NSFileManager watch directory

Question

How would you monitor a directory with NSFileManager?

I would like to able to detect when a file is deleted/added in my documents directory while my app is running.

Answer 1

Look Kernel Queues: An Alternative to File System Events in Apple documentation. There is an example for iOS in AVPlayerDemo (look DirectoryWatcher class).

Also, check Directory Monitor blog post.

Answer 2

Here's my version of DirectoryWatcher written in Swift using GCD instead of Mach and using a closure instead of a delegate

import Foundation

@objc public class DirectoryWatcher : NSObject {
    override public init() {
        super.init()
    }

    deinit {
        stop()
    }

    public typealias Callback = (_ directoryWatcher: DirectoryWatcher) -> Void

    @objc public convenience init(withPath path: String, callback: @escaping Callback) {
        init()
        if !watch(path: path, callback: callback) {
            assert(false)
        }
    }

    private var dirFD : Int32 = -1 {
        didSet {
            if oldValue != -1 {
                close(oldValue)
            }
        }
    }
    private var dispatchSource : DispatchSourceFileSystemObject?

    @objc public func watch(path: String, callback: @escaping Callback) -> Bool {
        // Open the directory
        dirFD = open(path, O_EVTONLY)
        if dirFD < 0 {
            return false
        }

        // Create and configure a DispatchSource to monitor it
        let dispatchSource = DispatchSource.makeFileSystemObjectSource(fileDescriptor: dirFD, eventMask: .write, queue: DispatchQueue.main)
        dispatchSource.setEventHandler {[unowned self] in
            callback(self)
        }
        dispatchSource.setCancelHandler {[unowned self] in
            self.dirFD = -1
        }
        self.dispatchSource = dispatchSource

        // Start monitoring
        dispatchSource.resume()

        // Success
        return true
    }

    @objc public func stop() {
        // Leave if not monitoring
        guard let dispatchSource = dispatchSource else {
            return
        }

        // Don't listen to more events
        dispatchSource.setEventHandler(handler: nil)

        // Cancel the source (this will also close the directory)
        dispatchSource.cancel()
        self.dispatchSource = nil
    }
}

Use it like Apple's DirectoryWatcher example, something like this:

let directoryWatcher = DirectoryWatcher(withPath: "/path/to/the/folder/you/want/to/monitor/", callback: {
    print("the folder changed")
})

Destroying the object will stop watching, or you can stop it explicitly

directoryWatcher.stop()

It should be compatible with Objective C they way it's written (untested). Using it would be like this:

DirectoryWatcher *directoryWatcher = [DirectoryWatcher.alloc initWithPath: @"/path/to/the/folder/you/want/to/monitor/" callback: ^(DirectoryWatcher *directoryWatcher) {
    NSLog(@"the folder changed")
}];

Stopping it is similar

[directoryWatcher stop];