UINavigationController crash because of pushing and poping UIViewControllers
- by Wayne Lo
My question is related to my discovery of a reason for UINavigationController to crash. So I will tell you about the discovery first. Please bare with me.
The issue:
I have a UINavigationController as as subview of UIWindow, a rootViewController class and a custom MyViewController class. The following steps will get a Exc_Bad_Access, 100% reproducible.:
[myNaviationController pushViewController:myViewController_1stInstance animated:YES];
[myNaviationController pushViewController:myViewController_2ndInstance animated:YES];
Hit the left back tapBarItem twice (pop out two of the myViewController instances) to show the rootViewController.
After a painful 1/2 day of try and error, I finally figure out the answer but also raise a question.
The Solutio: I declared many objects in the .m file as a lazy way of declaring private variables to avoid cluttering the .h file. For instance,
#impoart "MyViewController.h"
NSMutableString*variable1;
@implement ...
-(id)init
{
...
varialbe1=[[NSMutableString alloc] init];
...
}
-(void)dealloc
{
[variable1 release];
}
For some reasons, the iphone OS may loose track of these "lazy private" variables memory allocation when myViewController_1stInstance's view is unloaded (but still in the navigation controller's stacks) after loading the view of myViewController_2ndInstance. The first time to tap the back tapBarItem is ok since myViewController_2ndInstance'view is still loaded. But the 2nd tap on the back tapBarItem gave me hell because it tried to dealloc the 2nd instance. Executing [variable release] resulted in Exc_Bad_Access because it pointed randomly (loose pointer).
To fix this problem is simple, declare variable1 as a @private in the .h file.
Here is my Question:
I have been using the "lazy private" variables for quite some time without any issues until they are involved in UINavigationController. Is this a bug in iPhone OS? Or there is a fundamental misunderstanding on my part about Objective C? Please help.
