NoSuchMethodError 被拋出構建 StreamBuilder<documentsnapshot></documentsnapshot>

Question

實際上，我正在關注基於角色的身份驗證教程。 問題是當我第一次登錄時，我按預期導航到正確的屏幕並且快照確實有數據！ User: {role: admin, last_login: Timestamp(seconds=1610430754, nanoseconds=74000000), name: bilal, created_at: Timestamp(seconds=1610429038, nanoseconds=204000000), build_number: 1, email: bilalsaeed781833@gmail.com}

但是當我在移動的屏幕上創建另一個用戶並且同一個 Stream 構建器再次檢查基於角色的身份驗證時，快照沒有數據。

User:  null UserDoc: Instance of 'DocumentSnapshot'  

======== Exception caught by widgets library =======================================================
The following NoSuchMethodError was thrown building StreamBuilder<DocumentSnapshot>(dirty, state: _StreamBuilderBaseState<DocumentSnapshot, AsyncSnapshot<DocumentSnapshot>>#0f4cc):
The method '[]' was called on null.
Receiver: null
Tried calling: []("role")

我不知道為什么會出現該錯誤！

這是我的代碼：

return StreamBuilder<User>(
        stream: FirebaseAuth.instance.authStateChanges(),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.active) {
            return StreamBuilder<DocumentSnapshot>(
              stream: FirebaseFirestore.instance
                  .collection("users")
                  .doc(snapshot.data.uid)
                  .snapshots(),
              builder: (BuildContext context,
                  AsyncSnapshot<DocumentSnapshot> snapshot) {
                if (snapshot.connectionState == ConnectionState.active) {
                  final userDoc = snapshot.data;
                  final user = userDoc.data();
                  debugPrint("User:  ${user.toString()} UserDoc: ${userDoc.toString()}  ");
                  if (user['role'] == 'admin') {
                    return admin();

                  } else if (user['role'] == 'hod') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => hod()),
                    );
                  } else if (user['role'] == 'duo') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => duo()),
                    );
                  } else if (user['role'] == 'teacher') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => teacher()),
                    );
                  } else if (user['role'] == 'cr') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => cr()),
                    );
                  } else if (user['role'] == 'student') {
                    Navigator.push(
                      context,
                      MaterialPageRoute(builder: (context) => student()),
                    );
                  } else {
                    return Material(
                      child: Center(
                        child: CircularProgressIndicator(),
                      ),
                    );
                  }
                }

                return Material(
                  child: Center(
                    child: CircularProgressIndicator(),
                  ),
                );
              },
            );
          }
          return Material(
            child: Center(
              child: CircularProgressIndicator(),
            ),
          );
        });

Answer 1

在 if 語句中包裝 StreamBuilder 中的所有內容。

if (snapshot.connectionState == ConnectionState.active) {
  if(snapshot.hasData){
     ...
  } else {
      return CircularProgressIndicator()
  }
}

這應該可以防止構建器在沒有數據時嘗試構建某些東西。