Get notified when something is added to NSPasteboard

In my application I want to get notified if something is added into the NSPasteboard. If I copy an text from any other program I want my application to know about it.

Somewhere I read it can't be done that way. I should create a timer and check the content of the NSPasteboard myself.

Is this the way to do? Or are there any kin开发者_开发技巧d of notifications?

Yes, you essentially have to poll the pasteboard to see if its contents have changed. It's not ideal, but it's possible. Basically, you have a timer that fires once or twice a second and checks the -[NSPasteboard changeCount]. If the changeCount changes, that means the contents of the pasteboard have also changed (or there's at least a new owner).

Based on answer provided by Dave DeLong I came up with similar implementation but in swift, here is the link to its gist: PasteboardWatcher.swift

Code snippet from same:

class PasteboardWatcher : NSObject {

    // assigning a pasteboard object
    private let pasteboard = NSPasteboard.generalPasteboard()

    // to keep track of count of objects currently copied
    // also helps in determining if a new object is copied
    private var changeCount : Int

    // used to perform polling to identify if url with desired kind is copied
    private var timer: NSTimer?

    // the delegate which will be notified when desired link is copied
    var delegate: PasteboardWatcherDelegate?

    // the kinds of files for which if url is copied the delegate is notified
    private let fileKinds : [String]

    /// initializer which should be used to initialize object of this class
    /// - Parameter fileKinds: an array containing the desired file kinds
    init(fileKinds: [String]) {
        // assigning current pasteboard changeCount so that it can be compared later to identify changes
        changeCount = pasteboard.changeCount

        // assigning passed desired file kinds to respective instance variable
        self.fileKinds = fileKinds

        super.init()
    }
    /// starts polling to identify if url with desired kind is copied
    /// - Note: uses an NSTimer for polling
    func startPolling () {
        // setup and start of timer
        timer = NSTimer.scheduledTimerWithTimeInterval(2, target: self, selector: Selector("checkForChangesInPasteboard"), userInfo: nil, repeats: true)
    }

    /// method invoked continuously by timer
    /// - Note: To keep this method as private I referred this answer at stackoverflow - [Swift - NSTimer does not invoke a private func as selector](http://stackoverflow.com/a/30947182/217586)
    @objc private func checkForChangesInPasteboard() {
        // check if there is any new item copied
        // also check if kind of copied item is string
        if let copiedString = pasteboard.stringForType(NSPasteboardTypeString) where pasteboard.changeCount != changeCount {

            // obtain url from copied link if its path extension is one of the desired extensions
            if let fileUrl = NSURL(string: copiedString) where self.fileKinds.contains(fileUrl.pathExtension!){

                // invoke appropriate method on delegate
                self.delegate?.newlyCopiedUrlObtained(copiedUrl: fileUrl)
            }

            // assign new change count to instance variable for later comparison
            changeCount = pasteboard.changeCount
        }
    }
}

Note: in the shared code I am trying to identify if user has copied a file url or not, the provided code can easily be modified for other general purposes.