feat: simplify mechanism.

src/Ursa/Controls/DateTimePicker/CalendarContext.cs

@@ -1,21 +1,67 @@
 namespace Ursa.Controls;
 
-public class CalendarContext(int? year = null, int? month = null, int? day = null)
+public sealed class CalendarContext(int? year = null, int? month = null, int? startYear = null, int? endYear = null)
 {
-    public int? Year = year;
-    public int? Month = month;
-    public int? Day = day;
-    public int? StartYear;
-    public int? EndYear;
+    public int? Year { get; } = year;
+    public int? Month { get; } = month;
+    public int? StartYear { get; } = startYear;
+    public int? EndYear { get; } = endYear;
 
 
     public CalendarContext Clone()
     {
-        return new CalendarContext(Year, Month, Day) { StartYear = StartYear, EndYear = EndYear };
+        return new CalendarContext(Year, Month, StartYear, EndYear);
     }
 
     public static CalendarContext Today()
     {
-        return new CalendarContext(DateTime.Today.Year, DateTime.Today.Month, 1);
+        return new CalendarContext(DateTime.Today.Year, DateTime.Today.Month);
+    }
+
+    public CalendarContext With(int? year = null, int? month = null, int? startYear = null, int? endYear = null)
+    {
+        return new CalendarContext(year ?? Year, month ?? Month, startYear ?? StartYear, endYear ?? EndYear);
+    }
+
+    public CalendarContext NextMonth()
+    {
+        var year = Year;
+        var month = Month;
+        if (month == 12)
+        {
+            year++;
+            month = 1;
+        }
+        else
+        {
+            month++;
+        }
+        return new CalendarContext(year, month, StartYear, EndYear);
+    }
+
+    public CalendarContext PreviousMonth()
+    {
+        var year = Year;
+        var month = Month;
+        if (month == 1)
+        {
+            year--;
+            month = 12;
+        }
+        else
+        {
+            month--;
+        }
+        return new CalendarContext(year, month, StartYear, EndYear);
+    }
+
+    public CalendarContext NextYear()
+    {
+        return new CalendarContext(Year + 1, Month, StartYear, EndYear);
+    }
+
+    public CalendarContext PreviousYear()
+    {
+        return new CalendarContext(Year - 1, Month, StartYear, EndYear);
     }
 }