How can I make the combobox add items faster? - wpf

Question

I used a ComboBox to load all the font on the computer and preview it.
Here is the XAML:
<Window x:Class="Sample.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:Sample"
mc:Ignorable="d"
Title="Demo" Height="450" Width="800" WindowStartupLocation="CenterScreen" WindowStyle="None" Loaded="Window_Loaded" Background="White">
<Window.Resources>
<local:FontFamilyConverter x:Key="FontFamilyConverter"></local:FontFamilyConverter>
</Window.Resources>
<Grid>
<ComboBox Margin="10,10,0,10" HorizontalContentAlignment="Stretch" Name="FontFaimlyCB" Height="50" Width="250" ItemsSource="{Binding Source={x:Static Fonts.SystemFontFamilies}}">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" FontFamily="{Binding Converter={StaticResource FontFamilyConverter}}" FontSize="20"></TextBlock>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
</Grid>
</Window>
And here is code-behind:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using System.Threading;
using System.Globalization;
namespace Sample
{
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
}
}
public class FontFamilyConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return new FontFamily(value.ToString());
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
}
When the first time I click the ComboBox, it always takes a long time to add the items(There are almost one thousand fonts on my computer and it will takes 3-4 seconds to finish it). But any other software such as word/photoshop and so on won't like this.
How can I make it add faster? Please help me, thank you.

Answer 1

You could try to use a VirtualizingStackPanel as the ItemsPanel and set the MaxDropDownHeight to a fairly small value in order not to render all containers immediately. And you shouldn't have to use a converter:
<ComboBox Margin="10,10,0,10" HorizontalContentAlignment="Stretch" Name="FontFaimlyCB" Height="50" Width="250"
ItemsSource="{Binding Source={x:Static Fonts.SystemFontFamilies}}"
MaxDropDownHeight="50">
<ComboBox.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel />
</ItemsPanelTemplate>
</ComboBox.ItemsPanel>
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" FontFamily="{Binding}" FontSize="20" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>

Answer 2

Create your own observable collection that allows you to suspend notifications when adding items. E.g.:
static void Main(string[] args)
{
var fonts = new ObservableCollectionEx<string>();
using (fonts.DeferCollectionChanged())
{
for (int i = 0; i < 100000; i++)
{
fonts.Add(Guid.NewGuid().ToString());
}
}
}
public class ObservableCollectionEx<T> : ObservableCollection<T>
{
private int deferLevel;
private bool collectionUpdateNeeded;
public ObservableCollectionEx()
{
}
public ObservableCollectionEx(IEnumerable<T> collection)
: base(collection)
{
}
public ObservableCollectionEx(List<T> collection)
: base(collection)
{
}
public override event NotifyCollectionChangedEventHandler CollectionChanged;
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
if (deferLevel == 0)
{
CollectionChanged?.Invoke(this, e);
collectionUpdateNeeded = false;
}
else
{
collectionUpdateNeeded = true;
}
}
public IDisposable DeferCollectionChanged()
{
++deferLevel;
return new DeferHelper(this);
}
private void EndDefer()
{
--deferLevel;
if (deferLevel == 0 && collectionUpdateNeeded)
{
OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
}
}
private class DeferHelper : IDisposable
{
private ObservableCollectionEx<T> collection;
public DeferHelper(ObservableCollectionEx<T> collection)
{
this.collection = collection;
}
public void Dispose()
{
if (collection != null)
{
collection.EndDefer();
collection = null;
}
}
}
}