iOScaptureOutputdidOutputSampleBuffer不调用fromConnection

我想从AVCaptureSession的实时馈送中提取帧，我使用Apple的AVCam作为测试用例。以下是AVCam的链接：

I want to pull frames from the live-feed of AVCaptureSession and I am using Apple's AVCam as a test case. Here is the link to AVCam:

https://developer.apple.com/library/ios/samplecode/AVCam/Introduction/Intro.html

I发现 captureOutput：didOutputSampleBuffer：fromConnection 未被调用，我想知道为什么或我做错了什么。

I found that that captureOutput:didOutputSampleBuffer:fromConnection is NOT called and I would like to know why or what I am doing wrong.

以下是我所做的：

（1）我使 AVCamViewController 委托

@interface AVCamViewController () <AVCaptureFileOutputRecordingDelegate, AVCaptureVideoDataOutputSampleBufferDelegate>

（2）我创建了一个 AVCaptureVideoDataOutput 对象并将其添加到会话

(2) I created an AVCaptureVideoDataOutput object and add it to the session

AVCaptureVideoDataOutput *videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
if ([session canAddOutput:videoDataOutput])
     {
         [session addOutput:videoDataOutput];
     }

（3）我通过记录随机字符串添加了委托方法和测试测试

(3) I added the delegate method and test by logging a random string to test

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
    NSLog(@"I am called");

}

测试应用程序有效，但captureOutput：didOutputSampleBuffer：fromConnection不是叫做。

The test application works but captureOutput:didOutputSampleBuffer:fromConnection is not called.

（4）我在SO上读到 AVCaptureSession * session = [[AVCaptureSession alloc] init]中的会话变量; 在viewDidLoad中是本地的是一个可能的原因，为什么没有调用委托，我把它作为AVCamViewController类的实例变量，但它没有被调用。

(4) I read on SO that the session variable in AVCaptureSession *session = [[AVCaptureSession alloc] init]; being local in viewDidLoad is a possible reason why the delegate is not called and I made it an instance variable of the AVCamViewController class, yet it is not called.

这是我正在测试的viewDidLoad方法（取自AVCam），我在方法结束时添加了AVCaptureDataOutput：

Here is the viewDidLoad method I am testing with (taken from AVCam), I added AVCaptureDataOutput towards the end of the method:

- (void)viewDidLoad
{
    [super viewDidLoad];

    // Create the AVCaptureSession
    session = [[AVCaptureSession alloc] init];
    [self setSession:session];

    // Setup the preview view
    [[self previewView] setSession:session];

    // Check for device authorization
    [self checkDeviceAuthorizationStatus];

    // In general it is not safe to mutate an AVCaptureSession or any of its inputs, outputs, or connections from multiple threads at the same time.
    // Why not do all of this on the main queue?
    // -[AVCaptureSession startRunning] is a blocking call which can take a long time. We dispatch session setup to the sessionQueue so that the main queue isn't blocked (which keeps the UI responsive).

    dispatch_queue_t sessionQueue = dispatch_queue_create("session queue", DISPATCH_QUEUE_SERIAL);
    [self setSessionQueue:sessionQueue];

    dispatch_async(sessionQueue, ^{
        [self setBackgroundRecordingID:UIBackgroundTaskInvalid];

        NSError *error = nil;

        AVCaptureDevice *videoDevice = [AVCamViewController deviceWithMediaType:AVMediaTypeVideo preferringPosition:AVCaptureDevicePositionBack];
        AVCaptureDeviceInput *videoDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];

        if (error)
        {
            NSLog(@"%@", error);
        }

        if ([session canAddInput:videoDeviceInput])
        {
            [session addInput:videoDeviceInput];
            [self setVideoDeviceInput:videoDeviceInput];

            dispatch_async(dispatch_get_main_queue(), ^{
                // Why are we dispatching this to the main queue?
                // Because AVCaptureVideoPreviewLayer is the backing layer for AVCamPreviewView and UIView can only be manipulated on main thread.
                // Note: As an exception to the above rule, it is not necessary to serialize video orientation changes on the AVCaptureVideoPreviewLayer’s connection with other session manipulation.

                [[(AVCaptureVideoPreviewLayer *)[[self previewView] layer] connection] setVideoOrientation:(AVCaptureVideoOrientation)[self interfaceOrientation]];
            });
        }

        AVCaptureDevice *audioDevice = [[AVCaptureDevice devicesWithMediaType:AVMediaTypeAudio] firstObject];
        AVCaptureDeviceInput *audioDeviceInput = [AVCaptureDeviceInput deviceInputWithDevice:audioDevice error:&error];

        if (error)
        {
            NSLog(@"%@", error);
        }

        if ([session canAddInput:audioDeviceInput])
        {
            [session addInput:audioDeviceInput];
        }

        AVCaptureMovieFileOutput *movieFileOutput = [[AVCaptureMovieFileOutput alloc] init];
        if ([session canAddOutput:movieFileOutput])
        {
            [session addOutput:movieFileOutput];
            AVCaptureConnection *connection = [movieFileOutput connectionWithMediaType:AVMediaTypeVideo];
            if ([connection isVideoStabilizationSupported])
                [connection setEnablesVideoStabilizationWhenAvailable:YES];
            [self setMovieFileOutput:movieFileOutput];
        }

        AVCaptureStillImageOutput *stillImageOutput = [[AVCaptureStillImageOutput alloc] init];
        if ([session canAddOutput:stillImageOutput])
        {
            [stillImageOutput setOutputSettings:@{AVVideoCodecKey : AVVideoCodecJPEG}];
            [session addOutput:stillImageOutput];
            [self setStillImageOutput:stillImageOutput];
        }

        AVCaptureVideoDataOutput *videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
        [videoDataOutput setSampleBufferDelegate:self queue:sessionQueue];

        if ([session canAddOutput:videoDataOutput])
        {
            NSLog(@"Yes I can add it");
            [session addOutput:videoDataOutput];
        }

    });
}

- (void)viewWillAppear:(BOOL)animated
{
    dispatch_async([self sessionQueue], ^{
        [self addObserver:self forKeyPath:@"sessionRunningAndDeviceAuthorized" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:SessionRunningAndDeviceAuthorizedContext];
        [self addObserver:self forKeyPath:@"stillImageOutput.capturingStillImage" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:CapturingStillImageContext];
        [self addObserver:self forKeyPath:@"movieFileOutput.recording" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:RecordingContext];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(subjectAreaDidChange:) name:AVCaptureDeviceSubjectAreaDidChangeNotification object:[[self videoDeviceInput] device]];

        __weak AVCamViewController *weakSelf = self;
        [self setRuntimeErrorHandlingObserver:[[NSNotificationCenter defaultCenter] addObserverForName:AVCaptureSessionRuntimeErrorNotification object:[self session] queue:nil usingBlock:^(NSNotification *note) {
            AVCamViewController *strongSelf = weakSelf;
            dispatch_async([strongSelf sessionQueue], ^{
                // Manually restarting the session since it must have been stopped due to an error.
                [[strongSelf session] startRunning];
                [[strongSelf recordButton] setTitle:NSLocalizedString(@"Record", @"Recording button record title") forState:UIControlStateNormal];
            });
        }]];
        [[self session] startRunning];
    });
}

有人可以告诉我为什么以及如何解决它的建议？

Can someone please tell me why and suggestions on how to fix it?