NSNumber+Countdown.m

#import "NSNumber+Countdown.h"

@implementation NSNumber (Countdown)

- (dispatch_source_t)countdownWithInterval:(NSTimeInterval)interval
                    executing:(void (^)(NSTimeInterval))executing
                   completion:(void (^)(void))completion {
    __block NSTimeInterval timeout = self.doubleValue;
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(0, 0));
    dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), interval * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(timer, ^{
        timeout -= interval;
        if (timeout < 1) {
            dispatch_source_cancel(timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                !completion ?: completion();
            });
            return;
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            !executing ?: executing(timeout);
        });
    });
    dispatch_resume(timer);
    return timer;
}

@end

TimeInterval+Countdown.swift

extension TimeInterval {
    
    public func countdown(
        repeating: TimeInterval = 1,
        executing: @escaping (TimeInterval) -> Void,
        completion: @escaping () -> Void)
        -> DispatchSourceTimer
    {
        let timer = DispatchSource.makeTimerSource()
        timer.schedule(wallDeadline: .now(), repeating: repeating)
        
        var timeout = self
        timer.setEventHandler {
            timeout -= repeating
            
            guard timeout >= 1 else {
                timer.cancel()
                DispatchQueue.main.async { completion() }
                return
            }
            
            DispatchQueue.main.async { executing(timeout) }
        }
        timer.resume()
        
        return timer
    }
}

extension Int {
    
    public func countdown(
        repeating: TimeInterval = 1,
        executing: @escaping (TimeInterval) -> Void,
        completion: @escaping () -> Void)
        -> DispatchSourceTimer
    {
        return TimeInterval(self).countdown(
            repeating: repeating,
            executing: executing,
            completion: completion)
    }
}