自定义相机拍照和录制的时候，需求是横屏录制或拍照的时候，预览视频或图片要求竖屏显示。首先想到添加屏幕旋转通知,控制视屏或图片输入源方向。

添加屏幕旋转的通知

[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(handleDeviceOrientationChange:) name:UIDeviceOrientationDidChangeNotification object:nil];

但是当用户把竖屏锁定开启的时候，是无法接受到通知的，系统默认屏幕一直是竖屏状态。
这个时候就需要用到陀螺仪和加速器来判断屏幕的横竖屏状态。
首先引入CoreMotion.frameWork框架

    @property (nonatomic, strong) CMMotionManager * motionManager;

    - (void)startMotionManager{
          if (_motionManager == nil) {
                _motionManager = [[CMMotionManager alloc] init];
          }
           _motionManager.deviceMotionUpdateInterval = 1/15.0; // 每隔一个时间做一次轮询
          if (_motionManager.deviceMotionAvailable) {
                LRWeakSelf(self)
              [_motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue currentQueue]
                                         withHandler: ^(CMDeviceMotion *motion, NSError *error){
                                             [weakself performSelectorOnMainThread:@selector(handleDeviceMotion:) withObject:motion waitUntilDone:YES];

                                         }];
          } else {
               [self setMotionManager:nil];
          }
    }

    - (void)handleDeviceMotion:(CMDeviceMotion *)deviceMotion{
        double x = deviceMotion.gravity.x;
        double y = deviceMotion.gravity.y;
        AVCaptureVideoOrientation deviceOrientation;
        if (fabs(y) >= fabs(x))
        {
            if (y >= 0){
                // UIDeviceOrientationPortraitUpsideDown;  屏幕方向
                deviceOrientation = AVCaptureVideoOrientationPortraitUpsideDown; // 设置视频或图片的输入源的方向
           }else{
                  // UIDeviceOrientationPortrait;
                deviceOrientation = AVCaptureVideoOrientationPortrait;
            }
        }else{
            if (x >= 0){
                // UIDeviceOrientationLandscapeRight;
                deviceOrientation = AVCaptureVideoOrientationLandscapeLeft;
            }else{
                // UIDeviceOrientationLandscapeLeft;
                deviceOrientation = AVCaptureVideoOrientationLandscapeRight;
            }
        }
        self.deviceOrientation = deviceOrientation;
    }

当点击拍摄或录像的时候，设置下输入源的方向，控制视频或图片的方向。

结束的时候调用

[_motionManager stopDeviceMotionUpdates];