objective c delay in open page after scanning QR code - ios

Question

I'm quite new with ios developing (even though I have a lot of experience with other platforms) and I am facing an issue while I'm try to implement a QR code reader.
This is the piece of code I wrote:
- (BOOL)startReading {
NSError *error;
AVCaptureDevice *captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:captureDevice error:&error];
if (!input) {
NSLog(#"%#", [error localizedDescription]);
return NO;
}
_qrLB.text=#"Loading....";
_captureSession = [[AVCaptureSession alloc] init];
[_captureSession addInput:input];
AVCaptureMetadataOutput *captureMetadataOutput = [[AVCaptureMetadataOutput alloc] init];
[_captureSession addOutput:captureMetadataOutput];
dispatch_queue_t dispatchQueue;
dispatchQueue = dispatch_queue_create("myQueue", NULL);
[captureMetadataOutput setMetadataObjectsDelegate:self queue:dispatchQueue];
[captureMetadataOutput setMetadataObjectTypes:[NSArray arrayWithObject:AVMetadataObjectTypeQRCode]];
_videoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];
[_videoPreviewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
[_videoPreviewLayer setFrame:_viewPreview.layer.bounds];
[_viewPreview.layer addSublayer:_videoPreviewLayer];
[_captureSession startRunning];
return YES;
}
#pragma mark
#pragma mark Stop reading QR Code
-(void)stopReading{
[_captureSession stopRunning];
_captureSession = nil;
[_videoPreviewLayer removeFromSuperlayer];
}
#pragma mark
#pragma mark Capture QR Code
-(void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection{
NSLog(#"qrcode1");
if (metadataObjects != nil && [metadataObjects count] > 0) {
AVMetadataMachineReadableCodeObject *metadataObj = [metadataObjects objectAtIndex:0];
if ([[metadataObj type] isEqualToString:AVMetadataObjectTypeQRCode]) {
[_lblStatus performSelectorOnMainThread:#selector(setText:) withObject:[metadataObj stringValue] waitUntilDone:NO];
[self performSelectorOnMainThread:#selector(stopReading) withObject:nil waitUntilDone:NO];
_isReading = NO;
}
}
NSLog(#"Value of qrcodetext = %#", _lblStatus.text);
ReceiveVC *view = [self.storyboard instantiateViewControllerWithIdentifier:#"ReceiveVC"];
view.imageURL=_lblStatus.text;
view.title = #"";
[self.navigationController pushViewController:view animated:YES];
NSLog(#"qrcode2");
}
When I launch the app on the device (iphone 5s) and I scan a QR code this is the output I got:
screenshot
It works fine! it reads the value and it redirects the user to the page "ReceiveVC" ("ReceiveVC1" is a debug I print when the page at loading)
The problem is: it's taking like 20 seconds (sometime even more!!) from the moment in which the value is correctly read, to the moment in which the page is opened (check time of lines 2 and 3). Seems taht the navigationController is waiting for some thread to finish but I cannot figure out which one, since all the values I need are already there!
Can you help me?
Thank you