message [nfc]: Pull out some helpers for reconcileMessages; add TODO(zulip#1798)

Author: chrisbobbe

lib/model/message.dart

@@ -400,9 +400,23 @@ class MessageStoreImpl extends HasChannelStore with MessageStore, _OutboxMessage
 
   void reconcileMessages(List<Message> messages) {
     assert(!_disposed);
-    // What to do when some of the just-fetched messages are already known?
-    // This is common and normal: in particular it happens when one message list
-    // overlaps another, e.g. a stream and a topic within it.
+    for (int i = 0; i < messages.length; i++) {
+      final message = messages[i];
+
+      messages[i] = this.messages.update(message.id,
+        ifAbsent: () => _reconcileUnrecognizedMessage(message),
+        (current) => _reconcileRecognizedMessage(current, message));
+    }
+  }
+
+  Message _reconcileUnrecognizedMessage(Message incoming) {
+    return _stripMatchFields(incoming);
+  }
+
+  Message _reconcileRecognizedMessage(Message current, Message incoming) {
+    // This message is one we already know about. This is common and normal:
+    // in particular it happens when one message list overlaps another,
+    // e.g. a stream and a topic within it.
     //
     // Most often, the just-fetched message will look just like the one we
     // already have.  But they can differ: message fetching happens out of band
@@ -415,12 +429,8 @@ class MessageStoreImpl extends HasChannelStore with MessageStore, _OutboxMessage
     // we won't hear those events again; the only way to wind up with an
     // updated message is to use the version we have, that already reflects
     // those events' changes.  So we always stick with the version we have.
-    for (int i = 0; i < messages.length; i++) {
-      final message = messages[i];
-      messages[i] = this.messages.putIfAbsent(message.id, () {
-        return _stripMatchFields(message);
-      });
-    }
+    // TODO(#1798) consider unsubscribed channels
+    return current;
   }
 
   Message _stripMatchFields(Message message) {