string的length不可能等于 0的，好吧，

  NSString *searchText = [self.searchTextField.textstringByTrimmingCharactersInSet:[NSCharacterSetwhitespaceAndNewlineCharacterSet]];

    if (!searchText.length > 0) {

        [selfshowAlertWithTitle:nilmessage:@"请输入搜索关键字！"];

    }