a geek trapped in a cool guy’s body

The .NET framework is huge, but not so huge that it does everything for everyone; there are things that they in Redmond miss or don’t do for whatever reason but is still generally applicable to many developers. So, dear reader, I present to you a new series of posts on stuff I find missing in .NET, typically where even the Google fails to find the answer. It could be a useful class, a technique, a good practice or documentation that should be in the framework but isn’t.

In Part 1 last time, I started creating an AutoComplete TextBox in WPF using only the controls given to us by Microsoft and the concepts of WPF. I showed how you can quickly get the meat of the problem solved using a custom ICollectionView that filters based on the text in the TextBox. In this part, I’ll discuss what’s needed to make it reusable.

Reusa-ma-bility

Alright. So we have a pretty good idea of what’s required, how are we going to package this up to be reused elsewhere? Right now, my implementation is good for the window that I implemented it in. I could make a new control, and add this logic to it, but I already said at the outset that that wasn’t going to happen.

What I decided to use was an attached property. The guidance on using Attached Properties was what finally sold me. See, I also want to use this on ComboBoxes eventually, but ComboBox and TextBox, which both have a Text property, don’t have a common ancestor past Control. One of the scenarios that the docs describe, but don’t provide an example of is wanting to have a property on different, unrelated parts of the control hierarchy. I don’t know why Microsoft can’t provide an example, but I can: the TextSearch class defines two attached properties with this scenario in mind. (Seriously, the docs for .NET 1.1 were awesome. What the hell happened to msdn?)

OK, so I’m going to use an attached property. What that looks like is the following:

public sealed partial class AutoComplete
{
  public static readonly DependencyProperty SourceProperty;

  static AutoComplete()
  {
     SourceProperty = DependencyProperty.RegisterAttached("Source",
                        typeof (object), typeof (AutoComplete),
                        new FrameworkPropertyMetadata(null,OnSourcePropertyChanged));     
  }

  public static object GetSource(DependencyObject o)
  {
     return o.GetValue(SourceProperty);
  }

  public static void SetSource(DependencyObject o, object value)
  {
     o.SetValue(SourceProperty, value);
  }
}

This is pretty standard stuff that you can find in the docs. The one piece that’s missing in that snippet is the event handler method OnSourcePropertyChanged that’s declared in the call to DependencyProperty.RegisterAttached. I’ll get to that in Part 3.

What this allows is I can use this on any DependencyObject, like so:

<StackPanel >
    <TextBox Name="textBox1" 
         local:AutoComplete.Source="{Binding Source={StaticResource people}}"
         VerticalAlignment="Top" 
         HorizontalAlignment="Stretch" 
         TextChanged="textBox1_TextChanged" />    
</StackPanel>

where local is my declared namespace for my assembly (Visual Studio does this for you).

Awesome. I can put this on a ComboBox now, or anything else that allows text input. Now we have an auto-completing TextBox. Your welcome.

Yes, you have a question? There’s some parts missing? Like what? Oh, right the ListBox, it’s not there anymore.

A Look-less Autocomplete TextBox

I’ve already solved the hard problem of getting the List to show up when the user types something; I just need to make that work on any TextBox. The trouble is, I’m manipulating the visual tree, which, to my belated discovery, is really hard to do in code after the TextBox is created. Turns what I want to do is create a ControlTemplate that contains my ListBox and replace the TextBox’s Template with my own.

One of the key concepts in WPF is what the folks at Microsoft call "look-less controls". The canonical example is the button. There are a bajillion — at my last count — samples of making buttons look different than the standard Windows button in WPF. They do this by manipulating either the Content property of the button, or its ControlTemplate. Either way, you still have a button that can be clicked, but you’ve changed the look of the button. All WPF controls act like this, more or less, to some degree: I was asked to make a ListBox turn into a TabControl and that’s non-trivial, nigh impossible, even though they are related in the control hierarchy.

