feat(net)!: carry group ranges on track::Subscription

dart/moq/lib/src/aliases.dart

@@ -114,9 +114,18 @@ typedef Datagram = MoqDatagram;
 /// A payload plus the timestamp it should be presented at.
 typedef Frame = MoqFrame;
 
-/// Tunes how a track subscription is delivered: priority, group ordering, and range.
+/// Tunes how a track subscription is delivered: priority, latency budget, ranges, and group ordering.
 typedef Subscription = MoqSubscription;
 
+/// A span of a track to deliver: from `start` up to `end`, or onward when `end` is null.
+typedef Range = MoqRange;
+
+/// Where one end of a [Range] falls: absolute or relative to the latest group.
+typedef Anchor = MoqAnchor;
+
+/// Which pending group the publisher sends first.
+typedef Order = MoqOrder;
+
 /// Options for fetching one past group by sequence.
 typedef FetchGroupOptions = MoqFetchGroupOptions;
 

dart/moq/test/moq_test.dart

@@ -115,6 +115,39 @@ void main() {
     broadcast.close();
   });
 
+  test(
+    'an ascending subscription reads a cached backlog oldest first',
+    () async {
+      final broadcast = BroadcastProducer();
+      final track = broadcast.publishTrack(
+        name: 'backlog',
+        info: TrackInfo(timescale: 1000000),
+      );
+      for (var ms = 0; ms < 3; ms++) {
+        final group = track.appendGroup();
+        group.writeFrame(
+          frame: Frame(payload: utf8.encode('x'), timestampUs: ms * 1000),
+        );
+        group.finish();
+      }
+
+      final consumer = track.consume(
+        subscription: Subscription(
+          maxDelayUs: 10000000,
+          ranges: [Range(start: RelativeMoqAnchor(2))],
+          order: Order.ascending,
+        ),
+      );
+      final sequences = <int>[];
+      for (var i = 0; i < 3; i++) {
+        final group = await consumer.recvGroup().timeout(timeout);
+        sequences.add(group!.sequence());
+      }
+      expect(sequences, [0, 1, 2]);
+      broadcast.close();
+    },
+  );
+
   test('connects, announces, subscribes, and delivers a frame', () async {
     final relay = MoqOriginProducer(config: MoqOriginConfig());
     final server = MoqServer(