Swift / Cocoa: How to watch folder for changes?

Question

I'm writing a small macOS app, where I want to be able to watch a folder for changes. It doesn't need to watch subfolder, I only want to receive a notification if a file is added to the folder or removed.

It looks like NSFileCoordinator and/or NSFilePresenter could be used to achieve this, but I was not able to understand how to use them to achieve this.

Ideally this can be solved without having to include a third party framework.

Answer 1

You can do this using NSFilePresenter. The observing class must conform to NSFilePresenter as shown below.

The presentedItemURL would point to the folder you want to observe. If there is a change in the folder presentedSubitemDidChangeAtURL get called. The code snipped below could give you an idea how it can work.

class ObservingClass: NSObject, NSFilePresenter {

    lazy var presentedItemOperationQueue = NSOperationQueue.mainQueue()
    var presentedItemURL:NSURL?
    

    func presentedSubitemDidChangeAtURL(url: NSURL) {
        let pathExtension = url.pathExtension    
        if pathExtension == "png"{
            refreshImages()
        }
    }

   func refreshImages(){
        let path = snapshotPath
        var isDirectory: ObjCBool = ObjCBool(false)
        
        if NSFileManager.defaultManager().fileExistsAtPath(path!, isDirectory: &isDirectory){
            if isDirectory{
                do {
                    let list = try NSFileManager.defaultManager().contentsOfDirectoryAtPath(path!) as Array<String>
                    for filePath in list {
                        if filePath.hasSuffix(".png"){
                            if let snapshot = snapshotAtPath(path! + "/" + filePath){
                                newSnapshotArray += [snapshot]
                            }
                        }
                    }
                } catch {
                    // error handling 
                }
            }
        }
    }
}

Best wishes.

Answer 2

Marc T's answer still works and seems to be the easiest solution for this.

To make it work I needed to add the following line (could be at the init() of ObservingClass):

NSFileCoordinator.addFilePresenter(self)