Don't create a schema context each time.

The change in the previous commit to create a new `AvaloniaXamlSchemaContext` for each instance of `AvaloniaXamlLoader` had a negative performance effect. Instead, keep two instances of the schema context around: one for normal loading and one for design-time loading.

src/Markup/Avalonia.Markup.Xaml/AvaloniaXamlLoader.cs

@@ -23,13 +23,7 @@ namespace Avalonia.Markup.Xaml
     /// </summary>
     public class AvaloniaXamlLoader
     {
-        private readonly AvaloniaXamlSchemaContext _context = AvaloniaXamlSchemaContext.Create();
+        public bool IsDesignMode { get; set; }
-
-        public bool IsDesignMode
-        {
-            get => _context.IsDesignMode;
-            set => _context.IsDesignMode = value;
-        }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AvaloniaXamlLoader"/> class.
@@ -171,7 +165,8 @@ namespace Avalonia.Markup.Xaml
                 LocalAssembly = localAssembly
             };
 
-            var reader = new XamlXmlReader(stream, _context, readerSettings);
+            var context = IsDesignMode ? AvaloniaXamlSchemaContext.DesignInstance : AvaloniaXamlSchemaContext.Instance;
+            var reader = new XamlXmlReader(stream, context, readerSettings);
 
             object result = LoadFromReader(
                 reader,

src/Markup/Avalonia.Markup.Xaml/PortableXaml/AvaloniaXamlSchemaContext.cs

@@ -1,21 +1,48 @@
-using Avalonia.Data;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using Avalonia.Data;
 using Avalonia.Markup.Xaml.Context;
-using Avalonia.Markup.Data;
 using Avalonia.Markup.Xaml.MarkupExtensions;
 using Avalonia.Markup.Xaml.Styling;
 using Portable.Xaml;
-using Portable.Xaml.ComponentModel;
-using System.ComponentModel;
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Reflection;
 
 namespace Avalonia.Markup.Xaml.PortableXaml
 {
     internal class AvaloniaXamlSchemaContext : XamlSchemaContext
     {
-        public bool IsDesignMode { get; set; }
+        private static AvaloniaXamlSchemaContext s_instance;
+        private static AvaloniaXamlSchemaContext s_designInstance;
+
+        public static AvaloniaXamlSchemaContext Instance
+        {
+            get
+            {
+                if (s_instance == null)
+                {
+                    s_instance = Create();
+                }
+
+                return s_instance;
+            }
+        }
+
+        public static AvaloniaXamlSchemaContext DesignInstance
+        {
+            get
+            {
+                if (s_designInstance == null)
+                {
+                    s_designInstance = Create();
+                    s_designInstance.IsDesignMode = true;
+                }
+
+                return s_designInstance;
+            }
+        }
+
+        public bool IsDesignMode { get; private set; }
         public static AvaloniaXamlSchemaContext Create(IRuntimeTypeProvider typeProvider = null)
         {
             return new AvaloniaXamlSchemaContext(typeProvider ?? new AvaloniaRuntimeTypeProvider());