可以将无效的表达式可以用作for-In循环中的迭代器

发布于 2025-01-24 09:48:31 字数 1818 浏览 3 评论 0原文

我在使用Flutter＆amp; Firebase（Firestore），我进行了研究，无法找到任何有用的东西。
我想将我从firestore获得的流变成小部件流streambuilder（）。
这是firestore对象初始化最终FirebaseFirestore _firestore = firebasefirestore.instance;

这是我尝试从Firestore获取数据的方法

  void messagesstream（）async {
  等待（_firestore.collection（'message'）中的（var snapshot）。快照（））{
    对于（snapshot.docs中的var消息）{
      打印（message.data（））;
    }
  }
}

这是streambuilder widget insdie my widget tree =＆gt;

StreamBuilder<QuerySnapshot>(
              stream: _firestore.collection('message').snapshots(),
              builder: (context, snapshot) {
                if (!snapshot.hasData) {
                  return const Center(
                    child: CircularProgressIndicator(),
                  );
                }
                final messages = snapshot.data!;
                List<Text> messageWidgets = [];
                for (var message in messages) // <== I get My error here
                   *// The type 'QuerySnapshot<Object?>' used in the 'for' loop must 
                   // implement Iterable.*
                  {
                  final messageText = message.data['text'];
                  final messageSender = message.data['sender'];
                  final messageWidget =
                      Text('$messageText from @$messageSender');
                  messageWidgets.add(messageWidget);
                }
                return Column(
                  children: messageWidgets,
                );
              },
            ),

如果我从最终消息= snapshot.data;删除了Null检查操作员我得到了一个不同的错误，这是

一个无效的表达式无法用作for-In循环中的迭代器。尝试在将其用作迭代器之前检查值不是“ null”。

原文

I encountered this problem while working with Flutter & Firebase (firestore), I did my research and couldn`t find anything helpful.
I want to turn my streams I get from firestore into a stream of widgets StreamBuilder().
Here is Firestore object initialization final FirebaseFirestore _firestore = FirebaseFirestore.instance;

Here is the method where I try to get my data from firestore as a stream

void messagesStream() async {
  await for (var snapshot in _firestore.collection('message').snapshots()) {
    for (var message in snapshot.docs) {
      print(message.data());
    }
  }
}

Here is StreamBuilder widget insdie my widget tree =>

StreamBuilder<QuerySnapshot>(
              stream: _firestore.collection('message').snapshots(),
              builder: (context, snapshot) {
                if (!snapshot.hasData) {
                  return const Center(
                    child: CircularProgressIndicator(),
                  );
                }
                final messages = snapshot.data!;
                List<Text> messageWidgets = [];
                for (var message in messages) // <== I get My error here
                   *// The type 'QuerySnapshot<Object?>' used in the 'for' loop must 
                   // implement Iterable.*
                  {
                  final messageText = message.data['text'];
                  final messageSender = message.data['sender'];
                  final messageWidget =
                      Text('$messageText from @$messageSender');
                  messageWidgets.add(messageWidget);
                }
                return Column(
                  children: messageWidgets,
                );
              },
            ),

If I removed the null check operator from final messages = snapshot.data;
I get a different error which is

A nullable expression can't be used as an iterator in a for-in loop.
Try checking that the value isn't 'null' before using it as an iterator.

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黎夕旧梦 2025-01-31 09:48:31

对于循环，轻松处理中的无效迭代的一种方法是使用??提供不可删除的后备值。例如：

List<int>? nullableList;
for (var element in nullableList ?? <int>[]) {
  ...
}

另外，您可以使用foreach使用条件 - 访问操作员：

List<int>? nullableList;
nullableList?.forEach((element) {
  ...
});

尽管通常我建议常规对于 foreach 循环。

One way to easily deal with nullable iterables in a for loop is to use ?? to provide a non-nullable fallback value. For example:

List<int>? nullableList;
for (var element in nullableList ?? <int>[]) {
  ...
}

Alternatively you can use the conditional-member-access operator with forEach:

List<int>? nullableList;
nullableList?.forEach((element) {
  ...
});

although generally I recommend normal for loops over forEach.

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￡噩梦荏苒 2025-01-31 09:48:31

我认为您可以检查“ snapshot.data”是否在其他任何事情之前都没有零。

// This basically means that if you have data, you show the widget CircularProgressIndicator()
if (snapshot.hasData) {
    return const Center(
        child: CircularProgressIndicator(),
    );
}

您可能会这样尝试：

// Means that if the snapshot hasn't data show the widget
if (!snapshot.hasData) {
    return const Center(
        child: CircularProgressIndicator(),
    );
}

I think you may check if "snapshot.data" is not null before anything else.

// This basically means that if you have data, you show the widget CircularProgressIndicator()
if (snapshot.hasData) {
    return const Center(
        child: CircularProgressIndicator(),
    );
}

You may try like this :

// Means that if the snapshot hasn't data show the widget
if (!snapshot.hasData) {
    return const Center(
        child: CircularProgressIndicator(),
    );
}

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