User-6.swift

/// Mock Subclass of ImageDownloader
/// Fetch API will return what you were passed while init.
internal class MockImageDownloader: ImageFetcher {

    typealias MockResponse = (UIImage?, Error?)

    /// Mocked response. Pass this as part of init and this will be the response of fetch API
    let mockedResponse: MockResponse

    init(url: URL, response: MockResponse) {
        mockedResponse = response
        super.init(url: url)
    }

    override func fetch(then completion: @escaping (UIImage?, Error?) -> Void) {
        completion(mockedResponse.0, mockedResponse.1)
    }
}

let currentUser = User(name: "Tim Cook")
let imageFetcher = MockImageDownloader(url: URL(string: "https://somerandomapp.mlaz.im/images/GkdhaKGjhgj7yjHo")!, response: (UIImage(named: "timcook.png"), nil))
currentUser.fetchProfileImage(fetcher: imageFetcher) { (downloaded) in
    XCTAssertTrue(downloaded, "Downloaded is expected to be true when image is fetched")
    XCTAssertNotNil(currentUser.iProfileImage, "Fetched image should be available in the internal variable.")
    XCTAssertEqual(currentUser.iProfileImage, currentUser.profileImage, "Fetched image should be available in the public variable when fetch is successful")
}

// Or even you can do this if ImageFileFetcher is tested separately
let imageFileFetcher = ImageFileFetcher(imageName: "timcook.png")
currentUser.fetchProfileImage(fetcher: imageFetcher) { (downloaded) in
    XCTAssertTrue(downloaded, "Downloaded is expected to be true when image is fetched")
    XCTAssertNotNil(currentUser.iProfileImage, "Fetched image should be available in the internal variable.")
    XCTAssertEqual(currentUser.iProfileImage, currentUser.profileImage, "Fetched image should be available in the public variable when fetch is successful")
}