DirectoryMonitor.swift

/*
 Copyright (C) 2016 Apple Inc. All Rights Reserved.
 See LICENSE.txt for this sample’s licensing information
 Partially modified by Francesco Piraneo G.
 Original url: https://github.com/robovm/apple-ios-samples/blob/master/ListerforwatchOSiOSandOSX/Swift/ListerKit/DirectoryMonitor.swift
 
 Abstract:
 `DirectoryMonitor` is used to monitor the contents of the provided directory by using a GCD dispatch source.
 */

import Foundation

/// A protocol that allows delegates of `DirectoryMonitor` to respond to changes in a directory.
protocol DirectoryMonitorDelegate: class {
    func directoryMonitorDidObserveChange(directoryMonitor: DirectoryMonitor)
}

class DirectoryMonitor {
    // MARK: Properties
    
    /// The `DirectoryMonitor`'s delegate who is responsible for responding to `DirectoryMonitor` updates.
    weak var delegate: DirectoryMonitorDelegate?
    
    /// A file descriptor for the monitored directory.
    private var monitoredDirectoryFileDescriptor: CInt = -1
    
    /// A dispatch queue used for sending file changes in the directory.
    private let directoryMonitorQueue =  DispatchQueue(label: "ch.zriemann.AnoniCloud-Connect.DirectoryMonitor", attributes: .concurrent)
    
    /// A dispatch source to monitor a file descriptor created from the directory.
    private var directoryMonitorSource: DispatchSource?
    
    /// URL for the directory being monitored.
    private var url: URL?
    
    // MARK: Monitoring
    
    func startMonitoring(_ url: URL) {
        // Listen for changes to the directory (if we are not already).
        if directoryMonitorSource == nil && monitoredDirectoryFileDescriptor == -1 {
            // Save actual monitoring URL
            self.url =  url
            
            // Open the directory referenced by URL for monitoring only.
            monitoredDirectoryFileDescriptor = open((url as NSURL).fileSystemRepresentation, O_EVTONLY)
            
            // Define a dispatch source monitoring the directory for additions, deletions, and renamings.
            directoryMonitorSource = DispatchSource.makeFileSystemObjectSource(fileDescriptor: monitoredDirectoryFileDescriptor, eventMask: DispatchSource.FileSystemEvent.write, queue: directoryMonitorQueue) as? DispatchSource
            
            // Define the block to call when a file change is detected.
            directoryMonitorSource?.setEventHandler{
                // Call out to the `DirectoryMonitorDelegate` so that it can react appropriately to the change.
                self.delegate?.directoryMonitorDidObserveChange(directoryMonitor: self)
            }
            
            // Define a cancel handler to ensure the directory is closed when the source is cancelled.
            directoryMonitorSource?.setCancelHandler{
                close(self.monitoredDirectoryFileDescriptor)
                self.monitoredDirectoryFileDescriptor = -1
                self.directoryMonitorSource = nil
            }
            
            // Start monitoring the directory via the source.
            directoryMonitorSource?.resume()
        } else {
            directoryMonitorSource?.cancel()
            startMonitoring(url)
        }
    }
    
    func stopMonitoring() {
        // Stop listening for changes to the directory, if the source has been created.
        if directoryMonitorSource != nil {
            // Stop monitoring the directory via the source.
            directoryMonitorSource?.cancel()
            url = nil
        }
    }
    
    func getMonitoringURL() -> String {
        return self.url?.absoluteString ?? "NO URL MONITORED"
    }
}

This is a partially modified macOS directory monitor published by Apple during WWDC 2016.
You can find original Apple version at: Original Apple DirectoryMonitor
Original version cannot smootly stop and restart monitoring on a different directory. I've tested my modified version it and it's working. Feel free to contact me here if you have suggestions.