feat: DeviceListModifiedEvent now contains a delta

lib/src/helpers.dart

@@ -59,3 +59,27 @@ OmemoPublicKey? decodeKeyIfNotNull(
 int getTimestamp() {
   return DateTime.now().millisecondsSinceEpoch;
 }
+
+/// Describes the differences between two lists in terms of its items.
+class ListDiff<T> {
+  ListDiff(this.added, this.removed);
+
+  /// The items that were added.
+  final List<T> added;
+
+  /// The items that were removed.
+  final List<T> removed;
+}
+
+extension BeforeAfterListDiff<T> on List<T> {
+  /// Compute the set-based changes between this list and [newList].
+  ListDiff<T> diff(List<T> newList) {
+    final oldSet = Set<T>.from(this);
+    final newSet = Set<T>.from(newList);
+
+    return ListDiff(
+      newSet.difference(oldSet).toList(),
+      oldSet.difference(newSet).toList(),
+    );
+  }
+}

lib/src/omemo/events.dart

@@ -39,13 +39,16 @@ class RatchetRemovedEvent extends OmemoEvent {
 
 /// Triggered when the device map has been modified
 class DeviceListModifiedEvent extends OmemoEvent {
-  DeviceListModifiedEvent(this.jid, this.devices);
+  DeviceListModifiedEvent(this.jid, this.added, this.removed);
 
   /// The JID of the user.
   final String jid;
 
-  /// The list of devices for [jid].
-  final List<int> devices;
+  /// The list of added devices for [jid].
+  final List<int> added;
+
+  /// The list of removed devices for [jid].
+  final List<int> removed;
 }
 
 /// Triggered by the OmemoSessionManager when our own device bundle was modified

lib/src/omemo/omemo.dart

@@ -154,7 +154,7 @@ class OmemoManager {
         _deviceListRequested[jid] = true;
 
         _eventStreamController.add(
-          DeviceListModifiedEvent(jid, newDeviceList),
+          DeviceListModifiedEvent(jid, newDeviceList, []),
         );
       }
     }
@@ -709,9 +709,10 @@ class OmemoManager {
         }
 
         // Clear the device list
+        _eventStreamController
+            .add(DeviceListModifiedEvent(jid, [], _deviceList[jid]!));
         _deviceList.remove(jid);
         _deviceListRequested.remove(jid);
-        _eventStreamController.add(DeviceListModifiedEvent(jid, []));
       },
     );
   }
@@ -719,13 +720,26 @@ class OmemoManager {
   /// To be called when a update to the device list of [jid] is returned.
   /// [devices] is the list of device identifiers contained in the update.
   Future<void> onDeviceListUpdate(String jid, List<int> devices) async {
-    // Update our state
-    _deviceList[jid] = devices;
-    _deviceListRequested[jid] = true;
+    await _ratchetQueue.synchronized(
+      [jid],
+      () async {
+        // Compute the delta
+        ListDiff<int> delta;
+        if (_deviceList.containsKey(jid)) {
+          delta = _deviceList[jid]!.diff(devices);
+        } else {
+          delta = ListDiff(devices, []);
+        }
 
-    // Commit the device list
-    _eventStreamController.add(
-      DeviceListModifiedEvent(jid, devices),
+        // Update our state
+        _deviceList[jid] = devices;
+        _deviceListRequested[jid] = true;
+
+        // Commit the device list
+        _eventStreamController.add(
+          DeviceListModifiedEvent(jid, delta.added, delta.removed),
+        );
+      },
     );
   }