How to detect HDR playback capability

I found the following API for iOS 11.2+ from AVFoundation's AVPlayer.h:

/!
@typedef AVPlayerHDRMode
@abstract  A bitfield type that specifies an HDR mode.

@constant AVPlayerHDRModeHLG
@abstract Indicates that HLG (Hybrid Log-Gamma) HDR mode is available.
@constant AVPlayerHDRModeHDR10
@abstract Indicates that HDR10 HDR mode is available.
@constant AVPlayerHDRModeDolbyVision
@abstract Indicates that Dolby Vision HDR mode is available.
*/
typedef NS_OPTIONS(NSInteger, AVPlayerHDRMode) {
  AVPlayerHDRModeHLG = 0x1,
  AVPlayerHDRModeHDR10 = 0x2,
  AVPlayerHDRModeDolbyVision = 0x4,
} API_AVAILABLE(ios(11.2), tvos(11.2)) API_UNAVAILABLE(macos, watchos);
@interface AVPlayer (AVPlayerPlaybackCapabilities)
/!
  @property availableHDRModes
  @abstract An array of AVPlayerHDRMode values that indicates the HDR modes the device can play to an appropriate display.   A value of 0 indicates that no HDR modes are supported.

  @discussion
  This property indicates all of the HDR modes that the device can play.  Each value indicates that an appropriate HDR display is available for the specified HDR mode.  Additionally, the device must be capable of playing the specified HDR type.  This property does not indicate whether video contains HDR content, whether HDR video is currently playing, or whether video is playing on an HDR display.
*/
@property (class, nonatomic, readonly) AVPlayerHDRMode availableHDRModes API_AVAILABLE(ios(11.2), tvos(11.2)) API_UNAVAILABLE(macos, watchos);
/!
  @constant AVPlayerAvailableHDRModesDidChangeNotification
  @abstract A notification that fires whenever availableHDRModes changes.

  @discussion
  This notification fires when a value is added or removed from the list of availableHDRModes.  This can be caused by display connection/disconnection or resource changes.
*/
API_AVAILABLE(ios(11.2), tvos(11.2)) API_UNAVAILABLE(macos, watchos)
AVF_EXPORT NSNotificationName const AVPlayerAvailableHDRModesDidChangeNotification;
@end

But I tried to run the following code in ViewDidLoad, it always return 0 even when I run on iPhone X with iOS 11.2.6:

AVPlayerHDRMode HDRMode = AVPlayer.availableHDRModes;

iPhone X should have HDR support, and I expect the API return something other than 0. Or am I misunderstand the use of the API?