Hardened background work against disconnected circuits (#937)

app/MindWork AI Studio/Tools/Services/CircuitStateService.cs

@@ -0,0 +1,46 @@
+namespace AIStudio.Tools.Services;
+
+/// <summary>
+/// Knows whether the browser connection of one circuit is currently up.
+/// </summary>
+/// <remarks>
+/// There is one instance of this service per circuit, i.e. per browser window. It exists because the app
+/// keeps disconnected circuits around for a long time, cf. the retention settings in Program.cs: after a
+/// reload or while the machine sleeps, the components of the old circuit are still alive and still receive
+/// events. They may do their work as before — only JavaScript interop is impossible while the connection
+/// is gone. This is what tells them apart. Only the circuit handler changes this state.
+/// </remarks>
+public sealed class CircuitStateService
+{
+    private volatile bool isConnected = true;
+
+    /// <summary>
+    /// True, as long as the browser of this circuit is reachable, and thus JS interop is possible.
+    /// </summary>
+    /// <remarks>
+    /// This starts as true: a circuit is created for a connected browser, and the handler reports the
+    /// first connection only afterwards. Starting as false would block the interop of the first render.
+    /// </remarks>
+    public bool IsConnected => this.isConnected;
+
+    /// <summary>
+    /// The ID of this circuit, for logging purposes. It is "n/a" until the circuit was opened.
+    /// </summary>
+    public string CircuitId { get; private set; } = "n/a";
+
+    /// <summary>
+    /// Called by the circuit handler when the circuit was opened.
+    /// </summary>
+    /// <param name="circuitId">The ID of the opened circuit.</param>
+    public void AssignCircuit(string circuitId) => this.CircuitId = circuitId;
+
+    /// <summary>
+    /// Called by the circuit handler when the browser connection was established or restored.
+    /// </summary>
+    public void MarkAsConnected() => this.isConnected = true;
+
+    /// <summary>
+    /// Called by the circuit handler when the browser connection was lost or the circuit ended.
+    /// </summary>
+    public void MarkAsDisconnected() => this.isConnected = false;
+}