Support for pinned messages (#49)

The first draft to support pinned messages. Fixes #39.

Co-authored-by: Henry-Hiles <henry@henryhiles.com>
Reviewed-on: Nexus/nexus#49

lib/controllers/pinned_events_controller.dart

@@ -0,0 +1,30 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:hooks_riverpod/hooks_riverpod.dart";
+import "package:nexus/controllers/event_controller.dart";
+import "package:nexus/controllers/pinned_ids_controller.dart";
+import "package:nexus/models/event.dart";
+
+class PinnedEventsController extends AsyncNotifier<IList<Event>> {
+  final String roomId;
+  PinnedEventsController(this.roomId);
+
+  @override
+  Future<IList<Event>> build() async {
+    final pinIds = ref.watch(PinnedIdsController.provider(roomId));
+
+    return (await Future.wait(
+      pinIds.map(
+        (eventId) => ref.watch(
+          EventController.provider(
+            .new(eventId: eventId, roomId: roomId),
+          ).future,
+        ),
+      ),
+    )).nonNulls.toIList();
+  }
+
+  static final provider = AsyncNotifierProvider.family
+      .autoDispose<PinnedEventsController, IList<Event>, String>(
+        PinnedEventsController.new,
+      );
+}