问题

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我是Flutter的新手。在更新列表时遇到了一些问题。

目前，我有一个带有一些键和值的列表，如下所示：

[
  {filename: aa}, 
  {filename: bb.txt}, 
  {filename: cc.txt}
] 

我想通过添加一个名为isPresent的参数来更新上述列表，并通过对当前数据进行一些处理来进行更新，如下所示：

[
  {filename: aa,
   isPresent: false}, 
  {filename: bb.txt,
   isPresent: true}, 
  {filename: cc.txt,
  isPresent: false}
] 

如何做到这一点？ 

为了实现上述更新的列表，我正在使用以下代码：

final List<Map<String, dynamic>> list = [...fetchJsonData['data']];
list.asMap().forEach((key, value) async {
  Directory? directory;
  directory = await getExternalStorageDirectory();
  File file = File("${directory!.path}/${list[key]['filename']}");
  await io.File(file.path).exists();
  bool isExists = io.File(file.path).existsSync();
  print(isExists);
  list[key]['isPresent'] = isExists;
});
fileList = list;
print(fileList);

但是使用await时似乎不起作用。如果我漏掉了什么，请告诉我。

<details>
<summary>英文:</summary>

I am new to Flutter. I am facing some issues while updating a list. 

Currently, I have a list with some keys and values as follows:

[
{filename: aa},
{filename: bb.txt},
{filename: cc.txt}
]

And I want to update the above list by adding one more parameter called isPresent and update it as follows by doing some process on current data:

[
{filename: aa,
isPresent: false},
{filename: bb.txt,
isPresent: true},
{filename: cc.txt,
isPresent: false}
]

How to do that any idea?

To achieve the above updated list I am using the following code 

final List<Map<String, dynamic>> list = [...fetchJsonData['data']];
list.asMap().forEach((key, value) async {
Directory? directory;
directory = await getExternalStorageDirectory();
File file = File("${directory!.path}/${list[key]['filename']}");
await io.File(file.path).exists();
bool isExists = io.File(file.path).existsSync();
print(isExists);
list[key]['isPresent'] = isExists;
});
fileList = list;
print(fileList);

but does not work with Await I think. Let me know if I am missing something.

</details>


# 答案1
**得分**: 0

```dart
主函数() {
  List<Map<String, dynamic>> 数据 = [
    {'文件名': 'aa'},
    {'文件名': 'bb.txt'},
    {'文件名': 'cc.txt'}
  ];

  数据[0]['是否存在'] = false;
  数据[1]['是否存在'] = true;
  数据[2]['是否存在'] = false;

  打印(数据);
  //[{文件名: aa, 是否存在: false}, {文件名: bb.txt, 是否存在: true}, {文件名: cc.txt, 是否存在: false}]
}

main() {
  List<Map<String, dynamic>> data = [
    {'filename': 'aa'},
    {'filename': 'bb.txt'},
    {'filename': 'cc.txt'}
  ];

  data[0]['isPresent'] = false;
  data[1]['isPresent'] = true;
  data[2]['isPresent'] = false;

  print(data);
  //[{filename: aa, isPresent: false}, {filename: bb.txt, isPresent: true}, {filename: cc.txt, isPresent: false}]
}

</details>



# 答案2
**得分**: 0

List<Map<String, dynamic>> originalList = [
  {"filename": "aa"},
  {"filename": "bb.txt"},
  {"filename": "cc.txt"}
];

List<Map<String, dynamic>> updatedList = originalList.map((item) {
  String filename = item["filename"];
  bool isPresent = /* 根据文件名执行您的处理过程 */;
  
  return {
    "filename": filename,
    "isPresent": isPresent,
  };
}).toList();

List<Map<String, dynamic>> originalList = [
  {"filename": "aa"},
  {"filename": "bb.txt"},
  {"filename": "cc.txt"}
];

List<Map<String, dynamic>> updatedList = originalList.map((item) {
  String filename = item["filename"];
  bool isPresent = /* Perform your process here based on the filename */;
  
  return {
    "filename": filename,
    "isPresent": isPresent,
  };
}).toList();

void updateList() {
  
  List<Map<String, dynamic>> data = [
    {"filename": "aa"},
    {"filename": "bb.txt"},
    {"filename": "cc.txt"}
  ];

  for (var file in data) {
    file["isPresent"] = file["filename"].endsWith(".txt");
  }
  print(data);
}