GCD Timer using DispatchSourceTimer that is much simpler

timer.swift

import Foundation

class Timer {
    private let source: DispatchSourceTimer
    private let block: () -> ()
    
    public let isRepeating: Bool
    
    init(_ interval: DispatchTimeInterval, repeat shouldRepeat: Bool = false, block: @escaping () -> ()) {
        self.source = DispatchSource.makeTimerSource()
        self.block = block
        self.isRepeating = shouldRepeat
        
        let deadline = DispatchTime.now().advanced(by: interval)
        let repeating: DispatchTimeInterval = shouldRepeat ? interval : .never
        source.schedule(deadline: deadline, repeating: repeating)
        
        source.setEventHandler { [weak self] in self?.fire() }
        source.activate()
    }
    
    private func fire() {
        block()
        if !isRepeating { source.cancel() }
    }
    
    deinit {
        source.cancel()
    }
}