Using view controller containment, children lose parent

Question

New iOS developer writing first commissioned app for iPhone.

Modally called parent view controller has six children which are transitioned using a segmented control in the parent. I am getting the error:

Children view controllers and must have a common parent view controller when calling -[UIViewController transitionFromViewController:toViewController:duration:options:animations:completion:]

when I press a second button, ie i can press any button after the parent is presented and it works. But the next time I press a new button, I get the crash above. Using NSLogs I have determined that the reason for the crash is that both the current and new child have lost their parent between button presses. I have no idea why. Relevent code for parent follows:

Interface code:

#import <UIKit/UIKit.h>
#import "WorksheetScrollViewController.h"
#import "Worksheet1ViewController.h"
#import "Worksheet2ViewController.h"
#import "Worksheet3ViewController.h"
#import "Worksheet4ViewController.h"
#import "WorksheetEndViewController.h"


@interface WorkSheetParentViewController : UIViewController
{
IBOutlet UISegmentedControl *segControl;
IBOutlet UIBarButtonItem *doneButton;
IBOutlet UIView *childContainerView;

WorksheetScrollViewController *wStart;
Worksheet1ViewController *w1; 
Worksheet2ViewController *w2;
Worksheet3ViewController *w3;
Worksheet4ViewController *w4;
WorksheetEndViewController *wEnd;

UIViewController *currentVC;
UIViewController *newVC;

}


-(void)initToolbar;
-(IBAction) doneButtonPressed:(id)sender;
-(IBAction) segControlChanged:(id)sender;

-(void)swapViewControllers;

Implementation code:

- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view from its nib.
wStart = [[WorksheetScrollViewController alloc        ]initWithNibName:@"WorksheetScrollViewController" bundle:nil];

[self addChildViewController:wStart];
wStart.view.frame=childContainerView.bounds;

[childContainerView addSubview:wStart.view];
[wStart didMoveToParentViewController:self];
currentVC = wStart;

}

- (IBAction) segControlChanged:(id)sender

{

NSLog(@"selected tab bar item:%i",segControl.selectedSegmentIndex);
switch (segControl.selectedSegmentIndex) {
    case 0:
        if (!wStart) {
            wStart = [[WorksheetScrollViewController alloc ]initWithNibName:@"WorksheetScrollViewController" bundle:nil];
        }
        newVC = wStart;
        break;

    case 1:
        if (!w1) {
            w1 = [[Worksheet1ViewController alloc ]initWithNibName:@"Worksheet1ViewController" bundle:nil];
        }
        newVC = w1;
        break;

    case 2:
        if (!w2) {
            w2 = [[Worksheet2ViewController alloc ]initWithNibName:@"Worksheet2ViewController" bundle:nil];
        }
        newVC = w2;
       break;

    case 3:
        if (!w3) {
            w3 = [[Worksheet3ViewController alloc ]initWithNibName:@"Worksheet3ViewController" bundle:nil];
        }
        newVC = w3;
        break;

    case 4:
        if (!w4) {
            w4 = [[Worksheet4ViewController alloc ]initWithNibName:@"Worksheet4ViewController" bundle:nil];
        }
        newVC = w4;
        break;

    case 5:
        if (!wEnd) {
            wEnd = [[WorksheetEndViewController alloc ]initWithNibName:@"WorksheetEndViewController" bundle:nil];
        }
        newVC = wEnd;
        break;


    default:
        break;
}
[self  swapViewControllers];


}

-(void)swapViewControllers
{
NSLog(@"top current VC parent = %@",currentVC.parentViewController);
NSLog(@"top new VC parent = %@",newVC.parentViewController);
[currentVC willMoveToParentViewController:nil];
newVC.view.frame=childContainerView.bounds;

[self addChildViewController:newVC];

NSLog(@"current VC parent = %@",currentVC.parentViewController);
NSLog(@"new VC parent = %@",newVC.parentViewController);
[self transitionFromViewController:currentVC
                  toViewController:newVC
                          duration:1.0
                           options:UIViewAnimationOptionTransitionCurlUp
                        animations:nil
                        completion:^(BOOL finished) {
                            [currentVC removeFromParentViewController];
                            [newVC didMoveToParentViewController:self];

                        }];
currentVC = newVC;
NSLog(@"bottom current VC parent = %@",currentVC.parentViewController);
NSLog(@"bottom new VC parent = %@",newVC.parentViewController);

}

Nslog results for pressing button 1, then button 2:

2013-08-26 14:49:07.245 Alveolar Gas iPhone[556:707] selected tab bar item:1
2013-08-26 14:49:07.251 Alveolar Gas iPhone[556:707] top current VC parent =      <WorkSheetParentViewController: 0x198da0>
2013-08-26 14:49:07.256 Alveolar Gas iPhone[556:707] top new VC parent = (null)
2013-08-26 14:49:07.437 Alveolar Gas iPhone[556:707] current VC parent = <WorkSheetParentViewController: 0x198da0>
2013-08-26 14:49:07.440 Alveolar Gas iPhone[556:707] new VC parent = <WorkSheetParentViewController: 0x198da0>
2013-08-26 14:49:07.460 Alveolar Gas iPhone[556:707] bottom current VC parent = <WorkSheetParentViewController: 0x198da0>
2013-08-26 14:49:07.462 Alveolar Gas iPhone[556:707] bottom new VC parent = <WorkSheetParentViewController: 0x198da0>
2013-08-26 14:49:13.984 Alveolar Gas iPhone[556:707] selected tab bar item:2
2013-08-26 14:49:33.912 Alveolar Gas iPhone[556:707] top current VC parent = (null)
2013-08-26 14:49:50.038 Alveolar Gas iPhone[556:707] top new VC parent = (null)
2013-08-26 14:55:36.149 Alveolar Gas iPhone[556:707] current VC parent = (null)
2013-08-26 14:55:36.153 Alveolar Gas iPhone[556:707] new VC parent = <WorkSheetParentViewController: 0x198da0>
2013-08-26 14:55:36.160 Alveolar Gas iPhone[556:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Children view controllers <Worksheet1ViewController: 0x1ea780> and <Worksheet2ViewController: 0x1403b0> must have a common parent view controller when calling -[UIViewController transitionFromViewController:toViewController:duration:options:animations:completion:]'

Answer 1

I fixed it by moving currentVC = newVC; into the completion block. SwapViewControllers: now looks like this:

-(void)swapViewControllers
{
NSLog(@"top current VC parent = %@",currentVC.parentViewController);
NSLog(@"top new VC parent = %@",newVC.parentViewController);
[currentVC willMoveToParentViewController:nil];
newVC.view.frame=childContainerView.bounds;

[self addChildViewController:newVC];

NSLog(@"current VC parent = %@",currentVC.parentViewController);
NSLog(@"new VC parent = %@",newVC.parentViewController);
[self transitionFromViewController:currentVC
                  toViewController:newVC
                          duration:1.0
                           options:UIViewAnimationOptionTransitionCurlUp
                        animations:nil
                        completion:^(BOOL finished) {
                            [currentVC removeFromParentViewController];
                            [newVC didMoveToParentViewController:self];
                            currentVC = newVC;
                        }];

NSLog(@"bottom current VC parent = %@",currentVC.parentViewController);
NSLog(@"bottom new VC parent = %@",newVC.parentViewController);

}