When you have a portrait-only app you may still want to allow rotation when the app plays full screen media. This is a decent solution by creating a notification to signal a "full screen". Only tested on iOS9.

LandscapeOnFullScreen.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
  
  var isFullScreen = false
  
  func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    // create notifications from wherever to signal fullscreen
    NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(self.willEnterFullScreen(_:)), name: "MediaEnterFullScreen", object: nil)
    NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(self.willExitFullScreen(_:)), name: "MediaExitFullScreen", object: nil)
    
    return true
  }
  
  func willEnterFullScreen (notification: NSNotification) {
    isFullScreen = true
  }
  
  func willExitFullScreen (notification: NSNotification) {
    isFullScreen = false
  }
  
  func application(application: UIApplication, supportedInterfaceOrientationsForWindow window: UIWindow?) -> UIInterfaceOrientationMask {
    return isFullScreen == true ? UIInterfaceOrientationMask.All : UIInterfaceOrientationMask.Portrait
  }
}

note anyway those calls are deprecated since ios9 and as per Apple ADC docs, notifications are 👍

MPMoviePlayerWillEnterFullscreen
and so on.