feat: support creating challenge links

Closes #2015
Closes #856

Author: tom-anders

lib/src/model/challenge/challenge.dart

@@ -111,7 +111,8 @@ sealed class ChallengeRequest with _$ChallengeRequest, BaseChallenge implements
   const ChallengeRequest._();
 
   const factory ChallengeRequest({
-    required LightUser destUser,
+    // If null, it's an open challenge that anyone (even anonymous users) can accept.
+    LightUser? destUser,
     required Variant variant,
     required ChallengeTimeControlType timeControl,
     ({Duration time, Duration increment})? clock,

lib/src/model/challenge/challenge_preferences.dart

@@ -90,7 +90,7 @@ sealed class ChallengePrefs with _$ChallengePrefs implements Serializable {
       ? variant != Variant.fromPosition
       : timeControl == ChallengeTimeControlType.correspondence && variant == Variant.standard;
 
-  ChallengeRequest makeRequest(LightUser destUser, [String? initialFen]) {
+  ChallengeRequest makeRequest(LightUser? destUser, [String? initialFen]) {
     return ChallengeRequest(
       destUser: destUser,
       variant: variant,

lib/src/model/challenge/challenge_repository.dart

@@ -1,9 +1,12 @@
 import 'dart:async';
+import 'dart:math';
 
+import 'package:dartchess/dartchess.dart';
 import 'package:deep_pick/deep_pick.dart';
 import 'package:fast_immutable_collections/fast_immutable_collections.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:lichess_mobile/src/model/challenge/challenge.dart';
+import 'package:lichess_mobile/src/model/common/game.dart';
 import 'package:lichess_mobile/src/model/common/id.dart';
 import 'package:lichess_mobile/src/network/aggregator.dart';
 import 'package:lichess_mobile/src/network/http.dart';
@@ -40,17 +43,33 @@ class ChallengeRepository {
     return client.readJson(uri, mapper: Challenge.fromServerJson);
   }
 
-  Future<Challenge> create(ChallengeRequest challenge) {
-    final uri = Uri(path: '/api/challenge/${challenge.destUser.id}');
-    return client.postReadJson(
+  Future<Challenge> create(ChallengeRequest challengeReq) async {
+    final uri = Uri(path: '/api/challenge/${challengeReq.destUser?.id ?? 'open'}');
+    final challenge = await client.postReadJson(
       uri,
-      body: challenge.toRequestBody,
+      body: challengeReq.toRequestBody,
       mapper: Challenge.fromServerJson,
     );
+
+    // The API doesn't directly allow us to create an open challenge and also join it in one step,
+    // so after having created the challenge, we immediately accept it if it's an open challenge.
+    if (challengeReq.destUser == null) {
+      final side = switch (challengeReq.sideChoice) {
+        SideChoice.white => Side.white,
+        SideChoice.black => Side.black,
+        SideChoice.random => Side.values[Random().nextInt(Side.values.length)],
+      };
+      await accept(challenge.id, side: side);
+    }
+
+    return challenge;
   }
 
-  Future<void> accept(ChallengeId id) async {
-    final uri = Uri(path: '/api/challenge/$id/accept');
+  Future<void> accept(ChallengeId id, {Side? side}) async {
+    final uri = Uri(
+      path: '/api/challenge/$id/accept',
+      queryParameters: {if (side != null) 'color': side.name},
+    );
     await client.postRead(uri);
   }
 

lib/src/model/lobby/create_game_service.dart

@@ -119,23 +119,29 @@ class CreateGameService {
   }
 
   /// Create a new real time challenge.
-  ///
-  /// Will listen to the challenge socket and await the response from the destinated user.
-  Future<ChallengeResponse> newRealTimeChallenge(ChallengeRequest challengeReq) async {
+  Future<Challenge> newRealTimeChallenge(ChallengeRequest challengeReq) async {
     assert(challengeReq.timeControl == ChallengeTimeControlType.clock);
 
     if (_challengeConnection != null) {
       throw StateError('Already creating a challenge.');
     }
 
+    return await challengeRepository.create(challengeReq);
+  }
+
+  /// Wait for a response to a [challenge].
+  ///
+  /// Will listen to the challenge socket and await the response from the destinated user.
+  Future<ChallengeResponse> waitForChallengeResponse(Challenge challenge) {
+    assert(challenge.timeControl == ChallengeTimeControlType.clock);
+    if (_challengeConnection != null) {
+      throw StateError('Already creating a challenge.');
+    }
+
     // ensure the pending connection is closed in any case
     final completer = Completer<ChallengeResponse>()..future.whenComplete(dispose);
 
     try {
-      _log.info('Creating new challenge game');
-
-      final challenge = await challengeRepository.create(challengeReq);
-
       final socketPool = ref.read(socketPoolProvider);
       final socketClient = socketPool.open(
         Uri(

lib/src/view/board_editor/board_editor_screen.dart

@@ -347,7 +347,10 @@ class _BottomBar extends ConsumerWidget {
                             isScrollControlled: true,
                             useRootNavigator: true,
                             builder: (context) {
-                              return CreateChallengeBottomSheet(user, positionFen: editorState.fen);
+                              return CreateChallengeBottomSheet(
+                                user: user,
+                                positionFen: editorState.fen,
+                              );
                             },
                           );
                         },