Question

I have a class that uses automatic reference counting. Within the class, I have a property as described below. When I set the reminder for the class, I want the side effect of changing the button title to take place.

Here’s my code:

in the .h file:

@property(nonatomic,strong)Reminder* reminder;

in the .m file:
@synthesize reminder;

  -(void)setReminder:(Reminder *)reminder_
    {
//what else do I need to do here?
        reminder = reminder_;
         if(!reminder.useSound.boolValue)
        {
             onOffButton.title = NSLocalizedString(@"Off", @"Off title");

        }else
        {
            onOffButton.title = NSLocalizedString(@"On", @"On title");
        }
    }

I know that without ARC I would do something like this:

-(void)setReminder:(Reminder *)reminder_

{

[reminder release];
reminder = [reminder_ retain];


     if(!reminder.useSound.boolValue)
    {
         onOffButton.title = NSLocalizedString(@"Off", @"Off title");

    }else
    {
        onOffButton.title = NSLocalizedString(@"On", @"On title");
    }
}

Do I need to do anything else within my ARC enabled setter method to ensure that the strong variable is retained properly?

Answer 1

You don’t need to write your own setter for side effects like this. Use KVO!

Make the controller that owns the button observe this object’s reminder property:

[reminderOwner addObserver:self
                forKeyPath:@"reminder"
                   options:NSKeyValueObservingOptionNew
                   context:NULL];

The options argument specifies what information about the state of the observed object you’d like passed along when the observation is made.

Then your controller will be apprised whenever this property changes value (note that this will only occur if the property is set in a “Key-Value compliant” manner, the handiest way being via the setter method* — changing the ivar directly doesn’t count).

You need to implement observeValueForKeyPath:ofObject:change:context: in your controller:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    // Distinguish between observations if necessary; you can also
    // use the |object| argument for this.
    if( [keyPath isEqualToString:@"reminder"] ){
        // Update button text with contents of |change| dictionary
        Reminder * reminder = [change objectForKey:NSKeyValueChangeNewKey];
        if(!reminder.useSound.boolValue)
        {
             onOffButton.title = NSLocalizedString(@"Off", @"Off title");

        }else
        {
            onOffButton.title = NSLocalizedString(@"On", @"On title");
        }
    }
}

Oh, and to, uh, answer your actual question, I believe that your memory management is fine. reminder is synthesized to be a __strong-qualified variable, which means ARC will retain whatever you put there.

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*Which is why you’ve heard not to use setters in init and dealloc.