compose: Use topicsPolicy and realmTopicsPolicy for per-channel topics policy

[Ruhdee]

Fixes #1604.
Completes #1956.

First 3 commits are from #1956. This PR specifically addresses the second paragraph of this comment: #1956 (comment)

Changes Made:

1. Added topics policy based handling of topic input in compose box.
2. Updated existing tests to stop using deprecated realmMandatoryTopics and replace with realmTopicsPolicy.
3. Updated error string for 'mandatory but empty' validation error.

Doubts:

1. Are TODO comments added for realmMandatoryTopics considered unnecessary?
2. I have derived boolean mandatory from effective topics policy for code clarity, and because using switch statement on topicsPolicy is awkward due to topicsPolicy.unknown and topicsPolicy.inherit. Is current approach fine?
3. Has the previous author been credited correctly?

[chrisbobbe]

Thanks! Comments below.

Would you add a commit making InitialSnapshot.realmMandatoryTopics optional? Then the server will be free to drop it once most mobile users have upgraded their mobile-app installation to take that change. The UI should respond in the same way it does when new servers send a value for InitialSnapshot.realmTopicsPolicy that we don't recognize.

[chrisbobbe]

• Can tighten up the code in _effectiveTopicsPolicy; also remove a comment which repeats what's obvious in the code.
• For whether to use the new feature or the old feature, just check whether the new feature is available, i.e. check if store.realmMandatoryTopics is non-null, rather than an explicit feature-level check.
• Can pull out the "channel policy for realm policy" conversion into a method on TopicsPolicy.

E.g. like this:

  // TODO(#668): listen to [PerAccountStore] once we subscribe to topic policies.
  TopicsPolicy get _effectiveTopicsPolicy {
    final channelTopicsPolicy = store.streams[streamId]?.topicsPolicy;

    if (channelTopicsPolicy case .inherit || null) {
      final realmTopicsPolicy = store.realmTopicsPolicy
        ?? (store.realmMandatoryTopics
              ? .disableEmptyTopic
              : .allowEmptyTopic);
      return TopicsPolicy.forRealmPolicy(realmTopicsPolicy);
    }

    return channelTopicsPolicy;
  }

with this diff to add TopicsPolicy.forRealmPolicy:

diff --git lib/api/model/model.dart lib/api/model/model.dart
index e5c3f87c4..ddaeb24a3 100644
--- lib/api/model/model.dart
+++ lib/api/model/model.dart
@@ -811,6 +811,14 @@ enum TopicsPolicy {
   emptyTopicOnly,
   unknown;
 
+  /// The [TopicsPolicy] corresponding to the given [RealmTopicsPolicy].
+  static TopicsPolicy forRealmPolicy(RealmTopicsPolicy realmPolicy) =>
+    switch (realmPolicy) {
+      .allowEmptyTopic   => .allowEmptyTopic,
+      .disableEmptyTopic => .disableEmptyTopic,
+      .unknown           => .unknown,
+    };
+
   static TopicsPolicy fromApiValue(String value) => _byApiValue[value] ?? unknown;
 
   static final _byApiValue = _$TopicsPolicyEnumMap

[chrisbobbe]

Oh, also: let's instead make _effectiveTopicsPolicy a public method on ChannelStore, maybe just above selfHasContentAccess and selfCanSendMessage. That way we can offer it to other consumers, if desired, in the future.

[Ruhdee]

        ?? (store.realmMandatoryTopics
              ? .disableEmptyTopic
              : .allowEmptyTopic);

Is this formatting (no. of spaces) intentional in the suggested code?

[chrisbobbe]

Why this change?

[Ruhdee]

1. The following test: "TopicAutocomplete options appear, disappear, and change correctly" fails without the change because the test calls finder twice, once before any interaction with the topic input, and once after interaction.
2. Earlier, when realmMandatoryTopics was used, it defaulted to true. This meant that no matter the interaction status, hintText would be equal to composeBoxTopicHintText ("Topic").
3. Now that realmTopicsPolicy is used and it defaults to unknown, or allowEmptyTopic according to your suggestion, hintText changes according to interaction status. So now line 619 returns no element because the hintText changed
   Line 619 of test/widgets/autocomplete_test.dart:

check(tester.widget<TextField>(topicInputFinder).controller!.text)
  .equals(topic3.name.displayName!);

Thus, I made that change and used controller to identify the widget as I felt it is less brittle than matching hintText string. If there is a better way to make that change, please suggest it.

[Ruhdee]

Thank you for the review @chrisbobbe. I have addressed all the feedback, PTAL.

Doubt: Should the TODO comment about subscribing to store be changed now that effectiveTopicsPolicy is moved to ChannelStore?

// TODO(#668): listen to [PerAccountStore] once we subscribe to topic policies.

[Ruhdee]

@chrisbobbe It's been more than a week, would appreciate a review!

[Ruhdee]

1. Fixed merge conflict caused by Spanish translation update.
2. Switched to using channelId instead of streamId for ComposeTopicController and StreamComposeBoxController because of Rename "stream" within codebase to "channel"  #631.
3. Cleaned up the doTest helper in 'to ChannelNarrow, topic policy resolution' test group in compose_box_test.dart.