Implement XDND: drag-and-drop support for Linux

samples/ControlCatalog/Pages/DragAndDropPage.xaml

@@ -43,14 +43,14 @@
                 MaxWidth="260"
                 Background="{DynamicResource SystemAccentColorDark1}"
                 DragDrop.AllowDrop="True">
-          <TextBlock TextWrapping="Wrap">Drop some text or files here (Copy)</TextBlock>
+          <TextBlock TextWrapping="Wrap">Drop some text, files, bitmap or custom format here (Copy)</TextBlock>
         </Border>
         <Border Name="MoveTarget"
                 Padding="16"
                 MaxWidth="260"
                 Background="{DynamicResource SystemAccentColorDark1}"
                 DragDrop.AllowDrop="True">
-          <TextBlock TextWrapping="Wrap">Drop some text or files here (Move)</TextBlock>
+          <TextBlock TextWrapping="Wrap">Drop some text or custom format here (Move)</TextBlock>
         </Border>
       </StackPanel>
     </WrapPanel>

samples/ControlCatalog/Pages/DragAndDropPage.xaml.cs

@@ -124,6 +124,9 @@ async void Drop(object? sender, DragEventArgs e)
                     e.DragEffects = e.DragEffects & (DragDropEffects.Copy);
                 }
 
+                if (e.DragEffects == DragDropEffects.None)
+                    return;
+
                 if (e.DataTransfer.Contains(DataFormat.Text))
                 {
                     DropState.Content = e.DataTransfer.TryGetText();

src/Avalonia.Base/Input/AsyncDataTransferExtensions.cs

@@ -27,6 +27,9 @@ internal static IDataTransfer ToSynchronous(this IAsyncDataTransfer asyncDataTra
         return new AsyncToSyncDataTransfer(asyncDataTransfer);
     }
 
+    internal static IAsyncDataTransfer ToAsynchronous(this IDataTransfer dataTransfer)
+        => dataTransfer as IAsyncDataTransfer ?? new SyncToAsyncDataTransfer(dataTransfer);
+
     /// <summary>
     /// Gets whether a <see cref="IAsyncDataTransfer"/> supports a specific format.
     /// </summary>

src/Avalonia.Base/Input/SyncToAsyncDataTransfer.cs

@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+
+namespace Avalonia.Input;
+
+/// <summary>
+/// Wraps a <see cref="IDataTransfer"/> into a <see cref="IAsyncDataTransfer"/>.
+/// </summary>
+/// <param name="dataTransfer">The sync object to wrap.</param>
+internal sealed class SyncToAsyncDataTransfer(IDataTransfer dataTransfer)
+    : IDataTransfer, IAsyncDataTransfer
+{
+    private SyncToAsyncDataTransferItem[]? _items;
+
+    public IReadOnlyList<DataFormat> Formats
+        => dataTransfer.Formats;
+
+    public IReadOnlyList<SyncToAsyncDataTransferItem> Items
+        => _items ??= ProvideItems();
+
+    IReadOnlyList<IDataTransferItem> IDataTransfer.Items
+        => dataTransfer.Items;
+
+    IReadOnlyList<IAsyncDataTransferItem> IAsyncDataTransfer.Items
+        => Items;
+
+    private SyncToAsyncDataTransferItem[] ProvideItems()
+    {
+        var asyncItems = dataTransfer.Items;
+        var count = asyncItems.Count;
+        var syncItems = new SyncToAsyncDataTransferItem[count];
+
+        for (var i = 0; i < count; ++i)
+        {
+            var asyncItem = asyncItems[i];
+            syncItems[i] = new SyncToAsyncDataTransferItem(asyncItem);
+        }
+
+        return syncItems;
+    }
+
+    public void Dispose()
+        => dataTransfer.Dispose();
+}

src/Avalonia.Base/Input/SyncToAsyncDataTransferItem.cs

@@ -0,0 +1,21 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace Avalonia.Input;
+
+/// <summary>
+/// Wraps a <see cref="IDataTransferItem"/> into a <see cref="IAsyncDataTransferItem"/>.
+/// </summary>
+/// <param name="dataTransferItem">The sync item to wrap.</param>
+internal sealed class SyncToAsyncDataTransferItem(IDataTransferItem dataTransferItem)
+    : IDataTransferItem, IAsyncDataTransferItem
+{
+    public IReadOnlyList<DataFormat> Formats
+        => dataTransferItem.Formats;
+
+    public object? TryGetRaw(DataFormat format)
+        => dataTransferItem.TryGetRaw(format);
+
+    public Task<object?> TryGetRawAsync(DataFormat format)
+        => Task.FromResult(dataTransferItem.TryGetRaw(format));
+}

src/Avalonia.Controls/Platform/InProcessDragSource.cs

@@ -63,6 +63,7 @@ public async Task<DragDropEffects> DoDragDropAsync(
                     using (_result.Subscribe(new AnonymousObserver<DragDropEffects>(tcs)))
                     {
                         var effect = await tcs.Task;
+                        dataTransfer.Dispose();
                         return effect;
                     }
                 }