fix: Streamlines event scheduler API. Adds months to weekly recurrence (#2178)

Projects/UOContent/Engines/Events/BaseScheduledEvent.cs

@@ -0,0 +1,67 @@
+using System;
+using System.Runtime.CompilerServices;
+using Server.Logging;
+
+namespace Server.Engines.Events;
+
+public abstract class BaseScheduledEvent
+{
+    private static readonly ILogger logger = LogFactory.GetLogger(typeof(BaseScheduledEvent));
+
+    public TimeZoneInfo TimeZone { get; private set; } = TimeZoneInfo.Utc;
+    public DateTime NextOccurrence { get; private set; } = DateTime.MaxValue;
+    public bool Cancelled { get; private set; }
+    public EventScheduler Scheduler { get; private set; }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public void Schedule(DateTime startAfter, TimeZoneInfo timeZone = null) =>
+        Schedule(EventScheduler.Shared, startAfter, timeZone);
+
+    public void Schedule(EventScheduler scheduler, DateTime startAfter, TimeZoneInfo timeZone = null)
+    {
+        Cancel();
+        Scheduler = scheduler;
+        TimeZone = timeZone ?? TimeZoneInfo.Utc;
+        Schedule(startAfter, timeZone, true);
+    }
+
+    private void Schedule(DateTime startAfter, TimeZoneInfo timeZone, bool isFirst)
+    {
+        Cancelled = false;
+        var afterUtc = startAfter.Kind == DateTimeKind.Utc ? startAfter : startAfter.LocalToUtc(timeZone);
+
+        var next = GetNextOccurrence(afterUtc);
+
+        // For the first occurrence, we should set it to the startAfter date if we have no recurrence.
+        NextOccurrence = next == DateTime.MaxValue && isFirst ? afterUtc : next;
+
+        if (NextOccurrence != DateTime.MaxValue)
+        {
+            Scheduler.ScheduleEvent(this);
+        }
+    }
+
+    protected abstract DateTime GetNextOccurrence(DateTime after);
+
+    public void Cancel()
+    {
+        Cancelled = true;
+        Scheduler?.UnscheduleEvent(this);
+    }
+
+    public virtual void Advance()
+    {
+        try
+        {
+            OnEvent();
+        }
+        catch (Exception ex)
+        {
+            logger.Error(ex, "OnEvent failed to execute.");
+        }
+
+        Schedule(NextOccurrence, TimeZone, false);
+    }
+
+    public abstract void OnEvent();
+}