Objective-C object dealloc'd while other objects still has a delegate reference to it causes crashes. How to prevent this?

I have an app with a navigation controller as the root view. There are many views that can be pushed in. The user has to create an account to use the app. The user can then log into this account from other devices, but only one device can be logged onto the same account at a time. So if multiple devices try to log into an account, only the latest device will be logged in and the other devices are logged off (a push is sent to the devices).

Since there are multiple views that the device could be showing before it was logged off, I call popToRootViewControllerAnimated: to get back to the root view. This is because when the user logs in the next time I only want the root view to be shown (the new account might not have access to the previously shown view).

If the user has an alert view or action sheet presented (which uses the current view as its delegate) before the push is received, the view will still be shown after the popToRootViewControllerAnimate: method is called. If the user then taps on a button for the alert view or action sheet, it will send a message to the dealloc'd view and crash the app.

An example: myViewController is being shown to the user. myViewController create an action sheet prompting the user for a decision. T开发者_如何学Gohe push is received for the device to log out. The navigation controller pops all the views controllers and now shows myRootViewController. Since the view controllers are popped, myViewController is now dealloc'd. The action sheet from myViewController is still shown. When the user selects an option form the action sheet, a message is sent to myViewController, and since it is already dealloc'd, a crash will occur.

Is there any way to prevent this?

One solution I have considered would be to keep track of all the objects that uses a specific view controller as its delegate. Then when that view controller dealloc's it will also set all the object's delegates to nil. This requires me to manually take care of every view controller when they create an object that uses itself as the delegate, since I cannot think of a way to automatically create and update this list.

Any better solution (or improvement to mine) would be appreciated!

Edit: The alert view and action sheet are only examples of some objects that I would use myViewController as a delegate. I am also using a number of other classes (and third-party libraries) that implements this delegate pattern.

A few ideas:

1. you can encapsulate the alert/action sheet view and delegate in a single class. Then when you need an alert view, create MyAlertView instead, which will also be its own delegate and will do [self release] after the user taps a button.

2. make your App Delegate the only delegate for all your alert views and action sheets. App Delegate is always around while the application is running, so there won't be a problem with a released delegate.

The problem with both solutions is that if you need your application to know what happened in the alert view/action sheet, you somehow need to tell the interested class of the user's choice.

You can do that by either using delegates of your own - which would mean you're back to square one - or use notifications: when the alert view/action sheet delegate is called, it would post a notification ([[NSNotificationCenter defaultCenter] postNotificationName:NotificationName object:self userInfo:userInfo];), while the interested object would look for that notification ([[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(onNotification:) name:NotificationName object:nil];) and perform whatever tasks necessary in onNotification:(NSNotification*)aNotification method.

You'll be able to agree with yourself on what type of information is passed in those notifications (I would think the button number in a NSNumber class would be enough, or perhaps pass the button text, too). And you won't have to keep track of all alert views - just don't forget to remove the observer ([[NSNotificationCenter defaultCenter] removeObserver:self name:postNotificationName object:nil];) in the views' dealloc.

Edit:

"This requires me to manually take care of every view controller when they create an object that uses itself as the delegate, since I cannot think of a way to automatically create and update this list."

Actually you probably can do this in a semi-automated way: make a singleton object with a method like

-(id)delegate:(id)delegate for:(id)forWhom

And then instead of

someThingy.delegate = self;

you'd do

someThingy.delegate = [[DelegateLocker defaultLocker] delegate:self for:someThingy];

Inside the DelegateLocker you'd have a MutableDictionary with delegate class as a key and a MutableArray of someThingies as a value. Then in your view controllers' deallocs you'd call

[[DelegateLocker defaultLocker] delegateIsDying:self];

which would go through the thingies and assign delegate = nil for each

The drawback of course is that you'll be retaining all the thingies for an indefinite period of time instead of releasing them immediately.

So the ViewController that presented the action sheet iand set itself as the delegate right? So why dont you keep a reference to the ActionSheet in the ViewController, in the dealloc method of the view controller, you can check if the action sheet is visible, if it is then set the delegate of the action sheet to nil,and dismiss it...

so

-(void)dealloc
{
   if(myActionSheet && [myActionSheet visible])
   {
      [myActionSheet setDelegate: nil];
      //dismiss 
    }

}

Hope that helps

If you want automated solution, I think you can make a function to iterate through Ivars of your view controller to see if any Ivar has delegate property and set it to nil.