- (void)setText:(NSString *)text
{
    static NSString *key;
    if (!key) key = NSStringFromSelector(@selector(text));
    [self willChangeValueForKey:key];
    [self setPrimitiveText:text];
    self.textCount = text.count;
    [self didChangeValueForKey:key];
}

[someObject addObserver:self forKeyPath:NSStringFromSelector(@selector(text)) options:NSKeyValueObservingOptionNew context:someContext];