UIlabel is not getting updated inside AVCaptureSession Delegate

Question

I am learning objective c and doing a sample app to fetch video feed from iPhone camera. I was able to get the feeds from camera and display it on screen. Also I was trying to update some UILabel in screen for each frame from the video inside the delegate method. But the label value is not getting updated always. Here is the code I am using

This section will initialize the capture

   - (void)initCapture 
{
     NSError *error = nil;
    device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    if ([device isFocusModeSupported:AVCaptureFocusModeContinuousAutoFocus] && [device lockForConfiguration:&error]) {
        [device setFocusMode:AVCaptureFocusModeContinuousAutoFocus];
        [device unlockForConfiguration];
    }

    AVCaptureDeviceInput *captureInput = [AVCaptureDeviceInput deviceInputWithDevice:device error:nil];

     //AVCaptureStillImageOutput *imageCaptureOutput = [[AVCaptureStillImageOutput alloc] init];

     AVCaptureVideoDataOutput *captureOutput =[[AVCaptureVideoDataOutput alloc] init];

     captureOutput.alwaysDiscardsLateVideoFrames = YES;
     //captureOutput.minFrameDuration = CMTimeMake(1, 1);

     captureOutput.alwaysDiscardsLateVideoFrames = YES; 
     dispatch_queue_t queue;
     queue = dispatch_queue_create("cameraQueue", NULL);
     [captureOutput setSampleBufferDelegate:self queue:queue];
     dispatch_release(queue);
     // Set the video output to store frame in BGRA (It is supposed to be faster)
     NSString* key = (NSString*)kCVPixelBufferPixelFormatTypeKey; 
     NSNumber* value = [NSNumber numberWithUnsignedInt:kCVPixelFormatType_32BGRA]; 
     NSDictionary* videoSettings = [NSDictionary dictionaryWithObject:value forKey:key]; 
     [captureOutput setVideoSettings:videoSettings]; 

     self.captureSession = [[AVCaptureSession alloc] init];

     [self.captureSession addInput:captureInput];
     [self.captureSession addOutput:captureOutput];


     self.prevLayer = [AVCaptureVideoPreviewLayer layerWithSession: self.captureSession];
     self.prevLayer.frame = CGRectMake(0, 0, 320, 320);
     self.prevLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;



     [self.videoPreview.layer addSublayer: self.prevLayer];     

     [self.captureSession startRunning]; 


     }

This method is called for each video frame.

#pragma mark AVCaptureSession delegate
- (void)captureOutput:(AVCaptureOutput *)captureOutput 
     didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer 
     fromConnection:(AVCaptureConnection *)connection 
    { 

        i++;
        self.lblStatus.Text = [NSString stringWithFormat:@"%d",i];
    }

I am trying to print UILabel inside this method but it is not printed always. THere is much delay for the label text to change.

Could someone help please? Thanks.

Answer 1

Your sampleBufferDelegate's captureOutput is being called from a non-main thread - updating GUI objects from there can do no good. Try using performSelectorOnMainThread instead.