message: Identify moves by inequality new vs orig, not just non-nullity

This is NFC except in certain malformed cases: where either
newStreamId is present and equals origStreamId, or newTopic is
present and equals origTopic.  The API docs say neither of those
should happen.

This version is useful because it brings this code closer to how it
will look when we move this logic of "were the messages moved, and
how" into the API parsing code so that other parts of the data model
can reuse the results.

Author: gnprice

lib/model/message.dart

@@ -173,16 +173,14 @@ class MessageStoreImpl with MessageStore {
     // For reference, see: https://zulip.com/api/get-events#update_message
 
     final origStreamId = event.origStreamId;
-    final newStreamId = event.newStreamId; // null if topic-only move
     final origTopic = event.origTopic;
-    final newTopic = event.newTopic;
     final propagateMode = event.propagateMode;
 
     if (origTopic == null) {
       // There was no move.
       assert(() {
-        if (newStreamId != null && origStreamId != null
-            && newStreamId != origStreamId) {
+        if (event.newStreamId != null && origStreamId != null
+            && event.newStreamId != origStreamId) {
           // This should be impossible; `orig_subject` (aka origTopic) is
           // documented to be present when either the stream or topic changed.
           debugLog('Malformed UpdateMessageEvent: stream move but no origTopic'); // TODO(log)
@@ -192,12 +190,6 @@ class MessageStoreImpl with MessageStore {
       return;
     }
 
-    if (newStreamId == null && newTopic == null) {
-      // If neither the channel nor topic name changed, nothing moved.
-      // In that case `orig_subject` (aka origTopic) should have been null.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no newStreamId or newTopic')); // TODO(log)
-      return;
-    }
     if (origStreamId == null) {
       // The `stream_id` field (aka origStreamId) is documented to be present on moves.
       assert(debugLog('Malformed UpdateMessageEvent: move but no origStreamId')); // TODO(log)
@@ -209,8 +201,17 @@ class MessageStoreImpl with MessageStore {
       return;
     }
 
-    final wasResolveOrUnresolve = (newStreamId == null
-      && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic!));
+    final newStreamId = event.newStreamId ?? origStreamId;
+    final newTopic = event.newTopic ?? origTopic;
+    if (newStreamId == origStreamId && newTopic == origTopic) {
+      // If neither the channel nor topic name changed, nothing moved.
+      // In that case `orig_subject` (aka origTopic) should have been null.
+      assert(debugLog('Malformed UpdateMessageEvent: move but no newStreamId or newTopic')); // TODO(log)
+      return;
+    }
+
+    final wasResolveOrUnresolve = newStreamId == origStreamId
+      && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic);
 
     for (final messageId in event.messageIds) {
       final message = messages[messageId];
@@ -221,14 +222,14 @@ class MessageStoreImpl with MessageStore {
         continue;
       }
 
-      if (newStreamId != null) {
+      if (newStreamId != origStreamId) {
         message.streamId = newStreamId;
         // See [StreamMessage.displayRecipient] on why the invalidation is
         // needed.
         message.displayRecipient = null;
       }
 
-      if (newTopic != null) {
+      if (newTopic != origTopic) {
         message.topic = newTopic;
       }
 
@@ -241,9 +242,9 @@ class MessageStoreImpl with MessageStore {
     for (final view in _messageListViews) {
       view.messagesMoved(
         origStreamId: origStreamId,
-        newStreamId: newStreamId ?? origStreamId,
+        newStreamId: newStreamId,
         origTopic: origTopic,
-        newTopic: newTopic ?? origTopic,
+        newTopic: newTopic,
         messageIds: event.messageIds,
         propagateMode: propagateMode,
       );