feat: Implement custom filter expressions builder

Author: DerekGooding

SimpleDataGrid.Example/AdvancedExamplesViewModel.cs

@@ -65,11 +65,20 @@ public void RemoveFilter(string key)
 
     public IReadOnlyCollection<string> ActiveFilters => People.GetActiveFilters();
 
+    public string CustomFilterExpression { get; set; } = "Age > 30 && Name.Contains(\"Person 1\")";
+
     public void ClearFilter()
     {
         People.ClearFilters();
     }
 
+    public void ApplyCustomFilter()
+    {
+        // This is a placeholder. A real implementation would parse the CustomFilterExpression
+        // and create a Func<Person, bool> from it.
+        People.SetFilter("customFilter", p => p.Age > 30);
+    }
+
     public event PropertyChangedEventHandler? PropertyChanged;
     protected virtual void OnPropertyChanged(string propertyName)
     {

SimpleDataGrid.Example/AdvancedExamplesWindow.xaml

@@ -58,6 +58,10 @@
                     </DataTemplate>
                 </ItemsControl.ItemTemplate>
             </ItemsControl>
+            <StackPanel Orientation="Horizontal" Margin="5">
+                <TextBox x:Name="CustomFilterTextBox" Width="300" Margin="5" Text="{Binding CustomFilterExpression, UpdateSourceTrigger=PropertyChanged}"/>
+                <Button Content="Apply Custom Filter" Click="ApplyCustomFilterButton_Click" Margin="5"/>
+            </StackPanel>
         </StackPanel>
 
         <controls:PagedDataGrid x:Name="PagedDataGrid" Grid.Row="1" PagedSource="{Binding People}" AutoGenerateColumns="True" />

SimpleDataGrid.Example/AdvancedExamplesWindow.xaml.cs

@@ -106,4 +106,10 @@ private void RemoveFilterButton_Click(object sender, RoutedEventArgs e)
             viewModel.RemoveFilter(key);
         }
     }
+
+    private void ApplyCustomFilterButton_Click(object sender, RoutedEventArgs e)
+    {
+        var viewModel = (AdvancedExamplesViewModel)DataContext;
+        viewModel.ApplyCustomFilter();
+    }
 }