设备位置

该值为枚举值

@property(nonatomic, readonly) AVCaptureDevicePosition position;

typedef NS_ENUM(NSInteger, AVCaptureDevicePosition) {
    AVCaptureDevicePositionUnspecified = 0,
    AVCaptureDevicePositionBack        = 1,
    AVCaptureDevicePositionFront       = 2,
} NS_AVAILABLE(10_7, 4_0) __TVOS_PROHIBITED;