Flutter/Dart- 方法 'replaceAll' 被调用为 null。如何检查字符串是否存在？

Question

我有一个包含 3 个文本字段的表单，供用户插入 3 个标签。在上传到服务器之前，我需要用正则表达式清理它们。问题是，除非用户填写了所有三个文本字段，否则 Android Studio 会抛出以下错误；

E/flutter (13862): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'replaceAll' was called on null.
E/flutter (13862): Receiver: null
E/flutter (13862): Tried calling: replaceAll(RegExp: pattern=[# ] flags=, "") 

那么如何在发送前检查每个字段的存在呢？这是代码；

Future<String> uploadTags({String tag1, String tag2, String tag3,}) async {

  final serverurl = "http://example.com/example.php";
  
  final String cleantag1 = tag1.replaceAll(RegExp('[# ]'),'');
  final String cleantag2 = tag2.replaceAll(RegExp('[# ]'),'');
  final String cleantag3 = tag3.replaceAll(RegExp('[# ]'),'');

  var request = http.MultipartRequest('POST', Uri.parse(serverurl));
  
  request.fields['tag1'] = cleantag1;
  request.fields['tag2'] = cleantag2;
  request.fields['tag3'] = cleantag3;

  var multiPartFile = await http.MultipartFile.fromPath("audio", filepath, contentType: MediaType("audio", "mp4"));
  request.files.add(multiPartFile);
  request.send().then((result) async {
    http.Response.fromStream(result)
        .then((response) {
      if (response.statusCode == 200)
      {
        print('response.body '+response.body);
      }
      return response.body;
    });
  });
}

Answer 1

tag1或tag2或tag3参数值为NULL,

为了确定，在这一行加断点

final String cleantag1 = tag1.replaceAll(RegExp('[# ]'),'');

检查参数值

如果不需要这个值，你可以在变量之后使用 ? :

final String cleantag1 = tag1?.replaceAll(RegExp('[# ]'),'');

如果你想添加一个默认值这样写:

final String cleantag1 = tag1?.replaceAll(RegExp('[# ]'),'')??'defult_value';