Implement OAuth2 flow (#88)

* Move users router in sub-module

* Factorize UserRouter into EventHandlersRouter

* Implement OAuth registration/login router

* Apply isort/black

* Remove temporary pytest marker

* Fix httpx-oauth version in lock file

* Ensure ON_AFTER_REGISTER event is triggered on OAuth registration

* Add API on FastAPIUsers to generate an OAuth router

* Improve test coverage of FastAPIUsers

* Small fixes

* Write the OAuth documentation

* Fix SQL unit-tests by avoiding collisions in SQLite db files

fastapi_users/fastapi_users.py

@@ -1,9 +1,17 @@
-from typing import Callable, Sequence, Type
+from collections import defaultdict
+from typing import Callable, DefaultDict, List, Sequence, Type
+
+from httpx_oauth.oauth2 import BaseOAuth2
 
 from fastapi_users import models
 from fastapi_users.authentication import Authenticator, BaseAuthentication
 from fastapi_users.db import BaseUserDatabase
-from fastapi_users.router import Event, UserRouter, get_user_router
+from fastapi_users.router import (
+    Event,
+    EventHandlersRouter,
+    get_oauth_router,
+    get_user_router,
+)
 
 
 class FastAPIUsers:
@@ -20,12 +28,16 @@ class FastAPIUsers:
     :param reset_password_token_lifetime_seconds: Lifetime of reset password token.
 
     :attribute router: Router exposing authentication routes.
+    :attribute oauth_routers: List of OAuth routers created through `get_oauth_router`.
     :attribute get_current_user: Dependency callable to inject authenticated user.
     """
 
     db: BaseUserDatabase
     authenticator: Authenticator
-    router: UserRouter
+    router: EventHandlersRouter
+    oauth_routers: List[EventHandlersRouter]
+    _user_db_model: Type[models.BaseUserDB]
+    _event_handlers: DefaultDict[Event, List[Callable]]
 
     def __init__(
         self,
@@ -50,6 +62,9 @@ class FastAPIUsers:
             reset_password_token_secret,
             reset_password_token_lifetime_seconds,
         )
+        self.oauth_routers = []
+        self._user_db_model = user_db_model
+        self._event_handlers = defaultdict(list)
 
         self.get_current_user = self.authenticator.get_current_user
         self.get_current_active_user = self.authenticator.get_current_active_user
@@ -63,9 +78,40 @@ class FastAPIUsers:
         """Add an event handler on successful forgot password request."""
         return self._on_event(Event.ON_AFTER_FORGOT_PASSWORD)
 
+    def get_oauth_router(
+        self, oauth_client: BaseOAuth2, state_secret: str, redirect_url: str = None
+    ) -> EventHandlersRouter:
+        """
+        Return an OAuth router for a given OAuth client.
+
+        :param oauth_client: The HTTPX OAuth client instance.
+        :param state_secret: Secret used to encode the state JWT.
+        :param redirect_url: Optional arbitrary redirect URL for the OAuth2 flow.
+        If not given, the URL to the callback endpoint will be generated.
+        """
+        oauth_router = get_oauth_router(
+            oauth_client,
+            self.db,
+            self._user_db_model,
+            self.authenticator,
+            state_secret,
+            redirect_url,
+        )
+
+        for event_type in self._event_handlers:
+            for handler in self._event_handlers[event_type]:
+                oauth_router.add_event_handler(event_type, handler)
+
+        self.oauth_routers.append(oauth_router)
+
+        return oauth_router
+
     def _on_event(self, event_type: Event) -> Callable:
         def decorator(func: Callable) -> Callable:
+            self._event_handlers[event_type].append(func)
             self.router.add_event_handler(event_type, func)
+            for oauth_router in self.oauth_routers:
+                oauth_router.add_event_handler(event_type, func)
             return func
 
         return decorator