sane `fsevents` in Swift

FileSystemEventStream.swift

import Foundation
import Combine

public struct Event: Identifiable {
    public let id: FSEventStreamEventId
    public let path: String
    public let flags: Flags
}

public class FileSystemEventStream {
    
    public let events$ = PassthroughSubject<Event, Never>()
    
    /// An array of strings each specifying a path to a directory,
    /// signifying the root of a filesystem hierarchy to be watched
    /// for modifications.
    public let paths: [String]
    
    /// The number of seconds the service should wait after hearing about an
    /// event from the kernel before passing it along to the client via its
    /// callback. Specifying a larger value may result in more effective
    /// temporal coalescing, resulting in fewer callbacks and greater overall
    /// efficiency.
    public let latency: TimeInterval
    
    public init(_ paths: [String], latency: TimeInterval = 0) {
        self.paths = paths
        self.latency = latency
        start()
    }
    
    deinit { stop() }
    
    public private(set) var sinceWhen: FSEventStreamEventId = FSEventStreamEventId(kFSEventStreamEventIdSinceNow)
    private var reference: FSEventStreamRef!

    @discardableResult
    public func start(on runLoop: RunLoop = .main, mode: RunLoop.Mode = .default) -> Bool {
        guard reference == nil else { return false }
        
        var context = FSEventStreamContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
        context.info = Unmanaged.passUnretained(self).toOpaque()
        
        let flags = UInt32(kFSEventStreamCreateFlagUseCFTypes | kFSEventStreamCreateFlagFileEvents)
        reference = FSEventStreamCreate(kCFAllocatorDefault, { stream, context, count, paths, flags, ids in
            guard let paths = unsafeBitCast(paths, to: NSArray.self) as? [String] else { return }
            let stream = unsafeBitCast(context, to: FileSystemEventStream.self)
            for index in 0..<count {
                stream.process(id: ids[index], path: paths[index], flags: flags[index])
            }
        }, &context, paths as CFArray, sinceWhen, latency, flags)
        
        FSEventStreamSetDispatchQueue(reference, .main)
        return FSEventStreamStart(reference)
    }
    
    public func stop() {
        guard let reference = reference else { return }
        FSEventStreamStop(reference)
        FSEventStreamInvalidate(reference)
        FSEventStreamRelease(reference)
        self.reference = nil
    }
    
    func process(id: FSEventStreamEventId, path: String, flags: FSEventStreamEventFlags) {
        events$.send(Event(id: id, path: path, flags: flags))
        sinceWhen = id
    }
}

extension Event: Codable { }

extension Event.Flags: Codable {
    public init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        self.init(rawValue: try container.decode(UInt32.self))
    }
    public func encode(to encoder: Encoder) throws {
        try rawValue.encode(to: encoder)
    }
}

extension Event {
    public init(id: FSEventStreamEventId, path: String, flags: FSEventStreamEventFlags) {
        self.id = id
        self.path = path
        self.flags = .init(rawValue: flags)
    }
}

extension Event {

    public struct Flags: OptionSet {
        
        public let rawValue: UInt32
        
        public init(rawValue: UInt32) {
            self.rawValue = rawValue
        }
        
        static let none: Flags = 0x00000000;
        static let subdirectoryChanged: Flags = 0x00000001;
        static let userDropped: Flags = 0x00000002;
        static let kernelDropped: Flags = 0x00000004;
        static let eventIdsWrapped: Flags = 0x00000008;
        static let historyDone: Flags = 0x00000010;
        static let rootChanged: Flags = 0x00000020;
        static let mount: Flags = 0x00000040;
        static let unmount: Flags = 0x00000080;
        static let created: Flags = 0x00000100;
        static let removed: Flags = 0x00000200;
        static let indexNodeMetadata: Flags = 0x00000400;
        static let renamed: Flags = 0x00000800;
        static let modified: Flags = 0x00001000;
        static let finder: Flags = 0x00002000;
        static let changeOwner: Flags = 0x00004000;
        static let extendedFileAttributes: Flags = 0x00008000;
        static let isFile: Flags = 0x00010000;
        static let isDir: Flags = 0x00020000;
        static let isSymlink: Flags = 0x00040000;
        static let isHardlink: Flags = 0x00100000;
        static let isLastHardlink: Flags = 0x00200000;
        static let ownEvent: Flags = 0x00080000;
        static let cloned: Flags = 0x00400000;

    }
    
}

extension Event.Flags: ExpressibleByIntegerLiteral {
    public init(integerLiteral value: UInt32) {
        rawValue = value
    }
}

extension Data {
    public func string(encoding: String.Encoding = .utf8) -> String? {
        String(data: self, encoding: encoding)
    }
}

let watch = FileSystemEventStream(["/Users/oliver/"])
watch.events$.sink { event in
    // ...
}.store(in: &bag)