How to build a custom Email action handler in Flutter for Firebase Authentication

Question

I am very new to web development and i am building a website with Flutter.
I am aware of the Routing / Navigator system.
I would like to know how can i create such page in Flutter to handle Firebase email actions :

https://example.com/usermgmt?mode=resetPassword&oobCode=ABC123&apiKey=AIzaSy...&lang=fr

I am following this documentation.

It is stated here that:

Firebase adds several query parameters to your action handler URL when it generates user management emails.

However i don't understand if it's up to me to build the full url in my Flutter Routes or simply provide the endpoint /usermgmt and Firebase is automatically adding the params ?

I have tried to build a page in my website that addresses : https://example.com/usermgmt and it's working, the page exists hosted in my website.
Now if i add the Firebase params, it returns a 404 not found.

What should i do ?

EDIT : I have made the UI working, it doesn't return a 404 not found anymore.
I have done this for those interrested :

onGenerateRoute: (settings) {
          Widget? authPage;
          if (settings.name != null) {
            var uriData = Uri.parse(settings.name!);
            //uriData.path will be the path and uriData.queryParameters will hold query-params values

            print("Param in query : " + uriData.queryParameters.toString());
            switch (uriData.path) {
              case '/auth':
                authPage = AuthHandlerPage();
                break;
            }
          }
          if (authPage != null) {
            return MaterialPageRoute(
                builder: (BuildContext context) => authPage!);
          }
        },

I simply need to handle the links according to the params now.

Solution

To complete the email action, you need to fetch the oobCode from URL parameters and use applyActionCode method (checkActionCode can be used before applyActionCode to make sure the oobCode is valid, usually not required as applyActionCode will throw an error on getting an invalid code as well):

var actionCode = "" // Get from URL

try {
  await auth.checkActionCode(actionCode);
  await auth.applyActionCode(actionCode);

  // If successful, reload the user:
  auth.currentUser.reload();
} on FirebaseAuthException catch (e) {
  if (e.code == 'invalid-action-code') {
    print('The code is invalid.');
  }
}

If you are using dynamic links, you can get it this way:

//Get actionCode from the dynamicLink
final Uri deepLink = dynamicLink?.link;
var actionCode = deepLink.queryParameters['oobCode'];

You can refer the documentation for more information.

Alternatively, you can also use the Firebase REST API to verify the email links if you need to verify the oobCode from a Cloud function or server.