why nsstring is gone when used in another class? - objective-c

Question

in classA i have :
classB *classBI=[[classB alloc]init];
bits=[classBI data]; //bits has a property here in classA,it gets data from B/
UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(38.f, 20.f, 244.f, 43.f)];
label.text = bits;
in classB i have :
#property (nonatomic,retain) NSString *data; //in .h file
data=#"no data"; //at the init method of classB .
//then after a while when something is happen in classB , data is changed to :
data=[NSString stringWithFormat:#"data:%f,%f,%f,%f,%f,%f",
dataBits[0],dataBits[1],dataBits[2],dataBits[3],dataBits[4],dataBits[5] ];
classA is the main scene (cocos2d but it does not matter). at the start i can really see in the label that in classA the word "no data", but when data is changed in classB, i cant see that change in the label that is on screen. it stay with the word: "no data "
why is that ?
why i lost data ?
if i NSLOG data in classB ,right after it changed in there,i can see that its not null,and it has the new value. something is wrong with the instance of B ,in A, that get this string.

Answer 1

When you set the label's text to the string pointed to by bits, it is storing a reference to that string. Later when you change data, it is creating a new string which data is pointing to, but bits and the label both still have the original pointer which is looking at the original string. You want to update them whenever data is changed:
There are several approaches to take in this situation, but one of the easiest would be to observe the data property of classBI for changes, and update both bits and the label whenever it changes:
[classBI addObserver:self
forKeyPath:#"data"
options:NSKeyValueObservingOptionNew
context:NULL];
Then, whenever data changes, this method will be called:
// Note that you will need to get a reference to 'label' in order for this to work:
- (void) observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(NSDictionary*)change context:(void*)context {
if ([keyPath isEqual:#"data"]) {
bits = [change objectForKey:NSKeyValueChangeNewKey];
label.text = bits;
}
}