Another example is the ComboBox. I mentioned in the first edition of The Missing .NET that the traditional Win32 ComboBox is made up of three controls: a button, a listbox, and a textbox. Well, so is the WPF ComboBox! Only, the WPF is a little more explicit about it, and more flexible. The place where this is done is the Control.Template dependency property; it’s a dependency property hanging off Control that lets you set the visual tree of your control. The standard controls manipulate this property in their default Style. One tool you want in your WPF developer toolbox is something that will deconstruct the default Styles of the standard controls; I use XamlpadX. Bombing around the styles should give you an understanding of how they work.

Deconstructing the ComboBox in XamlPadX, sure enough, we see the TextBox, the ToggleButton and the ItemsControl, of which ListBox is a descendent.

We’re getting into some seriously advanced, yet fundamental WPF stuff here, which is a little beyond the scope of this blog post, at least if I wanted to finish quickly. If you’re interested in learning about Templates and Styles, Google can be big help; if you’d prefer to learn offline, then any of the WPF books will be quite helpful. Start small, and work your way up. Learning how templates and styles work is definitely worth the effort.

So the only way to stick my ListBox into the visual tree is to replace the ControlTemplate of the TextBox.

This is where the internets came up short for me and I went out on my own. I have no idea if my solution is a good idea. It seems to work, and work well, but I don’t know enough about WPF to tell if this will work in every situation. Every solution creates its own problems and I ran into a few while solving this one.

OK, so what did I do?

You’ll have to wait ’til next time. 🙂

The .NET framework is huge, but not so huge that it does everything for everyone; there are things that they in Redmond miss or don’t do for whatever reason but is still generally applicable to many developers. So, dear reader, I present to you a new series of posts on stuff I find missing in .NET, typically where even the Google fails to find the answer. It could be a useful class, a technique, a good practice or documentation that should be in the framework but isn’t.

The more I use WPF, the more I’m impressed and confounded by it. Impressed because it is really well-designed, thoughtful, and quite rich right out of the gate; I’m confounded by the things it’s lacking. I realize they had to ship something sooner than later, but some of the things Microsoft left out seem pretty trivial to implement given what I know about WPF. One of these is an auto-completing TextBox (It was even late to the Windows forms party, but still…). The most famous example is the address ComboBox in the browser. Start typing to see that the computer remembers what you typed previously. Auto-complete is everywhere in the OS, and you can see it on the web in spots too, now: Compose a message in Facebook and you’ll see your contact list drop down and filter out as you type.

I’ve seen suspiciously little from The Google on auto-completing text boxes using WPF. This can lead me to possible two explanations:

1. Auto-complete is obviously trivial and so everyone just implements it in two lines in their own projects, and I’m a complete idiot for searching The Google in the first place; or
2. No one is using WPF for anything  usable with normal-looking controls — they just want the 3-D light show with lasers.

In either case, I figure I’ll just keep writing this post. There are definitely more complete idiots out there, if 1 is the case. If 2 is the case, I just have to wait a few months or years for this post to be relevant.

When I set out, I had a goal to make a TextBox with auto-complete, I was going to use WPF’s idioms to the fullest, meaning no custom controls or inheriting from TextBox to add what I want. Also, I detest making custom controls because they are so hard to maintain and they aren’t just there in the IDE to use, like the default controls; the case would have to be pretty compelling to replace something as fundamental as the text box, and autocomplete ain’t it.

Besides, I think it’ll be a rare thing to create a custom control in WPF, at least a custom control the way a WinForms developer would think about a custom control: there’s just too much you can do with element composition and templates in WPF.

After some digging, I noticed that the bits and pieces of AutoComplete are already there in WPF; all I had to do was stitch them up together in a nice little package. So what I’d like to do is talk about those parts before I explain how I made the control. In the first part, I’ll talk about filtering the list as the user types; you know, the part that gives a textbox its autocomplete-y-ness.

