(optional) test: Add a test helper for inspecting logs.

This is inspired by `TestCase.assertLogs`, which is widely adapted
in the Zulip server tests.

This doesn't address the TODO comment in lib/log.dart, because the
feature is not really used for debugging.

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

Author: PIG208

lib/log.dart

@@ -1,9 +1,14 @@
+import 'package:flutter/foundation.dart';
 
 /// Whether [debugLog] should do anything.
 ///
 /// This has an effect only in a debug build.
 bool debugLogEnabled = false;
 
+/// Used for log inspection in tests.
+@visibleForTesting
+List<String>? debugLogHistory;
+
 /// Print a log message, if debug logging is enabled.
 ///
 /// In a debug build, if [debugLogEnabled] is true, this prints the given
@@ -26,6 +31,7 @@ bool debugLog(String message) {
     if (debugLogEnabled) {
       print(message); // ignore: avoid_print
     }
+    debugLogHistory?.add(message);
     return true;
   }());
   return true;

test/model/message_test.dart

@@ -12,6 +12,7 @@ import '../api/model/model_checks.dart';
 import '../api/model/submessage_checks.dart';
 import '../example_data.dart' as eg;
 import '../stdlib_checks.dart';
+import '../test_log.dart';
 import 'message_list_test.dart';
 import 'store_checks.dart';
 import 'test_store.dart';
@@ -700,12 +701,15 @@ void main() {
 
       test('ignore submessage event with malformed content', () async {
         final message = await preparePollMessage(question: 'Old question');
-        await store.handleEvent(eg.submessageEvent(message.id, eg.selfUser.userId,
-          content: {
-            'type': 'question',
-            // Invalid type for question
-            'question': 123,
-          }));
+        await checkLogs(() async {
+          await store.handleEvent(eg.submessageEvent(message.id, eg.selfUser.userId,
+            content: {
+              'type': 'question',
+              // Invalid type for question
+              'question': 123,
+            }));
+        })..length.equals(2)
+          ..last.contains('Malformed submessage event data for poll');
         checkNotifiedOnce();
         checkPoll(message).question.equals('Old question');
       });

test/test_log.dart

@@ -0,0 +1,15 @@
+import 'dart:async';
+
+import 'package:checks/checks.dart';
+import 'package:zulip/log.dart';
+
+Future<Subject<List<String>>> checkLogs(FutureOr<void> Function() callback) async {
+  assert(debugLogHistory == null);
+  debugLogHistory = [];
+  try {
+    await callback();
+    return check(debugLogHistory!);
+  } finally {
+    debugLogHistory = null;
+  }
+}