How do you test if an NSString is empty

-(BOOL) isEmptyString:(NSString*) string {
   if([string length]==0){//string is empty or nil
       return YES;
   }elseif([[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0){
     //string is all whitespace
       return YES;
   }
   return NO;
}