Dismissing UIAlertViews when entering background state

Apple recommends dismissing any UIAlertViews/UIActionSheets when entering background state in iOS 4. This is to avoid any confusion on the user's part when he relaunches the application later. I wonder how I could elegantly dismiss all UIAlertViews at once, without retaining a reference to it everytime I set one up...

Any idea ?

My call would be to add a category to UIAlertview adding the following function :

- (void) hide {
  [self dismissWithClickedButtonIndex:0 animated:YES];
}

And to suscribe to UIApplicationWillResignActiveNotification :

[[NSNotificationCenter defaultCenter] addObserver:alertView selector:@selector(hide) name:@"UIApplicationWillResignActiveNotification" object:nil];

I was intrigued by Dad's answer (funny username :), and curious why it was down-voted.

So I tried it.

Here is the .m part of a subclass of UIAlertView.

Edit: (Cédric) I have added a way to catch calls to delegate methods and remove the observer then to avoid multiple registrations to the notification center.

Everything bundled in a class in this github repo: https://github.com/sdarlington/WSLViewAutoDismiss

    #import "UIAlertViewAutoDismiss.h"
    #import <objc/runtime.h>

    @interface UIAlertViewAutoDismiss () <UIAlertViewDelegate> {
        id<UIAlertViewDelegate> __unsafe_unretained privateDelegate;
    }
    @end

    @implementation UIAlertViewAutoDismiss

    - (id)initWithTitle:(NSString *)title
                message:(NSString *)message
               delegate:(id)delegate
      cancelButtonTitle:(NSString *)cancelButtonTitle
      otherButtonTitles:(NSString *)otherButtonTitles, ...
    {
        self = [super initWithTitle:title
                            message:message
                           delegate:self
                  cancelButtonTitle:cancelButtonTitle
                  otherButtonTitles:nil, nil];

        if (self) {
            va_list args;
            va_start(args, otherButtonTitles);
            for (NSString *anOtherButtonTitle = otherButtonTitles; anOtherButtonTitle != nil; anOtherButtonTitle = va_arg(args, NSString *)) {
                [self addButtonWithTitle:anOtherButtonTitle];
            }
            privateDelegate = delegate;
        }
        return self;
    }

    - (void)dealloc
    {
        privateDelegate = nil;
        [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidEnterBackgroundNotification object:nil];
        [super dealloc];
    }

    - (void)setDelegate:(id)delegate
    {
        privateDelegate = delegate;
    }

    - (id)delegate
    {
        return privateDelegate;
    }

    - (void)show
    {
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(applicationDidEnterBackground:)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];

        [super show];
    }

    - (void)applicationDidEnterBackground:(NSNotification *)notification
    {
        [super dismissWithClickedButtonIndex:[self cancelButtonIndex] animated:NO];
        [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidEnterBackgroundNotification object:nil];
    }

    #pragma mark - UIAlertViewDelegate

    // The code below avoids to re-implement all protocol methods to forward to the real delegate.

    - (id)forwardingTargetForSelector:(SEL)aSelector
    {
        struct objc_method_description hasMethod = protocol_getMethodDescription(@protocol(UIAlertViewDelegate), aSelector, NO, YES);
        if (hasMethod.name != NULL) {
            // The method is that of the UIAlertViewDelegate.

            if (aSelector == @selector(alertView:didDismissWithButtonIndex:) ||
                aSelector == @selector(alertView:clickedButtonAtIndex:))
            {
                [[NSNotificationCenter defaultCenter] removeObserver:self
                                                                name:UIApplicationDidEnterBackgroundNotification
                                                              object:nil];
            }
            return privateDelegate;
        }
        else {
            return [super forwardingTargetForSelector:aSelector];
        }
    }

    @end

It works nicely. It's great, because you can just start using it the same way that you used to use UIAlertView.

I haven't had time to test it thoroughly, but I didn't notice any side effect.