Code sample #4 from https://medium.com/@johnsundell/simple-swift-dependency-injection-with-functions-7b0ec5b49e60

Simple-Swift-dependency-injection-with-functions-3.swift

class CardGameTests: XCTestCase {
    func testDrawingRandomCard() {
        var randomizationUpperBound: UInt32?
        
        let deck = Deck(cards: [Card(value: .ace, suite: .spades)])
        
        let game = CardGame(deck: deck, randomizer: { upperBound in            
            // Capture the upper bound to be able to assert it later
            randomizationUpperBound = upperBound
            
            // Return a constant value to remove randomness from
            // our test, making it run consistently.
            return 0
        })
        
        XCTAssertEqual(randomizationUpperBound, 1)
        XCTAssertEqual(game.drawRandomCard(), Card(value: .ace, suite: .spades))
    }
}