Delayed background color and label updating but NSLog works fine

Question

I am writing a program that reads a QR code and then uses the data collected. I am having a strange problem though. I believe it may have to do something with threads. Basically all the NSLogs in the if statement work fine and print rapidly every second yet when i try to update a label text or change the background color it takes ~40 seconds. I don't understand why. Here is the code.

#import "VerifyPassViewController.h"
#import "NewPassViewController.h"

@interface VerifyPassViewController ()



-(BOOL)startReading;
-(void)stopReading;
-(void)loadBeepSound;

@end

@implementation VerifyPassViewController



- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    // Initially make the captureSession object nil.
    _captureSession = nil;

    // Set the initial value of the flag to NO.
    _isReading = NO;


}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


#pragma mark - IBAction method implementation

- (IBAction)startStopReading:(id)sender {
    if (!_isReading) {
        // This is the case where the app should read a QR code when the start button is tapped.
        if ([self startReading]) {
            // If the startReading methods returns YES and the capture session is successfully
            // running, then change the start button title and the status message.
            [_bbITem setTitle:@"Stop"];
            [_bbITem setTitle:@"Scanning for QR Code..."];
        }
    }
    else{
        // In this case the app is currently reading a QR code and it should stop doing so.
        [self stopReading];
        // The bar button item's title should change again.
        [_bbITem setTitle:@"Start!"];
    }

    // Set to the flag the exact opposite value of the one that currently has.
    _isReading = !_isReading;
}


#pragma mark - Private method implementation

- (BOOL)startReading {
    NSError *error;

    // Get an instance of the AVCaptureDevice class to initialize a device object and provide the video
    // as the media type parameter.
    AVCaptureDevice *captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    // Get an instance of the AVCaptureDeviceInput class using the previous device object.
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:captureDevice error:&error];

    if (!input) {
        // If any error occurs, simply log the description of it and don't continue any more.
        NSLog(@"%@", [error localizedDescription]);
        return NO;
    }

    // Initialize the captureSession object.
    _captureSession = [[AVCaptureSession alloc] init];
    // Set the input device on the capture session.
    [_captureSession addInput:input];


    // Initialize a AVCaptureMetadataOutput object and set it as the output device to the capture session.
    AVCaptureMetadataOutput *captureMetadataOutput = [[AVCaptureMetadataOutput alloc] init];
    [_captureSession addOutput:captureMetadataOutput];

    // Create a new serial dispatch queue.
    dispatch_queue_t dispatchQueue;
    dispatchQueue = dispatch_queue_create("myQueue", NULL);
    [captureMetadataOutput setMetadataObjectsDelegate:self queue:dispatchQueue];
    [captureMetadataOutput setMetadataObjectTypes:[NSArray arrayWithObject:AVMetadataObjectTypeQRCode]];

    // Initialize the video preview layer and add it as a sublayer to the viewPreview view's layer.
    _videoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];
    [_videoPreviewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
    [_videoPreviewLayer setFrame:_viewPreview.layer.bounds];
    [_viewPreview.layer addSublayer:_videoPreviewLayer];


    // Start video capture.
    [_captureSession startRunning];

    return YES;
}


-(void)stopReading{
    // Stop video capture and make the capture session object nil.
    [_captureSession stopRunning];
    _captureSession = nil;

    // Remove the video preview layer from the viewPreview view's layer.
    [_videoPreviewLayer removeFromSuperlayer];
}




#pragma mark - AVCaptureMetadataOutputObjectsDelegate method implementation
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection
{
    for(AVMetadataObject *metadataObject in metadataObjects)
    {
        AVMetadataMachineReadableCodeObject *readableObject = (AVMetadataMachineReadableCodeObject *)metadataObject;
        if([metadataObject.type isEqualToString:AVMetadataObjectTypeQRCode])
        {
            NSLog(@"QR Code = %@", readableObject.stringValue);


            self.view.backgroundColor = [UIColor greenColor];

            NSLog(@"If the background isnt green i might throw up");





        }
        else if ([metadataObject.type isEqualToString:AVMetadataObjectTypeEAN13Code])
        {
            NSLog(@"EAN 13 = %@", readableObject.stringValue);
        }
    }
}





@end

Let me know if you have any ideas. Thank you

Answer 1

You can only update the UI from the main thread:

dispatch_async(dispatch_get_main_queue(), ^{
    ... update here your ui ..   
});

Answer 2

To state clearly what the other posters hint at: Your code is setting up a dispatch queue that runs on a background thread to process any QR codes it recognizes.

Your captureOutput:didOutputMetadataObjects:fromConnection: method will be run on a background thread.

UI code must be performed on the main thread. rafaperez posted code that you can use from a background thread to dispatch UI changes to a queue on the main thread.

You could also use the method performSelectorOnMainThread:withObject:waitUntilDone: in order to send single method calls to the main thread.