unreads [nfc]: Factor out countInAllDms, for public use and as a helper

Author: chrisbobbe

lib/model/unreads.dart

@@ -160,9 +160,7 @@ class Unreads extends PerAccountStoreBase with ChangeNotifier {
   // TODO(#370): maintain this count incrementally, rather than recomputing from scratch
   int countInCombinedFeedNarrow() {
     int c = 0;
-    for (final messageIds in dms.values) {
-      c = c + messageIds.length;
-    }
+    c += countInAllDms();
     for (final MapEntry(key: streamId, value: topics) in streams.entries) {
       for (final MapEntry(key: topic, value: messageIds) in topics.entries) {
         if (channelStore.isTopicVisible(streamId, topic)) {
@@ -230,6 +228,14 @@ class Unreads extends PerAccountStoreBase with ChangeNotifier {
   // TODO: Implement unreads handling?
   int countInKeywordSearchNarrow() => 0;
 
+  int countInAllDms() {
+    int c = 0;
+    for (final messageIds in dms.values) {
+      c += messageIds.length;
+    }
+    return c;
+  }
+
   int countInNarrow(Narrow narrow) {
     switch (narrow) {
       case CombinedFeedNarrow():

test/model/unreads_test.dart

@@ -271,6 +271,16 @@ void main() {
       ]);
       check(model.countInStarredMessagesNarrow()).equals(0);
     });
+
+    test('countInAllDms', () async {
+      prepare();
+      fillWithMessages([
+        eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: []),
+        eg.dmMessage(from: eg.thirdUser, to: [eg.selfUser], flags: []),
+        eg.dmMessage(from: eg.thirdUser, to: [eg.selfUser, eg.otherUser], flags: []),
+      ]);
+      check(model.countInCombinedFeedNarrow()).equals(3);
+    });
   });
 
   group('isUnread', () {