felix Mau. All rights reserved.
//

import Foundation
import XCTest

/// - Note: See `ScreenOverlayViewModelTests` for the Swift Testing variant of these tests.
final class ScreenOverlayViewModelTestCase: XCTestCase {

  // MARK: - Private Properties

  private var delegateMock: ScreenOverlayViewModelDelegateMock!
  private var viewModel: ScreenOverlayViewModel!

  // MARK: - Test Lifecycle

  override func setUp() {
    super.setUp()

    delegateMock = ScreenOverlayViewModelDelegateMock()

    viewModel = ScreenOverlayViewModel()
    viewModel.delegate = delegateMock
  }

  override func tearDown() {
    viewModel = nil
    delegateMock = nil

    super.tearDown()
  }

  // MARK: - Test Method `showOverlay()`

  func test_showOverlay_shouldInformDelegate_toUpdateAlpha_toOne() {
    // When
    viewModel.showOverlay()

    // Then
    XCTAssertEqual(delegateMock.invokedMethod, .screenOverlayViewModelDidUpdate(alpha: 1.0))
  }

  // MARK: - Test Method `hideOverlay()`

  func test_hideOverlay_shouldInformDelegate_toUpdateAlpha_toZero() {
    // When
    viewModel.hideOverlay()

    // Then
    XCTAssertEqual(delegateMock.invokedMethod, .screenOverlayViewModelDidUpdate(alpha: 0.0))
  }
}

// MARK: - Mocks

private final class ScreenOverlayViewModelDelegateMock: ScreenOverlayViewModelDelegate {

  // MARK: - Types

  enum DelegateMethod: Equatable {
    case screenOverlayViewModelDidUpdate(alpha: CGFloat)
  }

  // MARK: - Public properties

  private(set) var invokedMethod: DelegateMethod?

  // MARK: - `ScreenOverlayViewModelDelegate` Protocol

  func screenOverlayViewModelDidUpdate(alpha: CGFloat) {
    invokedMethod = .screenOverlayViewModelDidUpdate(alpha: alpha)
  }
}

ScreenOverlayViewModelTests.swift

//
//  ScreenOverlayViewModelTests.swift
//
//  Created by Felix Mau on 25.01.26.
//  Copyright © 2026 Felix Mau. All rights reserved.
//

import Foundation
import Testing

/// - Note: See `ScreenOverlayViewModelTestCase` for the XCTest variant of these tests.
struct ScreenOverlayViewModelTests {

  // MARK: - Private Properties

  private let delegateMock: ScreenOverlayViewModelDelegateMock
  private let viewModel: ScreenOverlayViewModel

  // MARK: - Initializer

  init() {
    delegateMock = ScreenOverlayViewModelDelegateMock()

    viewModel = ScreenOverlayViewModel()
    viewModel.delegate = delegateMock
  }

  // MARK: - Test Method `showOverlay()`

  @Test
  func showOverlay_shouldInformDelegate_toUpdateAlpha_toOne() {
    // When
    viewModel.showOverlay()

    // Then
    #expect(delegateMock.invokedMethod == .screenOverlayViewModelDidUpdate(alpha: 1.0))
  }

  // MARK: - Test Method `hideOverlay()`

  @Test
  func hideOverlay_shouldInformDelegate_toUpdateAlpha_toZero() {
    // When
    viewModel.hideOverlay()

    // Then
    #expect(delegateMock.invokedMethod == .screenOverlayViewModelDidUpdate(alpha: 0.0))
  }
}

// MARK: - Mocks

private final class ScreenOverlayViewModelDelegateMock: ScreenOverlayViewModelDelegate {

  // MARK: - Types

  enum DelegateMethod: Equatable {
    case screenOverlayViewModelDidUpdate(alpha: CGFloat)
  }

  // MARK: - Public properties

  private(set) var invokedMethod: DelegateMethod?

  // MARK: - `ScreenOverlayViewModelDelegate` Protocol

  func screenOverlayViewModelDidUpdate(alpha: CGFloat) {
    invokedMethod = .screenOverlayViewModelDidUpdate(alpha: alpha)
  }
}

In case you need to verify multiple delegate methods are called, you can change the type of the property invokedMethod to an array:

private(set) var invokedMethods = [DelegateMethod]()

And in the delegate protocol method use append() to add all invoked methods to that array:

  func screenOverlayViewModelDidUpdate(alpha: CGFloat) {
    invokedMethods.append(
      .screenOverlayViewModelDidUpdate(alpha: alpha)
    )
  }