Monitor updates to files is iOS and OSX

FileObserver.swift

import Foundation
// thanks to https://www.swiftbysundell.com/posts/a-deep-dive-into-grand-central-dispatch-in-swift

/// The FileObjserver watches a single File URL but only if the file exists
/// If the file is removed, you will get a delete event but nothing else
/// thereafter, even if the file is recreated at the same path
public class FileObserver {
    public typealias Event = DispatchSource.FileSystemEvent
    public let url: URL
    public var currentEvent: Event {
        return Event(rawValue: (source?.data ?? 0))
    }

    private let queue: DispatchQueue
    private var source: DispatchSourceFileSystemObject?

    public convenience init?(_ file: String) {
        self.init(URL(fileURLWithPath: file))
    }
    public init?(_ url: URL) {
        self.url = url
        guard (try? self.url.checkResourceIsReachable()) ?? false else { return nil }
        self.queue = DispatchQueue(label: "app.file_observer")
    }

    deinit {
        source?.cancel()
    }

    public func start(mask: Event? = nil, closure: @escaping (FileObserver) -> Void) {

        let mask = mask ?? .all
        let fd = open(url.path, O_EVTONLY)

        let source = DispatchSource
            .makeFileSystemObjectSource(fileDescriptor: fd, eventMask: mask, queue: queue)

        source.setEventHandler { [weak self] in
            guard let this = self else { return }
            closure(this)
        }
        self.source = source
        source.resume()
    }
}

extension DispatchSource.FileSystemEvent {
    public var name: String {
        switch self {
            case .all: return "all"
            case .attrib: return "attrib"
            case .delete: return "delete"
            case .extend: return "extend"
            case .funlock: return "funlock"
            case .link: return "link"
            case .rename: return "rename"
            case .revoke: return "revoke"
            case .write: return "write"
            default:
                return "<fsevent: \(rawValue)>"
        }
    }
}