Support multi selection

src/Calendar.Plugin/Shared/Controls/MultiSelectionCalendar.cs

@@ -0,0 +1,14 @@
+using Xamarin.Plugin.Calendar.Controls.SelectionEngines;
+
+namespace Xamarin.Plugin.Calendar.Controls
+{
+    public class MultiSelectionCalendar : Calendar
+    {
+        private readonly MultiSelectionEngine _multiSelectionEngine;
+
+        public MultiSelectionCalendar()
+        {
+            monthDaysView.CurrentSelectionEngine = _multiSelectionEngine = new MultiSelectionEngine();
+        }
+    }
+}

src/Calendar.Plugin/Shared/Controls/SelectionEngines/MultiSelectionEngine.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using Xamarin.Plugin.Calendar.Controls.Interfaces;
+using Xamarin.Plugin.Calendar.Models;
+
+namespace Xamarin.Plugin.Calendar.Controls.SelectionEngines
+{
+    public class MultiSelectionEngine : ISelectionEngine
+    {
+        private readonly HashSet<DateTime> _selectedDates;
+
+        public MultiSelectionEngine()
+        {
+            _selectedDates = new HashSet<DateTime>();
+        }
+
+        public string GetSelectedDateText(string selectedDateTextFormat, CultureInfo culture)
+        {
+            return _selectedDates
+                .Select(item => item.ToString(selectedDateTextFormat, culture))
+                .Aggregate((a, b) => $"{a}, {b}");
+        }
+
+        public bool TryGetSelectedEvents(EventCollection allEvents, out ICollection selectedEvents)
+        {
+            return allEvents.TryGetValues(_selectedDates, out selectedEvents);
+        }
+
+        public bool IsDateSelected(DateTime dateToCheck)
+        {
+            return _selectedDates.Contains(dateToCheck);
+        }
+
+        public List<DateTime> PerformDateSelection(DateTime dateToSelect)
+        {
+            if (_selectedDates.Contains(dateToSelect))
+                _selectedDates.Remove(dateToSelect);
+            else
+                _selectedDates.Add(dateToSelect);
+
+            return _selectedDates.ToList();
+        }
+
+        public void UpdateDateSelection(List<DateTime> datesToSelect)
+        {
+            datesToSelect.ForEach(date => _selectedDates.Add(date));
+        }
+    }
+}