local echo (7/7): Support simplified version of local echo

assets/l10n/app_en.arb

@@ -801,6 +801,10 @@
   "@messageIsMovedLabel": {
     "description": "Label for a moved message. (Use ALL CAPS for cased alphabets: Latin, Greek, Cyrillic, etc.)"
   },
+  "messageIsntSentLabel": "MESSAGE ISN'T SENT. CHECK YOUR CONNECTION.",
+  "@messageIsntSentLabel": {
+    "description": "Label for a message that isn't sent. (Use ALL CAPS for cased alphabets: Latin, Greek, Cyrillic, etc.)"
+  },
   "pollVoterNames": "({voterNames})",
   "@pollVoterNames": {
     "description": "The list of people who voted for a poll option, wrapped in parentheses.",

lib/api/model/model.dart

@@ -532,6 +532,15 @@ String? tryParseEmojiCodeToUnicode(String emojiCode) {
   }
 }
 
+/// The topic servers understand to mean "there is no topic".
+///
+/// This should match
+///   https://github.com/zulip/zulip/blob/6.0/zerver/actions/message_edit.py#L940
+/// or similar logic at the latest `main`.
+// This is hardcoded in the server, and therefore untranslated; that's
+// zulip/zulip#3639.
+const String kNoTopicTopic = '(no topic)';
+
 /// The name of a Zulip topic.
 // TODO(dart): Can we forbid calling Object members on this extension type?
 //   (The lack of "implements Object" ought to do that, but doesn't.)
@@ -586,6 +595,30 @@ extension type const TopicName(String _value) {
   /// using [canonicalize].
   bool isSameAs(TopicName other) => canonicalize() == other.canonicalize();
 
+  /// Convert this topic to match how it would appear on a message object from
+  /// the server, assuming the topic is originally for a send-message request.
+  ///
+  /// For a client that does not support empty topics,
+  /// a modern server (FL>=334) would convert "(no topic)" and empty topics to
+  /// `store.realmEmptyTopicDisplayName`.
+  ///
+  /// See also: https://zulip.com/api/send-message#parameter-topic
+  TopicName interpretAsServer({
+    required int zulipFeatureLevel,
+    required String? realmEmptyTopicDisplayName,
+  }) {
+    if (zulipFeatureLevel < 334) {
+      assert(_value.isNotEmpty);
+      return this;
+    }
+    if (_value == kNoTopicTopic || _value.isEmpty) {
+      // TODO(#1250): this assumes that the 'support_empty_topics'
+      //   client_capability is false; update this when we set it to true
+      return TopicName(realmEmptyTopicDisplayName!);
+    }
+    return TopicName(_value);
+  }
+
   TopicName.fromJson(this._value);
 
   String toJson() => apiName;
@@ -596,7 +629,10 @@ extension type const TopicName(String _value) {
 /// Different from [MessageDestination], this information comes from
 /// [getMessages] or [getEvents], identifying the conversation that contains a
 /// message.
-sealed class Conversation {}
+sealed class Conversation {
+  /// Whether [this] and [other] refer to the same Zulip conversation.
+  bool isSameAs(Conversation other);
+}
 
 /// The conversation a stream message is in.
 @JsonSerializable(fieldRename: FieldRename.snake, createToJson: false)
@@ -622,6 +658,13 @@ class StreamConversation extends Conversation {
 
   factory StreamConversation.fromJson(Map<String, dynamic> json) =>
     _$StreamConversationFromJson(json);
+
+  @override
+  bool isSameAs(Conversation other) {
+    return other is StreamConversation
+      && streamId == other.streamId
+      && topic.isSameAs(other.topic);
+  }
 }
 
 /// The conversation a DM message is in.
@@ -635,6 +678,21 @@ class DmConversation extends Conversation {
 
   DmConversation({required this.allRecipientIds})
     : assert(isSortedWithoutDuplicates(allRecipientIds.toList()));
+
+  bool _equalIdSequences(Iterable<int> xs, Iterable<int> ys) {
+    if (xs.length != ys.length) return false;
+    final xs_ = xs.iterator; final ys_ = ys.iterator;
+    while (xs_.moveNext() && ys_.moveNext()) {
+      if (xs_.current != ys_.current) return false;
+    }
+    return true;
+  }
+
+  @override
+  bool isSameAs(Conversation other) {
+    if (other is! DmConversation) return false;
+    return _equalIdSequences(allRecipientIds, other.allRecipientIds);
+  }
 }
 
 /// A message or message-like object, for showing in a message list.

lib/api/route/messages.dart

@@ -169,15 +169,6 @@ const int kMaxTopicLengthCodePoints = 60;
 // https://zulip.com/api/send-message#parameter-content
 const int kMaxMessageLengthCodePoints = 10000;
 
-/// The topic servers understand to mean "there is no topic".
-///
-/// This should match
-///   https://github.com/zulip/zulip/blob/6.0/zerver/actions/message_edit.py#L940
-/// or similar logic at the latest `main`.
-// This is hardcoded in the server, and therefore untranslated; that's
-// zulip/zulip#3639.
-const String kNoTopicTopic = '(no topic)';
-
 /// https://zulip.com/api/send-message
 Future<SendMessageResult> sendMessage(
   ApiConnection connection, {

lib/generated/l10n/zulip_localizations.dart

@@ -1169,6 +1169,12 @@ abstract class ZulipLocalizations {
   /// **'MOVED'**
   String get messageIsMovedLabel;
 
+  /// Label for a message that isn't sent. (Use ALL CAPS for cased alphabets: Latin, Greek, Cyrillic, etc.)
+  ///
+  /// In en, this message translates to:
+  /// **'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.'**
+  String get messageIsntSentLabel;
+
   /// The list of people who voted for a poll option, wrapped in parentheses.
   ///
   /// In en, this message translates to:

lib/generated/l10n/zulip_localizations_ar.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsAr extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'MOVED';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/generated/l10n/zulip_localizations_en.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsEn extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'MOVED';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/generated/l10n/zulip_localizations_ja.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsJa extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'MOVED';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/generated/l10n/zulip_localizations_nb.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsNb extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'MOVED';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/generated/l10n/zulip_localizations_pl.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsPl extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'PRZENIESIONO';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/generated/l10n/zulip_localizations_ru.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsRu extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'ПЕРЕМЕЩЕНО';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/generated/l10n/zulip_localizations_sk.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsSk extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'PRESUNUTÉ';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/generated/l10n/zulip_localizations_uk.dart

@@ -625,6 +625,9 @@ class ZulipLocalizationsUk extends ZulipLocalizations {
   @override
   String get messageIsMovedLabel => 'ПЕРЕМІЩЕНО';
 
+  @override
+  String get messageIsntSentLabel => 'MESSAGE ISN\'T SENT. CHECK YOUR CONNECTION.';
+
   @override
   String pollVoterNames(String voterNames) {
     return '($voterNames)';

lib/model/binding.dart

@@ -113,6 +113,11 @@ abstract class ZulipBinding {
   /// This wraps [url_launcher.closeInAppWebView].
   Future<void> closeInAppWebView();
 
+  /// Provides access to the current UTC date and time.
+  ///
+  /// Outside tests, this just calls [DateTime.timestamp].
+  DateTime utcNow();
+
   /// Provides access to a new stopwatch.
   ///
   /// Outside tests, this just calls the [Stopwatch] constructor.
@@ -365,6 +370,9 @@ class LiveZulipBinding extends ZulipBinding {
     return url_launcher.closeInAppWebView();
   }
 
+  @override
+  DateTime utcNow() => DateTime.timestamp();
+
   @override
   Stopwatch stopwatch() => Stopwatch();