narrow: Implement static SendableNarrow.ofMessage

Author: chrisbobbe

lib/model/narrow.dart

@@ -19,6 +19,18 @@ sealed class Narrow {
 /// A non-interleaved narrow, completely specifying a place to send a message.
 sealed class SendableNarrow extends Narrow {
   MessageDestination get destination;
+
+  static SendableNarrow ofMessage(Message message, {required int selfUserId}) {
+    switch (message) {
+      case StreamMessage(:var streamId, :var subject):
+        return TopicNarrow(streamId, subject);
+      case DmMessage(:var displayRecipient):
+        return DmNarrow(
+          allRecipientIds: displayRecipient.map((r) => r.id).toList(),
+          selfUserId: selfUserId,
+        );
+    }
+  }
 }
 
 /// The narrow called "All messages" in the UI.

test/model/narrow_test.dart

@@ -8,6 +8,39 @@ import '../example_data.dart' as eg;
 import 'narrow_checks.dart';
 
 void main() {
+  group('SendableNarrow', () {
+    test('stream message', () {
+      final stream = eg.stream();
+      final message = eg.streamMessage(inStream: stream);
+      final actual = SendableNarrow.ofMessage(message, selfUserId: eg.selfUser.userId);
+      check(actual).equals(TopicNarrow(stream.streamId, message.subject));
+    });
+
+    test('dm message: self-dm', () {
+      final message = eg.dmMessage(from: eg.selfUser, to: []);
+      final actual = SendableNarrow.ofMessage(message, selfUserId: eg.selfUser.userId);
+      check(actual).equals(DmNarrow(
+        allRecipientIds: [eg.selfUser.userId],
+        selfUserId: eg.selfUser.userId));
+    });
+
+    test('dm message: 1:1', () {
+      final message = eg.dmMessage(from: eg.selfUser, to: [eg.otherUser]);
+      final actual = SendableNarrow.ofMessage(message, selfUserId: eg.selfUser.userId);
+      check(actual).equals(DmNarrow(
+        allRecipientIds: [eg.selfUser.userId, eg.otherUser.userId],
+        selfUserId: eg.selfUser.userId));
+    });
+
+    test('dm message: group', () {
+      final message = eg.dmMessage(from: eg.selfUser, to: [eg.otherUser, eg.thirdUser]);
+      final actual = SendableNarrow.ofMessage(message, selfUserId: eg.selfUser.userId);
+      check(actual).equals(DmNarrow(
+        allRecipientIds: [eg.selfUser.userId, eg.otherUser.userId, eg.thirdUser.userId],
+        selfUserId: eg.selfUser.userId));
+    });
+  });
+
   group('DmNarrow', () {
     test('constructor assertions', () {
       check(() => DmNarrow(allRecipientIds: [2, 12], selfUserId: 2)).returnsNormally();