What is right way to pass error to widget - flutter

Question

What is the right way to pass exception/failure from services or other parts to your widget.
I would like to pass error code from register to onSignIn. Whats the right way. Is it ok to do it the way I am doing or should I
again throw exception from register and catch it in onSignIn.
don't catch in register but catch in onSignIn
File A
void onSignIn() async {
dynamic result = await _auth.register();
if (result.runtimeType == String) {
print(result);
}
}
File B
Future register() async {
try {
AuthResult result = await _auth.createUser();
return _user(result.user);
} catch (e) {
return e.code;
}
}

Answer 1

For error handling I usually use a wrapper object to help me out better handling, errors or even "loading". This is a patter from one of Google's Android examples and I've used on flutter with success. So basically you create a generic class, in my case I call it "Response" that has the structure of:
class Response<T> {
final ApiError error;
final T data;
final ResponseType type;
Response({
this.error,
this.data,
this.type,
});
bool get isOk {
return type == ResponseType.ok;
}
bool get isLoading {
return type == ResponseType.loading;
}
bool get hasError {
return type == ResponseType.error;
}
factory Response.loading() {
return Response<T>(type: ResponseType.loading);
}
factory Response.ok(T data) {
return Response<T>(type: ResponseType.ok, data: data);
}
factory Response.empty() {
return Response<T>.ok(null);
}
factory Response.error(ApiError data) {
return Response<T>(type: ResponseType.error, error: data);
}
}
enum ResponseType { loading, error, ok }
ApiError is my custom implementation that handles errors for a specific project, it can be replaced with a generic Error.
This approach is super helpful when you're using Streams, because it won't close a stream in case of error, you have the possibility to mark the beginning of the task by replying with a loading and it will handle succes/error properly.
This approach would also allow you to send a more well defined error to the UI, let's say you need the error code, error message, and maybe you get an input field name where you want to show that error on.
Altho, in your case it might be a bit of overkill, I would still use it like this:
Future<Response<AuthResult>> register() async {
try {
AuthResult result = await _auth.createUser();
return Response.ok(_user(result.user));
} catch (e) {
return Response.error(e);
}
}
void onSignIn() async {
Response<AuthResult> result = await _auth.register();
if (result.isOk) {
//TODO: all good let's use data with result.data
}
else if (result.isLoading) {
// TOOD: well...show a loading indicator
}
else {
//TODO: we got error handle it using result.error
}
}

Answer 2

In flutters' documentation, they almost always caught the error in the parent function(take a look at here as an example). Also using dynamic may be dangerous since it accepts all kinds of objects, a better approach would be using the final keyword. so the preferred way would be:
Future<AuthResult> register() async {
AuthResult result = await _auth.createUser();
return _user(result.user);
}
and then:
void onSignIn() async {
try{
final result = await _auth.register();
print(result);
} on Exception catch(ex){
// do something with error
}
}

Answer 3

I usually like to use Provider for such kind of things.
class Auth with ChangeNotifier{
AuthResult _authResult;
AuthResult get authResult => _authResult;
}
enum AuthResult{
Successful,
Error,
OTPError,
}
Then, I will use the provider package to get the data wherever needed.