Flutter登录验证后页面不加载及两类异常问题求助

Hey there! Let's break down your issues one by one and fix them step by step:

1. Fixing the ArgumentError in LoginScreenPresenter.doLogin #

The error message tells us your error handler in catchError doesn’t accept the correct arguments. Right now, you’re using (Exception error) but Dart’s catchError expects a handler that takes either an Object or an Object plus a StackTrace.

Update your login_screen_presenter.dart code:

doLogin(String username, String password) {
  api.login(username, password).then((User user) {
    _view.onLoginSuccess(user);
  }).catchError((Object error) => _view.onLoginError(error.toString()));
  // Optional: Capture stack trace for more detailed debugging
  // }).catchError((Object error, StackTrace stackTrace) => _view.onLoginError("$error\n$stackTrace"));
}

2. Fixing the "Boolean expression must not be null" Exception #

This comes from rest_ds.dart where you check if(res["error"]). On successful login, your API response doesn’t include an error key, so res["error"] returns null — and you can’t use a null value in a boolean check.

Modify the login method in rest_ds.dart to handle this safely:

Future<User> login(String username, String password) {
  return _netUtil.post(LOGIN_URL, body: {
    "username": username,
    "password": password
  }).then((dynamic res) {
    print(res.toString());
    // Check if "error" exists and is truthy
    if(res.containsKey("error") && res["error"]){ 
      throw new Exception(res["error_msg"]);
    }
    // Alternative: Use null coalescing to avoid null values
    // if(res["error"] ?? false){ throw new Exception(res["error_msg"]); }
    return new User.map(res["user"]);
  });
}

3. Cleaning Up Redundant Code in onLoginSuccess #

Your onLoginSuccess method has duplicate, unused code (the stray HomeScreen() calls) and you’re manually triggering onAuthStateChanged after notifying the provider. Let’s clean that up:

@override
void onLoginSuccess(User user) async {
  setState(() => _isLoading = false);
  _showSnackBar(user.toString());
  
  var db = new DatabaseHelper();
  await db.saveUser(user);
  
  var authStateProvider = new AuthStateProvider();
  authStateProvider.notify(AuthState.LOGGED_IN);
  // Remove the manual call — your listener will trigger navigation automatically
  // onAuthStateChanged(AuthState.LOGGED_IN); 
}

4. Verify Route Registration #

Double-check that you’ve properly registered the /home route in your app’s root MaterialApp:

MaterialApp(
  routes: {
    "/home": (context) => HomeScreen(),
    // Add other routes here
  },
)

5. Ensure Safe Navigation Context #

Instead of relying on the _ctx variable, use the widget’s built-in context directly in your auth state listener for more reliability:

@override
onAuthStateChanged(AuthState state) {
  if(state == AuthState.LOGGED_IN) {
    Navigator.of(context).pushReplacementNamed("/home");
  }
}

After applying these fixes, your loading animation should stop once login succeeds, and the app will navigate to the HomeScreen without those exceptions.

内容的提问来源于stack exchange，提问作者Sarang Rastogi