navigationController

Today, a quick note on an iPhone gotcha that recently bit me. The short version is that a UIViewControler's navigationController property is cleared when that view controller is removed from its nav. controller, which can trip you up if you’re manipulating that nav. controller in complex ways.

Motivation

I encountered this problem because I was reshuffling a navigation controller stack behind a modal view – i.e. the modal view was to scroll up over one screen, and then scroll down to reveal a different screen, depending upon what the user did in the modal view. The meat of the code looked something like this:

- (void)stageManage
{
	// Advance model to the next step, if any
	if ([self nextStep])
	{
		// Update this view
		[self.tableView reloadData];
	}
	else
	{
		// Dismiss this view, revealing it's parent
		[self.navigationController popViewControllerAnimated:NO];
	}
	
	// Dismiss modal view(s)
	[self.navigationController dismissModalViewControllerAnimated:YES];
}

(stageManage is a method of the view controller that’s atop the stack before the modal view is displayed, and it is called by the modal view controller via the target-action pattern.)

Difficulties

The preceeding code did not work. The use case in which popViewControllerAnimated: was called did not behave properly; the modal view was not dismissed.

It turned out that this was happening because the popViewControllerAnimated: call removed the self object from its navigation controller, clearing its navigationController property, and causing the dismissModalViewControllerAnimated: message to be sent to a nil object.

Fix

Caching the nav. controller fixed matters right up:

- (void)stageManage
{
	UINavigationController* nc = self.navigationController;

	// Advance model to the next step, if any
	if ([self nextStep])
	{
		// Update this view
		[self.tableView reloadData];
	}
	else
	{
		// Dismiss this view, revealing it's parent
		[nc popViewControllerAnimated:NO];
	}
	
	// Dismiss modal view(s)
	[nc dismissModalViewControllerAnimated:YES];
}

Nothing too complex, but puzzling if you’re not looking for it.