ios开发判断字符串是否为空

- (BOOL)isBlankString:(NSString *)string {
     if (string == nil || string == NULL) {
          return YES;
     }
     if ([string isKindOfClass:[NSNull class]]) {
          return YES;
     }
     if ([[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0) {
          return YES;
     }
     return NO;
}