[X11] Added support for filters in GTK file dialog

samples/ControlCatalog/Pages/DialogsPage.xaml

@@ -2,6 +2,7 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:Class="ControlCatalog.Pages.DialogsPage">
   <StackPanel Orientation="Vertical" Spacing="4" Margin="4">
+      <CheckBox Name="UseFilters">Use filters</CheckBox>
       <Button Name="OpenFile">Open File</Button>
       <Button Name="SaveFile">Save File</Button>
       <Button Name="SelectFolder">Select Folder</Button>

samples/ControlCatalog/Pages/DialogsPage.xaml.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using Avalonia.Controls;
 using Avalonia.Markup.Xaml;
 #pragma warning disable 4014
@@ -9,18 +10,39 @@ namespace ControlCatalog.Pages
         public DialogsPage()
         {
             this.InitializeComponent();
+
+            List<FileDialogFilter> GetFilters()
+            {
+                if (this.FindControl<CheckBox>("UseFilters").IsChecked != true)
+                    return null;
+                return  new List<FileDialogFilter>
+                {
+                    new FileDialogFilter
+                    {
+                        Name = "Text files (.txt)", Extensions = new List<string> {"txt"}
+                    },
+                    new FileDialogFilter
+                    {
+                        Name = "All files",
+                        Extensions = new List<string> {"*"}
+                    }
+                };
+            }
+
             this.FindControl<Button>("OpenFile").Click += delegate
             {
                 new OpenFileDialog()
                 {
-                    Title = "Open file"
+                    Title = "Open file",
+                    Filters = GetFilters()
                 }.ShowAsync(GetWindow());
             };
             this.FindControl<Button>("SaveFile").Click += delegate
             {
                 new SaveFileDialog()
                 {
-                    Title = "Save file"
+                    Title = "Save file",
+                    Filters = GetFilters()
                 }.ShowAsync(GetWindow());
             };
             this.FindControl<Button>("SelectFolder").Click += delegate

src/Avalonia.X11/NativeDialogs/Gtk.cs

@@ -190,6 +190,18 @@ namespace Avalonia.X11.NativeDialogs
         [DllImport(GtkName)]
         public static extern void gtk_file_chooser_set_filename(IntPtr chooser, Utf8Buffer file);
 
+        [DllImport(GtkName)]
+        public static extern IntPtr gtk_file_filter_new();
+        
+        [DllImport(GtkName)]
+        public static extern IntPtr gtk_file_filter_set_name(IntPtr filter, Utf8Buffer name);
+        
+        [DllImport(GtkName)]
+        public static extern IntPtr gtk_file_filter_add_pattern(IntPtr filter, Utf8Buffer pattern);
+        
+        [DllImport(GtkName)]
+        public static extern IntPtr gtk_file_chooser_add_filter(IntPtr chooser, IntPtr filter);
+        
         [DllImport(GtkName)]
         public static extern void gtk_widget_realize(IntPtr gtkWidget);
 

src/Avalonia.X11/NativeDialogs/GtkNativeFileDialogs.cs

@@ -16,7 +16,7 @@ namespace Avalonia.X11.NativeDialogs
     {
         private Task<bool> _initialized;
         private unsafe  Task<string[]> ShowDialog(string title, IWindowImpl parent, GtkFileChooserAction action,
-            bool multiSelect, string initialFileName)
+            bool multiSelect, string initialFileName, IEnumerable<FileDialogFilter> filters)
         {
             IntPtr dlg;
             using (var name = new Utf8Buffer(title))
@@ -35,6 +35,20 @@ namespace Avalonia.X11.NativeDialogs
                 foreach (var d in disposables) d.Dispose();
                 disposables.Clear();
             }
+            
+            if(filters != null)
+                foreach (var f in filters)
+                {
+                    var filter = gtk_file_filter_new();
+                    using (var b = new Utf8Buffer(f.Name))
+                        gtk_file_filter_set_name(filter, b);
+                    
+                    foreach (var e in f.Extensions)
+                        using (var b = new Utf8Buffer("*." + e))
+                            gtk_file_filter_add_pattern(filter, b);
+
+                    gtk_file_chooser_add_filter(dlg, filter);
+                }
 
             disposables = new List<IDisposable>
             {
@@ -90,7 +104,7 @@ namespace Avalonia.X11.NativeDialogs
                     dialog is OpenFileDialog ? GtkFileChooserAction.Open : GtkFileChooserAction.Save,
                     (dialog as OpenFileDialog)?.AllowMultiple ?? false,
                     Path.Combine(string.IsNullOrEmpty(dialog.InitialDirectory) ? "" : dialog.InitialDirectory,
-                        string.IsNullOrEmpty(dialog.InitialFileName) ? "" : dialog.InitialFileName)));
+                        string.IsNullOrEmpty(dialog.InitialFileName) ? "" : dialog.InitialFileName), dialog.Filters));
         }
 
         public async Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
@@ -99,7 +113,7 @@ namespace Avalonia.X11.NativeDialogs
             return await await RunOnGlibThread(async () =>
             {
                 var res = await ShowDialog(dialog.Title, parent,
-                    GtkFileChooserAction.SelectFolder, false, dialog.InitialDirectory);
+                    GtkFileChooserAction.SelectFolder, false, dialog.InitialDirectory, null);
                 return res?.FirstOrDefault();
             });
         }