add is-os selector

samples/ControlCatalog/MainView.xaml

@@ -26,6 +26,25 @@
       <Style Selector=":orientation(portrait) TextBlock#Portrait">
         <Setter Property="Foreground" Value="Red" />
       </Style>
+      <Style Selector=":is-os(windows) TextBlock#Windows">
+        <Setter Property="Foreground" Value="Red" />
+      </Style>
+      <Style Selector=":is-os(linux) TextBlock#Linux">
+        <Setter Property="Foreground"
+                Value="Red" />
+      </Style>
+      <Style Selector=":is-os(osx) TextBlock#OSX">
+        <Setter Property="Foreground"
+                Value="Red" />
+      </Style>
+      <Style Selector=":is-os(android) TextBlock#Android">
+        <Setter Property="Foreground"
+                Value="Red" />
+      </Style>
+      <Style Selector=":is-os(ios) TextBlock#iOS">
+        <Setter Property="Foreground"
+                Value="Red" />
+      </Style>
     </Grid.Styles>
     <StackPanel Spacing="10">
       <TextBlock Name="MinWidth" Classes="h2" Text="min-width" />
@@ -34,6 +53,11 @@
       <TextBlock Name="MaxHeight" Classes="h2" Text="max-height" />
       <TextBlock Name="Landscape" Classes="h2" Text="orientation(landscape)" />
       <TextBlock Name="Portrait" Classes="h2" Text="orientation(portrait)" />
+      <TextBlock Name="Windows" Classes="h2" Text="is-os(windows)" />
+      <TextBlock Name="Linux" Classes="h2" Text="is-os(linux)" />
+      <TextBlock Name="OSX" Classes="h2" Text="is-os(osx)" />
+      <TextBlock Name="Android" Classes="h2" Text="is-os(android)" />
+      <TextBlock Name="iOS" Classes="h2" Text="is-os(ios)" />
     </StackPanel>
   </Grid>
 </UserControl>

src/Android/Avalonia.Android/Platform/AndroidMediaProvider.cs

@@ -51,5 +51,10 @@ namespace Avalonia.Android.Platform
                 _ => DeviceOrientation.Portrait,
             };
         }
+
+        public string GetPlatform()
+        {
+            return "android";
+        }
     }
 }

src/Avalonia.Base/Platform/IMediaProvider.cs

@@ -4,6 +4,7 @@ namespace Avalonia.Platform
 {
     public interface IMediaProvider
     {
+        string GetPlatform();
         double GetScreenWidth();
 
         double GetScreenHeight();

src/Avalonia.Base/Platform/VisualMediaProvider.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading.Tasks;
 using Avalonia.Reactive;
@@ -53,5 +54,17 @@ namespace Avalonia.Platform
 
             return DeviceOrientation.Square;
         }
+
+        public string GetPlatform()
+        {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                return "windows";
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
+                return "linux";
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+                return "osx";
+
+            return "";
+        }
     }
 }

src/Avalonia.Base/Styling/Activators/DeviceActivator.cs

@@ -0,0 +1,14 @@
+namespace Avalonia.Styling.Activators
+{
+    internal sealed class IsOsActivator : MediaQueryActivatorBase
+    {
+        private readonly string _argument;
+
+        public IsOsActivator(Visual visual, string argument) : base(visual)
+        {
+            _argument = argument;
+        }
+
+        protected override bool EvaluateIsActive() => CurrentMediaInfoProvider != null && IsOsMediaSelector.Evaluate(CurrentMediaInfoProvider, _argument).IsMatch;
+    }
+}

src/Avalonia.Base/Styling/Activators/MediaQueryActivatorBase.cs

@@ -55,7 +55,7 @@ namespace Avalonia.Styling.Activators
             ReevaluateIsActive();
         }
 
-        private void OrientationChanged(object sender, EventArgs e)
+        private void OrientationChanged(object? sender, EventArgs e)
         {
             ReevaluateIsActive();
         }

src/Avalonia.Base/Styling/DeviceMediaSelector.cs

@@ -0,0 +1,45 @@
+using System;
+using Avalonia.Platform;
+using Avalonia.Styling.Activators;
+
+namespace Avalonia.Styling
+{
+    internal sealed class IsOsMediaSelector : MediaSelector<string>
+    {
+        public IsOsMediaSelector(Selector? previous, string argument) : base(previous, argument)
+        {
+        }
+
+        private protected override SelectorMatch Evaluate(StyledElement control, IStyle? parent, bool subscribe)
+        {
+            if (!(control is Visual visual))
+            {
+                return SelectorMatch.NeverThisType;
+            }
+
+            if (subscribe)
+            {
+                return new SelectorMatch(new IsOsActivator(visual, Argument));
+            }
+
+            if (visual.VisualRoot is IMediaProviderHost mediaProviderHost && mediaProviderHost.MediaProvider is { } mediaProvider)
+            {
+                return Evaluate(mediaProvider, Argument);
+            }
+
+            return SelectorMatch.NeverThisInstance;
+        }
+
+        internal static SelectorMatch Evaluate(IMediaProvider mediaProvider, string argument)
+        {
+            return mediaProvider.GetPlatform() == argument ? SelectorMatch.AlwaysThisInstance : SelectorMatch.NeverThisInstance;
+        }
+
+        public override string ToString() => "is-os";
+
+        public override string ToString(Style? owner)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}

