api: Simplify move data parsing

This drops the separate if-statements in favor of null-checks and
tries to rely on the natural semantics of the API.

This drops an expcetion that would have been thrown when propagate mode
is not present but the topic name/stream id changes.

See Greg's comment on this:
  zulip#1311 (comment)

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/model/events.dart

@@ -717,7 +717,7 @@ class UpdateMessageMoveData {
     required this.propagateMode,
     required this.origTopic,
     required this.newTopic,
-  });
+  }) : assert(origStreamId != newStreamId || origTopic != newTopic);
 
   /// Try to extract [UpdateMessageMoveData] from the JSON object for an
   /// [UpdateMessageEvent].
@@ -730,50 +730,33 @@ class UpdateMessageMoveData {
   // This may matter if we ever need 'stream_id' when no message move occurred.
   static UpdateMessageMoveData? tryParseFromJson(Object? json) {
     json as Map<String, Object?>;
-    final origStreamId = (json['stream_id'] as num?)?.toInt();
-    final newStreamId = (json['new_stream_id'] as num?)?.toInt();
     final propagateModeString = json['propagate_mode'] as String?;
     final propagateMode = propagateModeString == null ? null
       : PropagateMode.fromRawString(propagateModeString);
-    final origTopic = json['orig_subject'] == null ? null
-      : TopicName.fromJson(json['orig_subject'] as String);
-    final newTopic = json['subject'] == null ? null
-      : TopicName.fromJson(json['subject'] as String);
 
-    if (origTopic == null) {
+    if (propagateMode == null) {
       // There was no move.
-      assert(() {
-        if (newStreamId != null && origStreamId != null
-            && newStreamId != origStreamId) {
-          // This should be impossible; `orig_subject` (aka origTopic) is
-          // documented to be present when either the stream or topic changed.
-          throw FormatException('stream move but no origTopic');
-        }
-        return true;
-      }());
       return null;
     }
 
-    if (newStreamId == null && newTopic == null) {
+    final origStreamId = (json['stream_id'] as num).toInt();
+    final newStreamId = (json['new_stream_id'] as num?)?.toInt() ?? origStreamId;
+    final origTopic = TopicName.fromJson(json['orig_subject'] as String);
+    final newTopic = json['subject'] == null ? origTopic
+      : TopicName.fromJson(json['subject'] as String);
+
+    if (origStreamId == newStreamId && origTopic == newTopic) {
       // If neither the channel nor topic name changed, nothing moved.
-      // In that case `orig_subject` (aka origTopic) should have been null.
-      throw FormatException('move but no newStreamId or newTopic');
-    }
-    if (origStreamId == null) {
-      // The `stream_id` field (aka origStreamId) is documented to be present on moves.
-      throw FormatException('move but no origStreamId');
-    }
-    if (propagateMode == null) {
-      // The `propagate_mode` field (aka propagateMode) is documented to be present on moves.
-      throw FormatException('move but no propagateMode');
+      // In that case `propagate_mode` (aka propagateMode) should have been null.
+      throw FormatException('move but unchanged newStreamId and newTopic');
     }
 
     return UpdateMessageMoveData(
       origStreamId: origStreamId,
-      newStreamId: newStreamId ?? origStreamId,
+      newStreamId: newStreamId,
       propagateMode: propagateMode,
       origTopic: origTopic,
-      newTopic: newTopic ?? origTopic,
+      newTopic: newTopic,
     );
   }
 

test/api/model/events_test.dart

@@ -161,7 +161,7 @@ void main() {
         'orig_subject': null,
         'subject': null,
         'propagate_mode': 'change_all',
-      })).throws<FormatException>();
+      })).throws<void>();
     });
 
     test('move but no subject or new_stream_id', () {
@@ -181,17 +181,7 @@ void main() {
         'orig_subject': 'foo',
         'subject': 'bar',
         'propagate_mode': 'change_all',
-      })).throws<FormatException>();
-    });
-
-    test('move but no propagate_mode', () {
-      check(() => Event.fromJson({...baseJson,
-        'stream_id': 1,
-        'new_stream_id': 2,
-        'orig_subject': 'foo',
-        'subject': 'bar',
-        'propagate_mode': null,
-      })).throws<FormatException>();
+      })).throws<void>();
     });
   });