How can I check if my AVPlayer is buffering? for AVPlayer

Question

I want to detect if my AVPlayer is buffering for the current location so that I can show a loader or something. But I can't seem to find anything in the documentation for AVPlayer. I am using this code but it's not working :

playerItem.addObserver(self, forKeyPath: "playbackBufferEmpty", options: .New, context: nil)
playerItem.addObserver(self, forKeyPath: "playbackLikelyToKeepUp", options: .New, context: nil)
playerItem.addObserver(self, forKeyPath: "playbackBufferFull", options: .New, context: nil)

override public func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
    if object is AVPlayerItem {
        switch keyPath {
            case "playbackBufferEmpty":
               // Show loader

            case "playbackLikelyToKeepUp":
                // Hide loader

            case "playbackBufferFull":
                // Hide loader
        }
    }
}