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.
//

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 XCTest

final class ScreenOverlayViewModelTestCase: XCTestCase {

    // MARK: - Private properties

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

    // MARK: - Public methods

    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 testShowOverlayShouldInformDelegateToUpdateAlphaToOne() {
        // When
        viewModel.showOverlay()

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

    // MARK: - Test method `hideOverlay`

    func testHideOverlayShouldInformDelegateToUpdateAlphaToZero() {
        // 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)
    }
}

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)
        )
    }