Add event-driven state sync with ETag optimization

Infrastructure/StateChangeNotifier.cs

@@ -0,0 +1,69 @@
+using Microsoft.Extensions.Primitives;
+
+namespace GameList.Infrastructure;
+
+public sealed class StateChangeNotifier
+{
+    private readonly string _instanceId = Guid.NewGuid().ToString("N");
+    private long _version = 1;
+    private TaskCompletionSource<long> _nextChange = CreateWaiter();
+
+    public long CurrentVersion => Interlocked.Read(ref _version);
+
+    public string CurrentEtag => $"\"{_instanceId}:{CurrentVersion}\"";
+
+    public long NotifyChange()
+    {
+        var newVersion = Interlocked.Increment(ref _version);
+
+        while (true)
+        {
+            var waiter = Volatile.Read(ref _nextChange);
+            var replacement = CreateWaiter();
+            if (Interlocked.CompareExchange(ref _nextChange, replacement, waiter) != waiter)
+                continue;
+
+            waiter.TrySetResult(newVersion);
+            return newVersion;
+        }
+    }
+
+    public bool MatchesCurrentEtag(StringValues ifNoneMatchValues)
+    {
+        if (StringValues.IsNullOrEmpty(ifNoneMatchValues))
+            return false;
+
+        var current = CurrentEtag;
+        foreach (var raw in ifNoneMatchValues)
+        {
+            if (string.IsNullOrWhiteSpace(raw))
+                continue;
+
+            var parts = raw.Split(',', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries);
+            foreach (var part in parts)
+            {
+                if (part == "*" || string.Equals(part, current, StringComparison.Ordinal))
+                    return true;
+            }
+        }
+
+        return false;
+    }
+
+    public async Task<long> WaitForChangeAsync(long observedVersion, CancellationToken cancellationToken)
+    {
+        while (true)
+        {
+            var current = CurrentVersion;
+            if (current > observedVersion)
+                return current;
+
+            var waiter = Volatile.Read(ref _nextChange);
+            var signaled = await waiter.Task.WaitAsync(cancellationToken);
+            if (signaled > observedVersion)
+                return signaled;
+        }
+    }
+
+    private static TaskCompletionSource<long> CreateWaiter() => new(TaskCreationOptions.RunContinuationsAsynchronously);
+}