Demonstration of a mocked Timer

Timing.swift

import Foundation

protocol Timing {
    associatedtype TimerType: Timing
    static func scheduledTimer(withTimeInterval: TimeInterval, repeats: Bool, block: @escaping (TimerType) -> Void) -> TimerType
    func invalidate()
    func fire()
}

extension Timer: Timing {}

struct MockTimer: Timing {
    typealias TimerType = Self
    static var timerCreated: ((Self) -> Void)?
    
    var timeInterval: TimeInterval
    var repeats: Bool
    var block: (Self) -> Void
    var mockInvalidate: () -> Void
    
    static func scheduledTimer(
        withTimeInterval timeInterval: TimeInterval,
        repeats: Bool,
        block: @escaping (TimerType) -> Void
    ) -> TimerType {
        var invalidatableBlock: ((TimerType) -> Void)? = block
        
        let timer = MockTimer(
            timeInterval: timeInterval,
            repeats: repeats,
            block: { invalidatableBlock?($0) },
            mockInvalidate: { invalidatableBlock = nil }
        )
        
        timerCreated?(timer)
        return timer
    }
    
    func invalidate() {
        mockInvalidate()
    }
    
    func fire() {
        block(self)
    }
}