AVCamViewController没有可见的@interface声明选择器层

Question

- (void)setSession:(AVCaptureSession *)session {
    ((AVPlayerLayer *)[self layer]).videoGravity = AVLayerVideoGravityResizeAspectFill;
    ((AVPlayerLayer *)[self layer]).bounds = ((AVPlayerLayer *)[self layer]).bounds;
    [(AVCaptureVideoPreviewLayer *)[self layer] setSession:session];
}

当我尝试使用视频重力代码时，它显示错误，No visible @interface for AVCamViewController声明选择器层，当它表示＆＃34; self layer＆＃34;在AVCAM样本中？我该如何解决这个问题？