FileWatcher / FileListener

FileWatcherFileListener.swift

import Foundation

fileprivate let queue = DispatchQueue(label: "FileMonitorQueue", target: .main)

enum FileWatcherError: Error {
    case directoryOpenFailed(Int32)
    case directoryURLInvalid(URL)
}

final class FileWatcher {
    let fsSource: DispatchSourceFileSystemObject
    var readSource: DispatchSourceRead!
    
    init(in directory: URL, filename: String, errorHandler: ((Error) -> Void)?) throws {
        let fileDescriptor: Int32 = try directory.withUnsafeFileSystemRepresentation { path in
            guard let path = path else { throw FileWatcherError.directoryURLInvalid(directory) }
            
            let result = open(path, O_EVTONLY)
            guard result >= 0 else { throw FileWatcherError.directoryOpenFailed(errno) }
            
            return result
        }
        
        fsSource = DispatchSource.makeFileSystemObjectSource(
            fileDescriptor: fileDescriptor,
            eventMask: .write,
            queue: queue
        )
        
        fsSource.setEventHandler { [unowned self] in
            do {
                let fileHandle = try FileHandle(
                    forReadingFrom: directory.appendingPathComponent(filename)
                )
                
                self.readSource = DispatchSource.makeReadSource(fileDescriptor: fileHandle.fileDescriptor, queue: queue)
                self.readSource.setEventHandler {
                    fileHandle.readToEndOfFileInBackgroundAndNotify()
                }
                self.readSource.resume()
                
                self.readSource.setCancelHandler {
                    fileHandle.closeFile()
                }
            } catch {
                errorHandler?(error)
            }
        }
        
        fsSource.setCancelHandler {
            close(fileDescriptor)
        }
        
        fsSource.resume()
    }
    
    deinit {
        fsSource.cancel()
    }
}

final class FileListener {
    init() {
        NotificationCenter.default.addObserver( 
            self,
            selector: #selector(handler),
            name: .NSFileHandleReadToEndOfFileCompletion,
            object: nil
        )
    }
    
    @objc func handler() {
        /* file is available for reading here */
    }
}

Sid,

I solved it myself :)
I needed to change line 26 from:

eventMask: .write,

to

eventMask: .all,