Add FolderProvider

Toolkit.Avalonia/FolderProvider.cs

@@ -0,0 +1,25 @@
+using Avalonia.Controls;
+using Avalonia.Platform.Storage;
+using Toolkit.Foundation;
+
+namespace Toolkit.Avalonia;
+
+public class FolderProvider(ITopLevelProvider topLevelProvider) :
+    IFolderProvider
+{
+    public async Task<IReadOnlyCollection<string>> SelectFolders(FolderFilter filter)
+    {
+        if (topLevelProvider.Get() is TopLevel topLevel)
+        {
+            IReadOnlyList<IStorageFolder> folders = await topLevel.StorageProvider.OpenFolderPickerAsync(new FolderPickerOpenOptions()
+            {
+                AllowMultiple = filter.AllowMultiple  
+            });
+
+
+            return folders.Select(x => x.Path.LocalPath).ToList();
+        }
+
+        return [];
+    }
+}