Question

I have looked over some ideas for how to supply a context to a UIAlertView. The common answers are save it in a dictionary or subclass UIAlertView. I don’t like the idea of saving the context in a dictionary, it’s the wrong place for the data. Subclassing UIAlertView is not supported by Apple, so by my standard, is not a good solution.

I came up with an idea, but I’m not sure what to make of it. Create an instance of a context object that is the delegate of UIAlertView. The alert view context, in turn, has it’s own delegate which is the view controller.

The trouble is releasing memory. I set alertView.delegate to nil and call [self autorelease] to free the context object in -alertView:didDismissWithButtonIndex:.

THE QUESTION IS: What problems am I causing myself? I have a suspicion that I’m setting myself up for a subtle memory error.

Here is the simple version which only supports -alertView:clickedButtonAtIndex:

Use

- (void)askUserIfTheyWantToSeeRemoteNotification:(NSDictionary *)userInfo
{
    [[[[UIAlertView alloc] initWithTitle:[userInfo valueForKey:@"action"]
                                 message:[userInfo valueForKeyPath:@"aps.alert"]
                                delegate:[[WantAlertViewContext alloc] initWithDelegate:self context:userInfo]
                       cancelButtonTitle:@"Dismiss"
                       otherButtonTitles:@"View", nil] autorelease] show];
}
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex withContext:(id)context
{
    if (buttonIndex != alertView.cancelButtonIndex)
        [self presentViewForRemoteNotification:context];
}

Interface

@protocol WantAlertViewContextDelegate <NSObject>
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex withContext:(id)context;
@end

@interface WantAlertViewContext : NSObject <UIAlertViewDelegate>
- (id)initWithDelegate:(id<WantAlertViewContextDelegate>)delegate context:(id)context;
@property (assign, nonatomic) id<WantAlertViewContextDelegate> delegate;
@property (retain, nonatomic) id context;
@end

Implementation

@implementation WantAlertViewContext
- (id)initWithDelegate:(id<WantAlertViewContextDelegate>)delegate context:(id)context
{
    self = [super init];
    if (self) {
        _delegate = delegate;
        _context  = [context retain];
    }
    return self;
}
- (void)dealloc
{
    [_context release];
    [super dealloc];
}
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    [self.delegate alertView:alertView clickedButtonAtIndex:buttonIndex withContext:self.context];
}
- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex
{
    alertView.delegate = nil;
    [self autorelease];
}
@synthesize delegate = _delegate;
@synthesize context  = _context;
@end

Answer 1

You can use the concept of associated objects. Using the functions objc_setAssociatedObject() and objc_getAssociatedObject(). You can use these properties to essentially add a new property, in your case to hold an NSDictionary, to an object through a category.

Here is an example of a UIAlertView category. These files should be compiled without ARC, -fno-objc-arc flag set if the project is using ARC.

UIAlertView+WithContext.h:

#import <UIKit/UIKit.h>
@interface UIAlertView (Context)
@property (nonatomic, copy) NSDictionary *userInfo;
@end

UIAlertView+WithContext.m:

#import "UIAlertView+WithContext.h"
// This enum is actually declared elseware
enum {
    OBJC_ASSOCIATION_ASSIGN = 0,
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
    OBJC_ASSOCIATION_RETAIN = 01401,
    OBJC_ASSOCIATION_COPY = 01403
};
@implementation UIAlertView (Context) 
static char ContextPrivateKey;
-(void)setUserInfo:(NSDictionary *)userInfo{
    objc_setAssociatedObject(self, &ContextPrivateKey, userInfo, 3);
}
-(NSDictionary *)userInfo{
    return objc_getAssociatedObject(self, &ContextPrivateKey);
}
@end

This category is easily used.

SomeViewController.m: a UIAlertViewDelegate using ARC or not.

-(void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Title" message:@"Message" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
    alert.userInfo = [NSDictionary dictionaryWithObject:@"Hello" forKey:@"Greeting"];// autorelease if MRC
    [alert show]; // release if MRC
}

-(void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex{
    NSLog(@"userInfo:%@",alertView.userInfo);
}

When you press the alertview’s OK button you will see:

userInfo:{
    Greeting = Hello;
}

A couple of notes:

1) Make sure the association type matches the property declaration so things behave as expected.

2) You probably shouldn’t use userInfo for the property/association since Apple may well decide to add a userInfo property to UIAlertView in the future.

Edit To address your concerns about your [self autorelease];

It is imperative that you balance your implicit alloc retain from this line: delegate:[[WantAlertViewContext alloc] initWithDelegate:self context:userInfo]. You achieve this balance by calling [self autorelease]; in the final UIAlertView delegate method.

Granted, this does feel wrong. Mostly because there is no way when looking at this that it doesn’t at first blush look like memory mis-management. But there is one simple way to avoid this “controlled leak” API you are creating; Have the instance of WantAlertViewContext explicitly retain itself. For example:

-(id)initWithDelegate:(id<WantAlertViewContextDelegate>)delegate context:(id)context{
    self = [super init];
    if (self) {
        _delegate = delegate;
        _context  = [context retain];
    }
    return [self retain]; // Explicitly retain self
}

-(void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex{
    alertView.delegate = nil;
    [self autorelease]; // Or just [self release]; doesn't make much difference at this point
}

Now your class has some internal harmony. I say some because this is still not perfect. For example, if an instance is never an alert-view delegate it will never be released. It is still just a “semi-controlled” memory leak.

Anyway, now your instantiation call can look more logical:

delegate:[[[WantAlertViewContext alloc] initWithDelegate:self context:userInfo] autorelease];

I think that this particular design pattern is fraught with danger. If you do end up using it keep a close eye on it.