Initial version

DataTransfer.Client/Services/ServerStatusMonitor.cs

@@ -0,0 +1,63 @@
+using System.Net.Http.Json;
+using DataTransfer.Client.Shared;
+
+namespace DataTransfer.Client.Services;
+
+/// <summary>Periodically fetches the load of the relay server; shared by all components of the page.</summary>
+public sealed class ServerStatusMonitor(HttpClient http, ILogger<ServerStatusMonitor> logger) : IDisposable
+{
+    private static readonly TimeSpan PollInterval = TimeSpan.FromSeconds(5);
+
+    private readonly CancellationTokenSource _stopping = new();
+    private Task? _polling;
+
+    public event Action? Changed;
+
+    /// <summary>The latest status, or <c>null</c> while it is unknown.</summary>
+    public ServerStatus? Current { get; private set; }
+
+    public void EnsureStarted() => _polling ??= PollAsync(_stopping.Token);
+
+    public async Task RefreshAsync(CancellationToken cancellationToken = default)
+    {
+        ServerStatus? status;
+        try
+        {
+            status = await http.GetFromJsonAsync<ServerStatus>(TransferProtocol.StatusPath.TrimStart('/'), cancellationToken);
+        }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            return;
+        }
+        catch (Exception ex)
+        {
+            logger.LogDebug(ex, "Fetching the server status failed");
+            status = null;
+        }
+
+        if (status != Current)
+        {
+            Current = status;
+            Changed?.Invoke();
+        }
+    }
+
+    public void Dispose()
+    {
+        _stopping.Cancel();
+        _stopping.Dispose();
+    }
+
+    private async Task PollAsync(CancellationToken cancellationToken)
+    {
+        using var timer = new PeriodicTimer(PollInterval);
+        try
+        {
+            do
+            {
+                await RefreshAsync(cancellationToken);
+            } while (await timer.WaitForNextTickAsync(cancellationToken));
+        }
+        catch (OperationCanceledException) { }
+    }
+}