// assume this text field is defined elsewhere:
UITextField *field = self.someTextField;

[field becomeFirstResponder];

// create the scanning view controller and a navigation controller in which to present it:
CDZQRScanningViewController *scanningVC = [CDZQRScanningViewController new];
UINavigationController *scanningNavVC = [[UINavigationController alloc] initWithRootViewController:scanningVC];

// configure the scanning view controller:
scanningVC.resultBlock = ^(NSString *result) {
    field.text = result;
    [scanningNavVC dismissViewControllerAnimated:YES completion:nil];
};
scanningVC.cancelBlock = ^() {
    [scanningNavVC dismissViewControllerAnimated:YES completion:nil];
};
scanningVC.errorBlock = ^(NSError *error) {
    // todo: show a UIAlertView orNSLog the error
    [scanningNavVC dismissViewControllerAnimated:YES completion:nil];
};

// present the view controller full-screen on iPhone; in a form sheet on iPad:
scanningNavVC.modalPresentationStyle = UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone ? UIModalPresentationFullScreen : UIModalPresentationFormSheet;
[self presentViewController:scanningNavVC animated:YES completion:nil];