api: Add get-message route (for single message)

Fixes: zulip#140

Author: chrisbobbe

lib/api/route/messages.dart

@@ -6,6 +6,35 @@ import '../model/narrow.dart';
 
 part 'messages.g.dart';
 
+/// https://zulip.com/api/get-message
+///
+/// This binding only supports feature levels 120+.
+// TODO(server-5) remove FL 120+ mention in doc, and the related `assert`
+Future<GetMessageResult> getMessage(ApiConnection connection, {
+  required int messageId,
+  bool? applyMarkdown,
+}) {
+  assert(connection.zulipFeatureLevel! >= 120);
+  return connection.get('getMessage', GetMessageResult.fromJson, 'messages/$messageId', {
+    if (applyMarkdown != null) 'apply_markdown': applyMarkdown,
+  });
+}
+
+@JsonSerializable(fieldRename: FieldRename.snake)
+class GetMessageResult {
+  // final String rawContent; // deprecated; ignore
+  final Message message;
+
+  GetMessageResult({
+    required this.message,
+  });
+
+  factory GetMessageResult.fromJson(Map<String, dynamic> json) =>
+    _$GetMessageResultFromJson(json);
+
+  Map<String, dynamic> toJson() => _$GetMessageResultToJson(this);
+}
+
 /// https://zulip.com/api/get-messages
 Future<GetMessagesResult> getMessages(ApiConnection connection, {
   required ApiNarrow narrow,

lib/api/route/messages.g.dart

@@ -13,6 +13,56 @@ import '../fake_api.dart';
 import 'route_checks.dart';
 
 void main() {
+  group('getMessage', () {
+    Future<GetMessageResult> checkGetMessage(
+      FakeApiConnection connection, {
+      required int messageId,
+      bool? applyMarkdown,
+      required Map<String, String> expected,
+    }) async {
+      final result = await getMessage(connection,
+        messageId: messageId,
+        applyMarkdown: applyMarkdown,
+      );
+      check(connection.lastRequest).isNotNull().isA<http.Request>()
+        ..method.equals('GET')
+        ..url.path.equals('/api/v1/messages/$messageId')
+        ..url.queryParameters.deepEquals(expected);
+      return result;
+    }
+
+    final fakeResult = GetMessageResult(message: eg.streamMessage());
+
+    test('applyMarkdown true', () {
+      return FakeApiConnection.with_((connection) async {
+        connection.prepare(json: fakeResult.toJson());
+        await checkGetMessage(connection,
+          messageId: 1,
+          applyMarkdown: true,
+          expected: {'apply_markdown': 'true'});
+      });
+    });
+
+    test('applyMarkdown false', () {
+      return FakeApiConnection.with_((connection) async {
+        connection.prepare(json: fakeResult.toJson());
+        await checkGetMessage(connection,
+          messageId: 1,
+          applyMarkdown: false,
+          expected: {'apply_markdown': 'false'});
+      });
+    });
+
+    test('Throws assertion error when FL <120', () {
+      return FakeApiConnection.with_(zulipFeatureLevel: 119, (connection) async {
+        connection.prepare(json: fakeResult.toJson());
+        check(() => getMessage(connection,
+          messageId: 1,
+        )).throws<AssertionError>();
+      });
+    });
+  });
+
   group('getMessages', () {
     Future<GetMessagesResult> checkGetMessages(
       FakeApiConnection connection, {