Objective c 委托回调中的CMSampleBufferRef参数为空

我正在使用此代码创建AVCaptureSession，但当调用

-（void）captureOutput:（AVCaptureOutput*）captureOutput didOutputSampleBuffer:（CMSampleBufferRef）SampleBufferfromConnection:（AVCaptureConnection*）连接时，sampleBuffer大小为0

- (BOOL)createCaptureSession:(NSError **)outError{
/* Create a capture session. */
self.captureSession = [[AVCaptureSession alloc] init];
if ([self.captureSession canSetSessionPreset:AVCaptureSessionPresetHigh])
{
    /* Specifies capture settings suitable for high quality video and audio output. */
    [self.captureSession setSessionPreset:AVCaptureSessionPresetHigh];
}

/* Add the main display as a capture input. */
display = CGMainDisplayID();
self.captureScreenInput = [[AVCaptureScreenInput alloc] initWithDisplayID:display];
if ([self.captureSession canAddInput:self.captureScreenInput])
{
    [self.captureSession addInput:self.captureScreenInput];
}
else
{   NSLog(@"nevyslo");
    return NO;
}

/* Register for notifications of errors during the capture session so we can display an alert. */
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(captureSessionRuntimeErrorDidOccur:) name:AVCaptureSessionRuntimeErrorNotification object:self.captureSession];


AVCaptureVideoDataOutput *output = [[AVCaptureVideoDataOutput alloc] init];
[self.captureSession addOutput:output];

dispatch_queue_t queue = dispatch_queue_create("myQueue", NULL);
[output setSampleBufferDelegate:self queue:queue];

[self.captureSession startRunning];
    return YES;
}

我的方法有什么问题吗？这个错误可能在别的地方吗？
我正在使用ARC

编辑
缓冲区不是空的，但它立即被释放。通过保留缓冲区来解决。谢谢
您想如何处理样本缓冲区？您应该在问题中包括您的
captureOutput:didOutputSampleBuffer:fromConnection:
方法。您想如何处理示例缓冲区？您应该在问题中包括您的
captureOutput:didOutputSampleBuffer:fromConnection:
方法。