TW-858: Add ADR for how to detect unread message

docs/adr/0015-callback-for-when-messages-become-visible-or-scroll-out-of-view.md

@@ -0,0 +1,61 @@
+# 14. Add a callback for when messages become visible or scroll out of view
+
+Date: 2023-12-05
+
+## Status
+
+Accepted
+
+## Context
+
+- We have a lot of unread messages in the room.
+- When we open the room, we need to set messages is read.
+
+## Decision
+
+- We use `visibility_detector` to detect when messages become visible or scroll out of view.
+- We use `onVisibilityChanged` to listen callback when messages become visible or scroll out of
+  view.
+
+```
+VisibilityDetector(
+  key: Key(message.id),
+  onVisibilityChanged: (visibilityInfo) {
+    if (visibilityInfo.visibleFraction == 1.0) {
+      /// set message is read  
+      handleMessageVisibilityChanged(message, true);
+    }
+  },
+```
+
+```
+void handleMessageVisibilityChanged(Event event, bool visible) {
+    if (_isJumpingToLastReadEvent) return;
+    Logs().d(
+      'Chat::handleMessageVisibilityChanged() - isVisible $visible',
+    );
+    final isUnreadEvent = _unreadEvents.toList().firstWhereOrNull(
+          (event) => event?.eventId == event?.eventId,
+        );
+    final inputBarHasChange = inputFocus.hasFocus;
+    if (isUnreadEvent != null && inputBarHasChange) {
+      _setReadMarkerEvent(isUnreadEvent);
+    }
+    if (isUnreadEvent != null && visible && !_isJumpingToLastReadEvent) {
+      Logs().d(
+        'Chat::handleMessageVisibilityChanged() - Set read Event ${isUnreadEvent.eventId}',
+      );
+      _setReadMarkerEvent(isUnreadEvent);
+    }
+  }
+```
+
+## Consequences
+
+- Continuous listening will affect the app's performance
+- The processing mechanism is not really effective because the SDK only supports setting markers for
+  1 event
+
+## Docs
+
+https://pub.dev/packages/visibility_detector