Swift code to play YouTube videos on AppleTV tvOS via a TVJS call using XCDYouTubeKit

YouTubePlayerViaTVJS.swift

//
//  Created by Nick Vance on 12/4/15.
//  Copyright © 2015 ToWatchList. All rights reserved.
//

import AVKit
import XCDYouTubeKit
import TVMLKit

class YTPlayerViewController: AVPlayerViewController, AVPlayerViewControllerDelegate {

    //call this method once after setting up your appController.
    func createPlayYT( appController:TVApplicationController ){

        //allows us to access the javascript context
        appController.evaluateInJavaScriptContext({(evaluation: JSContext) -> Void in

            //this is the block that will be called when javascript calls playYTblock(videoIdentifier)
            let playYTblock : @convention(block) (String) -> Void = {
                (videoIdentifier : String) -> Void in

                print("Playing YouTube Video with ID:", videoIdentifier)

                let playerViewController = AVPlayerViewController()

                if var topController = UIApplication.sharedApplication().keyWindow?.rootViewController {
                    while let presentedViewController = topController.presentedViewController {
                        topController = presentedViewController
                    }
                    // topController should now be your topmost view controller
                    topController.presentViewController(playerViewController, animated: true, completion: nil)

                    XCDYouTubeClient.defaultClient().getVideoWithIdentifier(videoIdentifier) { [weak playerViewController] (video: XCDYouTubeVideo?, error: NSError?) in
                        if let streamURL = (video?.streamURLs[XCDYouTubeVideoQualityHTTPLiveStreaming] ??
                            video?.streamURLs[XCDYouTubeVideoQuality.HD720.rawValue] ??
                            video?.streamURLs[XCDYouTubeVideoQuality.Medium360.rawValue] ??
                            video?.streamURLs[XCDYouTubeVideoQuality.Small240.rawValue]) {
                                playerViewController?.player = AVPlayer(URL: streamURL)
                                playerViewController?.player?.play()
                                playerViewController?.player?.actionAtItemEnd = AVPlayerActionAtItemEnd.None

                                NSNotificationCenter.defaultCenter().addObserver(self,
                                    selector: "playerItemDidReachEnd:",
                                    name: AVPlayerItemDidPlayToEndTimeNotification,
                                    object: playerViewController?.player?.currentItem)
                        } else {
                            self.dismissViewControllerAnimated(true, completion: nil)
                        }
                    }
                }
            }

            //this creates a function in the javascript context called "playYTblock".
            //calling playYTblock(videoIdentifier) in javascript will call the block we created above.
            evaluation.setObject(unsafeBitCast(playYTblock, AnyObject.self), forKeyedSubscript: "playYTblock")
            }, completion: {(Bool) -> Void in
                //evaluation block finished running
        })
    }

    func playerItemDidReachEnd(notification: NSNotification) {
        print("Video reached end! Dismiss player")

        if var topController = UIApplication.sharedApplication().keyWindow?.rootViewController {
            while let presentedViewController = topController.presentedViewController {
                topController = presentedViewController
            }
            // topController should now be your topmost view controller
            topController.dismissViewControllerAnimated(true, completion: nil)
        }
    }
}

Here's a gist updated for TVOS 10 based on 0xced's Stack Overflow comment.