refactor: page factory and less mvvm-breaking

Needlework.Net/ViewModels/Pages/PageFactory.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Needlework.Net.ViewModels.Pages
+{
+    public class PageFactory
+    {
+        private readonly IEnumerable<PageBase> _pages;
+
+        public PageFactory(IEnumerable<PageBase> pages)
+        {
+            _pages = pages;
+        }
+
+        public PageBase GetPage<T>() where T : PageBase
+        {
+            var page = _pages.Where(page => typeof(T) == page.GetType())
+                .FirstOrDefault() ?? throw new NotSupportedException(typeof(T).FullName);
+            Task.Run(page.InitializeAsync);
+            return page;
+        }
+    }
+}