src/Avalonia.Base/Styling/Selectors.cs

@@ -221,6 +221,11 @@ namespace Avalonia.Styling
         {
             return new OrientationMediaSelector(previous, (DeviceOrientation)argument);
         }
+        
+        public static Selector IsOs(this Selector? previous, string argument)
+        {
+            return new IsOsMediaSelector(previous, argument);
+        }
 
         /// <summary>
         /// Returns a selector which matches a control with the specified property value.

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlSelectorTransformer.cs

@@ -162,6 +162,9 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                         case SelectorGrammar.OrientationSyntax orientation:
                             result = new XamlIlOrientationSelector(result, orientation.Argument);
                             break;
+                        case SelectorGrammar.IsOsSyntax isOs:
+                            result = new XamlIlIsOsSelector(result, isOs.Argument);
+                            break;
                         case SelectorGrammar.CommaSyntax comma:
                             if (results == null) 
                                 results = new XamlIlOrSelectorNode(node, selectorType);
@@ -490,6 +493,24 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                 m => m.Name == "Orientation" && m.Parameters.Count == 2);
         }
     }
+    
+    class XamlIlIsOsSelector : XamlIlSelectorNode
+    {
+        private string _argument;
+        
+        public XamlIlIsOsSelector(XamlIlSelectorNode previous, string argument) : base(previous)
+        {
+            _argument = argument;
+        }
+
+        public override IXamlType TargetType => Previous?.TargetType;
+        protected override void DoEmit(XamlEmitContext<IXamlILEmitter, XamlILNodeEmitResult> context, IXamlILEmitter codeGen)
+        {
+            codeGen.Ldstr(_argument);
+            EmitCall(context, codeGen,
+                m => m.Name == "IsOs" && m.Parameters.Count == 2);
+        }
+    }
 
     class XamlIlPropertyEqualsSelector : XamlIlSelectorNode
     {

src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs

@@ -183,6 +183,7 @@ namespace Avalonia.Markup.Parsers
             const string NthChildKeyword = "nth-child";
             const string NthLastChildKeyword = "nth-last-child";
             const string OrientationKeyword = "orientation";
+            const string IsOsKeyword = "is-os";
 
             if (identifier.SequenceEqual(IsKeyword.AsSpan()) && r.TakeIf('('))
             {
@@ -253,6 +254,14 @@ namespace Avalonia.Markup.Parsers
                 var syntax = new OrientationSyntax { Argument = argument };
                 return (State.Middle, syntax);
             }
+            if (identifier.SequenceEqual(IsOsKeyword.AsSpan()) && r.TakeIf('('))
+            {
+                var argument = ParseString(ref r);
+                Expect(ref r, ')');
+
+                var syntax = new IsOsSyntax { Argument = argument };
+                return (State.Middle, syntax);
+            }
             else
             {
                 return (
@@ -341,6 +350,11 @@ namespace Avalonia.Markup.Parsers
 
             throw new ExpressionParseException(r.Position, $"Expected a {typeof(T)} after.");
         }
+        
+        private static string ParseString(ref CharacterReader r) 
+        {
+            return r.ParseIdentifier().ToString();
+        }
 
         private static (State, ISyntax) ParseTypeName(ref CharacterReader r)
         {
@@ -763,6 +777,16 @@ namespace Avalonia.Markup.Parsers
             }
         }
 
+        public class IsOsSyntax : ISyntax
+        {
+            public string Argument { get; set; } = string.Empty;
+
+            public override bool Equals(object? obj)
+            {
+                return (obj is IsOsSyntax orientation) && orientation.Argument == Argument;
+            }
+        }
+
         public class CommaSyntax : ISyntax
         {
             public override bool Equals(object? obj)

src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs

@@ -173,6 +173,9 @@ namespace Avalonia.Markup.Parsers
                     case SelectorGrammar.OrientationSyntax orientation:
                         result = result.Orientation(orientation.Argument);
                         break;
+                    case SelectorGrammar.IsOsSyntax isOs:
+                        result = result.IsOs(isOs.Argument);
+                        break;
                     case SelectorGrammar.CommaSyntax comma:
                         if (results == null)
                         {