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 */
    }
}

@DevyV Not sure if you've seen it, but I wrote an accompanying blog post that might be helpful https://thoughtbot.com/blog/waiting-for-file-write-completion-on-ios

errorHandler is an optional callback function that handles any errors that might occur during file reading.

You'll want to listen to notifications for .NSFileHandleReadToEndOfFileCompletion, and update your label in the notification handler function. Line 62 shows an example class that calls the handler function when it receives a notification.

Hello Sid,

Thanks for your help, I managed to make it work :)

Actually, I am facing a strange issue now:
as long as the monitored file is being edited and saved by a text editor (e.g. SubEthaEdit) Filewatcher can detect changes.
But when I change the contents of the monitored file with AppleScript (see example below) then Filewatcher does NOT detect the change.

Why? Do you have any idea how to fix it?

Here is my AppleScript used to change the contents of the file:

set documentsPath to POSIX path of (path to documents folder from user domain)
set my_file to (documentsPath & "test_file.txt")
set newContent to "new text"

set fileHandle to open for access my_file with write permission
set eof of fileHandle to 0 -- make sure to overwrite the whole contents in the file
write ((newContent as string) & return) to fileHandle --- starting at eof
close access fileHandle

Sid,

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

eventMask: .write,

to

eventMask: .all,