Question

实际上，我正在学习基于角色的身份验证教程。问题是，当我第一次登录时，我按预期导航到了正确的屏幕，并且快照确实有数据！ 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}

但是当我在移动的屏幕上创建另一个用户并且同一个流构建器再次检查基于角色的身份验证时，快照没有数据。

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")

我不知道为什么会出现这个错误！

这是我的代码：

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

将 StreamBuilder 中的所有内容都包含在 if 语句中。

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

这应该可以防止构建器在没有数据时尝试构建某些东西。

构建 StreamBuilder<DocumentSnapshot> 时抛出 NoSuchMethodError

1 个答案: