compose: Prototype compose box, using Material TextField widget

lib/api/route/messages.dart

@@ -46,3 +46,56 @@ class GetMessagesResult {
 
   Map<String, dynamic> toJson() => _$GetMessagesResultToJson(this);
 }
+
+// https://zulip.com/api/send-message#parameter-topic
+const int kMaxTopicLength = 60;
+
+// https://zulip.com/api/send-message#parameter-content
+const int kMaxMessageLengthCodePoints = 10000;
+
+/// The topic servers understand to mean "there is no topic".
+///
+/// This should match
+///   https://github.com/zulip/zulip/blob/6.0/zerver/actions/message_edit.py#L940
+/// or similar logic at the latest `main`.
+// This is hardcoded in the server, and therefore untranslated; that's
+// zulip/zulip#3639.
+const String kNoTopicTopic = '(no topic)';
+
+/// https://zulip.com/api/send-message
+// TODO currently only handles stream messages; fix
+Future<SendMessageResult> sendMessage(
+  ApiConnection connection, {
+  required String content,
+  required String topic,
+}) async {
+  // assert() is less verbose but would have no effect in production, I think:
+  //   https://dart.dev/guides/language/language-tour#assert
+  if (Uri.parse(connection.auth.realmUrl).origin != 'https://chat.zulip.org') {
+    throw Exception('This binding can currently only be used on https://chat.zulip.org.');
+  }
+
+  final data = await connection.post('messages', {
+    'type': RawParameter('stream'), // TODO parametrize
+    'to': 7, // TODO parametrize; this is `#test here`
+    'topic': RawParameter(topic),
+    'content': RawParameter(content),
+  });
+  return SendMessageResult.fromJson(jsonDecode(data));
+}
+
+@JsonSerializable()
+class SendMessageResult {
+  final int id;
+  final String? deliver_at;
+
+  SendMessageResult({
+    required this.id,
+    this.deliver_at,
+  });
+
+  factory SendMessageResult.fromJson(Map<String, dynamic> json) =>
+      _$SendMessageResultFromJson(json);
+
+  Map<String, dynamic> toJson() => _$SendMessageResultToJson(this);
+}

lib/model/store.dart

@@ -9,6 +9,7 @@ import '../api/model/events.dart';
 import '../api/model/initial_snapshot.dart';
 import '../api/model/model.dart';
 import '../api/route/events.dart';
+import '../api/route/messages.dart';
 import '../credential_fixture.dart' as credentials;
 import 'message_list.dart';
 
@@ -106,6 +107,11 @@ class PerAccountStore extends ChangeNotifier {
     }
   }
 
+  Future<void> sendStreamMessage({required String topic, required String content}) {
+    // TODO implement outbox; see design at
+    //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M3881.20Sending.20outbox.20messages.20is.20fraught.20with.20issues/near/1405739
+    return sendMessage(connection, topic: topic, content: content);
+  }
 }
 
 /// A scaffolding hack for while prototyping.

lib/widgets/app.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 
+import 'compose_box.dart';
 import 'message_list.dart';
 import '../model/store.dart';
 
@@ -163,9 +164,6 @@ class MessageListPage extends StatelessWidget {
 
                       child: const Expanded(
                           child: MessageList())),
-                  const SizedBox(
-                      height: 80,
-                      child: Center(
-                          child: Text("(Compose box goes here.)")))]))));
+                  const StreamComposeBox()]))));
   }
 }