JAPF

The last couple of days, I’ve been busy writing an article about MVVM in French. I don’t know yet if I’ll translate it to English, but I’m sure I can share the demo application, and this is the goal of this post !

I think the best way to understand how things fit together is to explore the source code of the application.

It’s a very small demo because I wanted to demonstrate only a subset of MVVM facilities, so you’ll find:

• an easy way to work with commands in MVVM using Josh Smith RelayCommand class
• the power of the ICollectionView interface to implement navigation and search feature
• empty code-behind file for my views
• a base class for all my ViewModel objects (again, based on the work of Josh)

You can download the source code here. Enjoy

Article updated ! I find out that the Command property of the KeyBinding class is not a DependencyProperty… This prevent using the syntax I wanted (binding to an ICommand exposed in the ViewModel). There are several workarounds you can find across the web (using a MarkupExtension, an attached properies…) but because I didn’t have much time, I finally put some glue in my code-behind ;(

I’m still playing with MVVM those days and today, I needed a simple behavior I didn’t know how to do with MVVM. I finally found a simple solution and as I love them, it’s 100% XAML :p

The problem is simple, I have a ListBox where the user can type date in it.
When the User Validate his input, I need to process the text entered and return another value.
The User can validate the input by pressing the Enter key while the TextBox has the focus.

My first attempt was the subscribe to the KeyUp event, and in the code-behind, delegate the work to the associated ViewModel class. I don’t like that solution a lot (events handler in code-behind are sort of ugly…).

I know I could define an ICommand in my ViewModel and do the work directly there (using the excellent RelayCommand class written by Josh Smith). The problem was how to trigger this command when the Enter key is pressed. well, actually it’s pretty simple:

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    <TextBox Text="Hello world" Width="100" Height="40">
        <TextBox.InputBindings>
            <!-- update: we cannot use Binding to the Command property as it's not a DP -->
            <!-- workarounds exist such a MarkupExtension, AttachedProperty... ->
            <!-- personally, I did that in the code behind to make it works :( -->
            <KeyBinding Command="{Binding ProcessInput}" Key="Enter"/>
        </TextBox.InputBindings>            
    </TextBox>

As I updated the article (because Command is not a DP), I finally did the same thing in C# in the code-behind…

As you might already know, I’m a big fan of the Model-View-ViewModel pattern. I’m using it extensively on the current I’m working on at work. Today I had to face a simple problem that was tricky to solve.

In the application I’m building, I have a “Library panel”. This panel contains a set of items that are used in my application. Because the number of items can be very large, I decided to add a filter capability. Filtering to a collection is very straightforward once you get familiar with the ICollectionView interface. If you want more details about it, check out Marlon’s blog post about it.

To give the user the possibility to filter the items, I added a ComboBox control. Of course, because I’m using the MVVM pattern, I’m not creating the ComboBoxItem myself, the databinding mechanism of WPF handle this (to be precise, the databinding handles collecting the item, and the ComboBox handles the creation of the ComboBoxItem to wrap them…).

The problem

In my ViewModel, I create a ObservableCollection<string> property that I called “Categories”.Then, in my view (XAML), I databound the ComboBox’s ItemsSource property to this “Categories”. Ok, it works fine and took me about 10min to do it.

Now, I want to add separator between some ComboBoxItem… Hmmm, how should I do that… I cannot do combobox.Items.Add(something) anymore because the ItemsSource property is databound… Well, I could as something in my ViewModel, but what ?

My solution

I wanted to keep the logical information about where Separator are in the ViewModel. This is typically an example of something that should stays in the ViewModel. I decided to add an empty items in my ObservableCollection for every Separator I wanter to have in the view.

In the view, I set up an ItemContainerStyle for my ComboBoxItem. The tricky part is, how could I replace my ComboBoxItem with a Separator… Well actually, we can’t. But what we can do is to change the entire template of the ComboBoxItem when the content is empty:

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        <ComboBox ItemsSource="{Binding Categories}">
            <ComboBox.ItemContainerStyle>
                <Style TargetType="{x:Type ComboBoxItem}" BasedOn="{StaticResource {x:Type ComboBoxItem}}">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding}" Value="">
                            <Setter Property="Template">
                                <Setter.Value>
                                    <ControlTemplate TargetType="{x:Type ComboBoxItem}">
                                        <Separator HorizontalAlignment="Stretch" IsEnabled="False"/>
                                    </ControlTemplate>
                                </Setter.Value>
                            </Setter>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </ComboBox.ItemContainerStyle>
        </ComboBox>

And voila ! It works fine and keeps the logic in the ViewModel. If you have any other idea about how we can solve this issue, please feel free to comment