你可以使用GCD的Dispatch Source来监控一个文件。下面是一个简单的例子，我们监控一个文件，并在每次更新后用该文件的内容更新一个NSTextView。

class ViewController: NSViewController {
    @IBOutlet var textView: NSTextView!

    // Create a dispatch_source_t to monitor the file
    func dispatchSoureForFile(at path: String) -> dispatch_source_t? {
        let fileHandle = open(path.cStringUsingEncoding(NSUTF8StringEncoding)!, O_RDONLY)

        // Cannot open file
        guard fileHandle != -1 else {
            return nil
        }

        // The queue where the event handler will execute. Don't set to the main queue
        let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

        // The events we are interested in
        let mask = DISPATCH_VNODE_DELETE | DISPATCH_VNODE_WRITE | DISPATCH_VNODE_EXTEND

        return dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, UInt(fileHandle), mask, queue)
    }

    // The function to use for monitoring a file
    func startMonitoringFile(at path: String) {
        guard let dispatchSource = dispatchSoureForFile(at: path) else {
            print("Cannot create dispatch source to monitor file at '\(path)'")
            return
        }

        let eventHandler = {
            let data = dispatch_source_get_data(dispatchSource)

            // Tell what change happened to the file. Delete it if you want
            if data & DISPATCH_VNODE_WRITE != 0 {
                print("File is written to")
            }
            if data & DISPATCH_VNODE_EXTEND != 0 {
                print("File is extended to")
            }
            if data & DISPATCH_VNODE_DELETE != 0 {
                print("File is deleted")
            }

            // Sometimes the old version of the file is deleted before the new version is written
            // to disk. This happens when you call `writeToFile(_, atomically: true)` for example.
            // In that case, we want to stop monitoring at the old node and start at the new node
            if data & DISPATCH_VNODE_DELETE == 1 {
                dispatch_source_cancel(dispatchSource)
                self.startMonitoringFile(at: path)
                return
            }

            // Always update the GUI from the main queue
            let fileContent = try! String(contentsOfFile: path)
            dispatch_async(dispatch_get_main_queue()) {
                self.textView.string = fileContent
            }
        }

        // When we stop monitoring a vnode, close the file handle
        let cancelHandler = {
            let fileHandle = dispatch_source_get_handle(dispatchSource)
            close(Int32(fileHandle))
        }

        dispatch_source_set_registration_handler(dispatchSource, eventHandler)
        dispatch_source_set_event_handler(dispatchSource, eventHandler)
        dispatch_source_set_cancel_handler(dispatchSource, cancelHandler)
        dispatch_resume(dispatchSource)
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        self.startMonitoringFile(at: "/path/to/file.txt")
    }
}

要触发DISPATCH_VNODE_EXTEND事件，您可以在终端上尝试以下操作：

echo "Hello world" >> /path/to/file.txt

我设法让下面的代码（在Swift 3.x中）在一个非常相似的用例中工作。希望它能有所帮助。

override func viewDidLoad() {
    super.viewDidLoad()

    NotificationCenter.default.addObserver(
        self,
        selector: #selector(ViewController.commandOutputNotification(_:)),
        name: NSNotification.Name.NSFileHandleDataAvailable,
        object: nil)

    self.startTasks()
}

func startTasks() {
    self.task = Process()
    self.task!.terminationHandler = self.commandTerminationHandler
    let argumentsString = "tail -n 1 -f /path/to/file/file.log"

    self.task!.launchPath = "/bin/sh"
    self.task!.arguments = ["-c", argumentsString]

    let pipe = Pipe()

    task!.standardOutput = pipe
    task!.standardError = pipe

    pipe.fileHandleForReading.waitForDataInBackgroundAndNotify()

    task!.launch()
}

func commandTerminationHandler(_ task: Process) -> Void {
    // TODO: finish this
}

func commandOutputNotification(_ notification: Notification) {
    let fileHandle = notification.object as! FileHandle
    let data = fileHandle.availableData

    if data.count > 0 {
        processLogData(data:(String.init(data: data, encoding: String.Encoding.utf8)))
        fileHandle.waitForDataInBackgroundAndNotify()
    }
}

func processLogData(data:String?) {
    // Handle data String
}