为什么文档不存在。但所有时间 !snapshot.hasData 都是假的

Question

我制作了注册系统和管理用户信息。

注册时需要填写用户名，保存在firestore中

用户数据是这样存储的；

并尝试使用代码获得 userName；

CollectionReference users = FirebaseFirestore.instance.collection('users');

return FutureBuilder<DocumentSnapshot>(
  future: users.doc(FirebaseAuth.instance.currentUser!.uid).get(),
  builder:
      (BuildContext context, AsyncSnapshot<DocumentSnapshot> snapshot) {
    if (snapshot.hasError) {
      return Text("Something went wrong");
    }

    print("${!snapshot.hasData} ${!snapshot.data!.exists}"); // always "false true"
    if (!snapshot.hasData && !snapshot.data!.exists) {
      return Text("Document does not exist");
    }

    if (snapshot.connectionState == ConnectionState.done) {
      Map<String, dynamic> data =
          snapshot.data!.data() as Map<String, dynamic>;
      return Text("User Name: ${data['userName']}");
    }

    return const CircularProgressIndicator();
  },
);

但是总是调用return Text("Document does not exist");，userName不显示。

为什么 userName 没有返回？我搜索了一个拼写错误，但找不到。这个问题今天花了很多时间。谢谢。

Answer 1

想在snapshot.hasData

之前使用!

if (!snapshot.hasData && !snapshot.data!.exists) {
      return Text("Document does not exist");
    }

Answer 2

你说的snapshot.data就是用来监控异步操作状态的AsyncSnapshot。当异步操作完成时，它的 hasData 变为真：所以当它完成从数据库读取 DocumentSnapshot 时。

要检测DocumentSnapshot是否真的存在，需要检查它的exists 属性.

常见状态的快速总结：

Object.property	Value	Explanation
`AsyncSnapshot.hasData`	`false`	the asynchronous operation has either not yet completed, or did not result in a `DocumentSnapshot` (for example: if there was a network error). Calling `snapshot.data` will result in an error message.
`AsyncSnapshot.hasData`	`true`	the asynchronous operation has completed and you can safely call `snapshot.data` on it to get the `DocumentSnapshot`.
`DocumentSnapshot.exists`	`false`	the document you requested does not exist, and calling `document.data` on the snapshot will result in an error.
`DocumentSnapshot.exists`	`true`	the document you requested exists, and you can access its data.

因此，一旦 AsyncSnapshot 完成加载 DocumentSnapshot，您的 !snapshot.hasData 将为假。要确定文档是否实际存在，请使用 snapshot.hasData && snapshot.data!.exists.

我还建议您在这里查看我对 Flutter 代码中各种快照类型的解释：

为什么文档不存在。但所有时间 !snapshot.hasData 都是假的

Why document doesn't exist. But all time !snapshot.hasData is false

dart

firebase

flutter

google-cloud-firestore

flutter-web