api [nfc]: Factor out logic that supports serializing ApiNarrowDm

Also pull out narrow.toJson tests.

Author: sirpengi

lib/api/model/narrow.dart

@@ -1,5 +1,18 @@
 typedef ApiNarrow = List<ApiNarrowElement>;
 
+/// Resolve any [ApiNarrowDm] elements appropriately.
+///
+/// This encapsulates a server-feature check.
+ApiNarrow resolveDmElements(ApiNarrow narrow, int zulipFeatureLevel) {
+  final supportsOperatorDm = zulipFeatureLevel >= 177; // TODO(server-7)
+  return (narrow.any((element) => element is ApiNarrowDm))
+    ? narrow.map((element) => switch (element) {
+        ApiNarrowDm() => element.resolve(legacy: !supportsOperatorDm),
+        _             => element,
+      }).toList()
+    : narrow;
+}
+
 /// An element in the list representing a narrow in the Zulip API.
 ///
 /// Docs: <https://zulip.com/api/construct-narrow>
@@ -51,6 +64,8 @@ class ApiNarrowTopic extends ApiNarrowElement {
 /// An instance directly of this class must not be serialized with [jsonEncode],
 /// and more generally its [operator] getter must not be called.
 /// Instead, call [resolve] and use the object it returns.
+///
+/// If part of [ApiNarrow] use [resolveDmElements].
 class ApiNarrowDm extends ApiNarrowElement {
   @override String get operator {
     assert(false,

lib/api/route/messages.dart

@@ -90,15 +90,8 @@ Future<GetMessagesResult> getMessages(ApiConnection connection, {
   bool? applyMarkdown,
   // bool? useFirstUnreadAnchor // omitted because deprecated
 }) {
-  if (narrow.any((element) => element is ApiNarrowDm)) {
-    final supportsOperatorDm = connection.zulipFeatureLevel! >= 177; // TODO(server-7)
-    narrow = narrow.map((element) => switch (element) {
-      ApiNarrowDm() => element.resolve(legacy: !supportsOperatorDm),
-      _             => element,
-    }).toList();
-  }
   return connection.get('getMessages', GetMessagesResult.fromJson, 'messages', {
-    'narrow': narrow,
+    'narrow': resolveDmElements(narrow, connection.zulipFeatureLevel!),
     'anchor': switch (anchor) {
       NumericAnchor(:var messageId) => messageId,
       AnchorCode.newest             => RawParameter('newest'),

test/api/route/messages_test.dart

@@ -172,6 +172,34 @@ void main() {
     });
   });
 
+  test('Narrow.toJson', () {
+    return FakeApiConnection.with_((connection) async {
+      void checkNarrow(ApiNarrow narrow, String expected) {
+        narrow = resolveDmElements(narrow, connection.zulipFeatureLevel!);
+        check(jsonEncode(narrow)).equals(expected);
+      }
+
+      checkNarrow(const AllMessagesNarrow().apiEncode(), jsonEncode([]));
+      checkNarrow(const StreamNarrow(12).apiEncode(), jsonEncode([
+        {'operator': 'stream', 'operand': 12},
+      ]));
+      checkNarrow(const TopicNarrow(12, 'stuff').apiEncode(), jsonEncode([
+        {'operator': 'stream', 'operand': 12},
+        {'operator': 'topic', 'operand': 'stuff'},
+      ]));
+
+      checkNarrow([ApiNarrowDm([123, 234])], jsonEncode([
+        {'operator': 'dm', 'operand': [123, 234]},
+      ]));
+
+      connection.zulipFeatureLevel = 176;
+      checkNarrow([ApiNarrowDm([123, 234])], jsonEncode([
+        {'operator': 'pm-with', 'operand': [123, 234]},
+      ]));
+      connection.zulipFeatureLevel = eg.futureZulipFeatureLevel;
+    });
+  });
+
   group('getMessages', () {
     Future<GetMessagesResult> checkGetMessages(
       FakeApiConnection connection, {
@@ -215,38 +243,20 @@ void main() {
       });
     });
 
-    test('narrow', () {
-      return FakeApiConnection.with_((connection) async {
-        Future<void> checkNarrow(ApiNarrow narrow, String expected) async {
-          connection.prepare(json: fakeResult.toJson());
-          await checkGetMessages(connection,
-            narrow: narrow,
-            anchor: AnchorCode.newest, numBefore: 10, numAfter: 20,
-            expected: {
-              'narrow': expected,
-              'anchor': 'newest',
-              'num_before': '10',
-              'num_after': '20',
-            });
-        }
-
-        await checkNarrow(const AllMessagesNarrow().apiEncode(), jsonEncode([]));
-        await checkNarrow(const StreamNarrow(12).apiEncode(), jsonEncode([
-          {'operator': 'stream', 'operand': 12},
-        ]));
-        await checkNarrow(const TopicNarrow(12, 'stuff').apiEncode(), jsonEncode([
-          {'operator': 'stream', 'operand': 12},
-          {'operator': 'topic', 'operand': 'stuff'},
-        ]));
-
-        await checkNarrow([ApiNarrowDm([123, 234])], jsonEncode([
-          {'operator': 'dm', 'operand': [123, 234]},
-        ]));
-        connection.zulipFeatureLevel = 176;
-        await checkNarrow([ApiNarrowDm([123, 234])], jsonEncode([
-          {'operator': 'pm-with', 'operand': [123, 234]},
-        ]));
-        connection.zulipFeatureLevel = eg.futureZulipFeatureLevel;
+    test('narrow uses resolveLegacyElements to encode', () {
+      return FakeApiConnection.with_(zulipFeatureLevel: 176, (connection) async {
+        connection.prepare(json: fakeResult.toJson());
+        await checkGetMessages(connection,
+          narrow: [ApiNarrowDm([123, 234])],
+          anchor: AnchorCode.newest, numBefore: 10, numAfter: 20,
+          expected: {
+            'narrow': jsonEncode([
+              {'operator': 'pm-with', 'operand': [123, 234]},
+            ]),
+            'anchor': 'newest',
+            'num_before': '10',
+            'num_after': '20',
+          });
       });
     });