NSCell -stringValue calls -setStringValue?

Question

In essence, here is my code in a subclass of NSTextFieldCell:

- (void)setStringValue:(NSString *)aString {
    [super setStringValue:aString];
    [self doSomething];
}

- (void)doSomething {
    NSLog(@"%@", [self stringValue]);
}

In essence, I'm trying to get my subclass notified of a change in the string value, and when the value changes, I want the subclass of NSCell to be able to do something with that new value (Using -attributedStringValue would be even better, because I want to cache it for special drawing). The problem is, for some reason, calling -(NSString *)stringValue somehow results in a call to -(void)setStringValue which ends up becoming...

...an infinite loop. Can someone enlighten me on this - and a possible workaround?

How are you calling stringValue? I tried your code, and I didn't get any loop, it seemed to work fine. — rdelmar, Apr 08 '12 at 23:34
@rdelmar I'm using the default implementation of stringValue. Maybe I missed something obvious in my code, but I'm just writing a subclass for a NSTextfieldCell which happens to be located inside an NSTableView (with the NSTextField bound to the TableCellView) — Vervious, Apr 09 '12 at 00:18

score 1 · Accepted Answer · answered Apr 08 '12 at 23:00

Probably stringValue is doing some lazy initialization. If it detects that it has no stringValue but can derive the value from something else (perhaps attributedStringValue?), then it derives the value, calls [self setStringValue:derivedValue], then returns the value.

This does make life tricky, though. Try something like this?

@implementation MyCell ()
{
    BOOL gettingStringValue;
}
@end

- (NSString*)stringValue {
    gettingStringValue = YES;
    NSString* stringValue = [super stringValue];
    gettingStringValue = NO;
    return stringValue; 
}

- (void)setStringValue:(NSString *)aString {
    [super setStringValue:aString];
    if (!gettingStringValue)
        [self doSomething];
}

That's a really nice solution - I wish I had thought of that beforehand - thank you! — Vervious, Apr 09 '12 at 00:23
It's not my favorite technique -- this sort of thing tends to be pretty fragile -- but sometimes it's the only way... — Kurt Revis, Apr 09 '12 at 00:29

NSCell -stringValue calls -setStringValue?

1 Answers1