List-Filtering: CollectionViewSource

People like lists in their blog posts I’ve found; so here’s a list: the Best Thing about WPF

• Data-binding

The end.

You can write an RSS reader in like 100 lines of XAML and zero lines of code, all with data-binding. Representing custom classes is a matter of a moment, creating a DataTemplate for them. Hierarchical data? No problem. XML? No problem. Database? Um, not sure, don’t use ’em, but probably.

WPF was designed with data-driven apps as one of the key scenarios, and it shows. What’s even nicer is that you can get started right away without knowing too much about how databinding works. But dig a little deeper and you can see how well-designed it is.

For instance, the ListBox can represent a list of items and, with DataTemplates, you can customize the look of each item. You can also track the currently selected item and bind other controls to that item. What if you had two ListBoxes, though, both pointing to the same collection, and you wanted one to show a subset of the collection? Could you do that?

Absolutely.

When you bind to a list source WPF wraps the source in an ICollectionView. The ICollectionView manages the currency (the currently selected item) of the list for the ListBox. Each ListBox gets a different ICollectionView instance. If you look at the API for ICollectionView, you’ll see some methods and properties for grouping, sorting, and filtering.

We’re interested in filtering lists! If you want to filter a list, you can create your own ICollectionView through it’s XAML representation, CollectionViewSource; bind it to your source; bind your list box to your CollectionViewSource and then register for the Filter event on the view.

The XAML below provides an example of this.

<Grid Name="grid1">
     <Grid.Resources>
        <ObjectDataProvider x:Key="people" 
                            ObjectType="{x:Type local:People}" 
                            MethodName="GetPeople" />
        <CollectionViewSource x:Key="viewSource" 
                              Source="{StaticResource people}" 
                              Filter="CollectionViewSource_Filter"/>
    </Grid.Resources>
    <ListBox ItemsSource="{Binding Source={StaticResource viewSource}}" />
</Grid>

The CollectionViewSource_Filter event handler is where your filtering code goes. If you hook it up in XAML, you get a nice event with specific event args, but you could also do it in code, in which case, the Filter property is a Predicate<T> delegate. The event deals with each item in the data source at a time.

So where am I going with this ICollectionView thing? Well, if you look at what constitutes an autocomplete textbox in Win32, it’s a just a textbox with a listbox below that shows up when typing happens and filters the items in the list based on what’s typed.

So why not start there in WPF?

Autocomplete TextBox First Draft

My first iteration wasn’t a control or anything more complicated than the XAML below

<StackPanel >
    <TextBox Name="textBox1" 
         VerticalAlignment="Top" 
         HorizontalAlignment="Stretch" 
         TextChanged="textBox1_TextChanged" />
    <ListBox Name="listBox1" 
         Visibility="Hidden"
         ItemsSource="{Binding Source={StaticResource viewSource}}"
         HorizontalAlignment="Stretch" 
         Focusable="False"/>
</StackPanel>

The viewSource in the ListBox.ItemsSource Binding is the same as the previous XAML snippet. In the code behind file, I have the following event handlers:

private void textBox1_TextChanged(object sender, TextChangedEventArgs e)
{
    CollectionViewSource viewSource = (CollectionViewSource) grid1.FindResource("viewSource");
    this.listBox1.Visibility = this.textBox1.Text != "" 
                                    ? Visibility.Visible : Visibility.Hidden;

    viewSource.View.Refresh();
}

private void CollectionViewSource_Filter(object sender, FilterEventArgs e)
{
    e.Accepted = e.Item.ToString().StartsWith(textBox1.Text, 
                        StringComparison.CurrentCultureIgnoreCase);
}

When the text changes, I ask the viewSource’s ICollectionView to Refresh. This in turn raises the Filter event that determines if each item in the list should still be there based on the text in the TextBox. Cool, eh? That’s all that’s needed

Run it with your favourite IDE with some suitable data, start typing and you have the basic concept of an autocompleting TextBox. That was easy.

Next time: we package this up in a reusable way.