画中画(PiP)：AVPictureInPictureContr

 // 当前设备是否支持画中画
 open class func isPictureInPictureSupported() -> Bool
 // 创建画中画控制器,初始化方法
 public convenience init?(playerLayer: AVPlayerLayer)
 // 播放媒体的播放器层
 open var playerLayer: AVPlayerLayer { get }
 // 委托对象
 weak open var delegate: AVPictureInPictureControllerDelegate?
 // 系统默认的“画中画”开始模板图像，用于应用程序的“画中画”按钮。
 open class func pictureInPictureButtonStartImage(compatibleWith traitCollection: UITraitCollection?) -> UIImage
 @available(iOS 13.0, *)
 open class var pictureInPictureButtonStartImage: UIImage { get }
 //系统默认的画中画停止模板图像，用于应用程序的画中画按钮。
 open class func pictureInPictureButtonStopImage(compatibleWith traitCollection: UITraitCollection?) -> UIImage
 @available(iOS 13.0, *)
 open class var pictureInPictureButtonStopImage: UIImage { get }
 // 对于可能是示例缓冲区显示层或播放器层的内容，请使用此初始值设定项
 @available(iOS 15.0, *)
 public init(contentSource: AVPictureInPictureController.ContentSource)
 //接收者的内容源
 @available(iOS 15.0, *)
 open var contentSource: AVPictureInPictureController.ContentSource?
 // 开始画中画播放
 open func startPictureInPicture()
 // 停止画中画播放(如果当前处于活跃状态)
 open func stopPictureInPicture()
 // 当前是否可以进行画中画播放
 open var isPictureInPicturePossible: Bool { get }
 // 控制器的画中画窗口是否在屏幕上,是否是活跃状态
 open var isPictureInPictureActive: Bool { get }
 // 系统是否挂起控制器的画中画窗口
 open var isPictureInPictureSuspended: Bool { get }
 // 是否允许用户跳过媒体内容
 @available(iOS 14.0, *)
 open var requiresLinearPlayback: Bool
 // 表示进入后台是否自动开启画中画
 @available(iOS 14.2, *)
 open var canStartPictureInPictureAutomaticallyFromInline: Bool

// 画中画即将开始
optional func pictureInPictureControllerWillStartPictureInPicture(_ pictureInPictureController: AVPictureInPictureController)
// 画中画已经开始
optional func pictureInPictureControllerDidStartPictureInPicture(_ pictureInPictureController: AVPictureInPictureController)
// 画中画无法启动，失败原因
optional func pictureInPictureController(_ pictureInPictureController: AVPictureInPictureController, failedToStartPictureInPictureWithError error: Error)
// 画中画即将停止
optional func pictureInPictureControllerWillStopPictureInPicture(_ pictureInPictureController: AVPictureInPictureController)
// 画中画已经停止
optional func pictureInPictureControllerDidStopPictureInPicture(_ pictureInPictureController: AVPictureInPictureController)
// 关闭画中画且恢复播放界面
optional func pictureInPictureController(_ pictureInPictureController: AVPictureInPictureController, restoreUserInterfaceForPictureInPictureStopWithCompletionHandler completionHandler: @escaping (Bool) -> Void)