iOS observeValueForKeyPath not getting called

Question

I had a problem getting observeValueForKeyPath to be called but solved it using this:

However, I'd like to know how to handle this in general for future reference.

The code that works looks like this:

 - (void) input: (NSString*) digit
 {
     NSLog(@"input() - Entering... digit=%@", digit);

     if ([digits length] <  MAX_DIGITS_LENGTH)
     {
         self.digits = [[[ self.digits autorelease] stringByAppendingString:digit] retain];
         NSLog(@"digits is now %@", digits);
     }

 }

Prior to this I was using an NSMutableString instead of NSString and just said used appendString. I didn't do an assignment and I wasn't appending "self" to the digits variable. Are there any good websites/tutorials that explain this more in depth so I know how to do this in general for any type of object?

score 4 · Accepted Answer · answered Jul 12 '11 at 03:08

4

KVO works by method swizzling and notifying on value changes. If you access your properties with out self then you are accessing an iVar directly and not using the method. When not using a method there is no way to send KVO notifications to your observer. The other issue you ran into is mutable vs immutable data. When you are appending a string you not changing the object but you are changing the data it is pointing at and that is why you were not getting any notifications. Only the get accessor was being called to get the string then you were calling append data on that.

answered Jul 12 '11 at 03:08

Joe

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So am I doing it right now? I'm not really happy with the autorelease/retain thing for NSString. Would it be better to do this with NSMutableString? If so, how do I write it and still get the methods to work? – gonzobrains Jul 12 '11 at 03:33
oh, I forgot to use my "swizzle stick," too. hahaha – gonzobrains Jul 12 '11 at 03:34
You wont be able to do it unless you create a custom append method and send a delegate message. – Joe Jul 12 '11 at 03:53

score 0 · Answer 2 · answered Jul 12 '11 at 03:43

Your memory mgmt seems off. Assuming 'digits' is a @property declared with the retain or copy attribute, and you are using the synthesized setter (or wrote an equivalent one yourself), then you should use the setter like this:

self.digits = [self.digits stringByAppendingString:digit];

You shouldn't autorelease the old value or retain the new one, since the setter does that for you.

If you would like to use an NSMutableString there instead, you could trigger the KVO notifications manually like so:

 [self willChangeValueForKey:@"digits"];
 [self.digits appendString:whatever]; // assumes digits is a mutable string now
 [self didChangeValueForKey:@"digits"];

Hope that helps.

iOS observeValueForKeyPath not getting called

2 Answers2