interface - 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 videogravity 代码时，它显示错误，No visible @interface for AVCamViewController声明选择器层，在 AVCAM 示例中显示“self layer”时的三个底行？我该如何解决？