[WPF] Checkboxes in ComboBox - display number of checked items when closed

[NickThissen]

Hi,

I need to have a ComboBox that displays a list of CheckBoxes when dropped down so the user can check the relevant ones and then close it again. When closed, I want the ComboBox (the 'Text' area) to display the number of checked items (for example "Checked items: 3").

I cannot seem to figure this out... I can get the CheckBoxes in the ComboBox, that's easy enough using a DataTemplate, but then it also uses the same template for the 'selected item' and thus shows the last clicked ("selected") CheckBox in the text area. Selecting an item is actually not really defined for this ComboBox, I just need the user to check some items and be done with it, he doesn't need to select any items.

I've tried looking for solutions, and there seem to be a few different solutions to having a different template for the selected item as for the dropdown items, but none seem to work for me...

The code-behind for all of my attempts is:

Imports System.Collections.ObjectModel
 
Class MainWindow
 
    Private items As ObservableCollection(Of ComboBoxItem)
 
    Public Sub New()
 
        ' This call is required by the designer.
        InitializeComponent()
 
        ' Add any initialization after the InitializeComponent() call.
 
        items = New ObservableCollection(Of ComboBoxItem)
        For i As Integer = 1 To 5
            items.Add(New ComboBoxItem() With {.Text = "Item " & i})
        Next
 
        comboBox.ItemsSource = items
 
    End Sub
 
    Private ReadOnly Property CheckedItems As Integer
        Get
            Return items.Where(Function(cbi) cbi.IsChecked).Count()
        End Get
    End Property
 
End Class
 
Public Class ComboBoxItem
 
    Private _Text As String
    Public Property Text() As String
        Get
            Return _Text
        End Get
        Set(ByVal value As String)
            _Text = value
        End Set
    End Property
 
    Private _IsChecked As Boolean
    Public Property IsChecked() As Boolean
        Get
            Return _IsChecked
        End Get
        Set(ByVal value As Boolean)
            _IsChecked = value
        End Set
    End Property
 
End Class

Attempt 1:
Source
My XAML code:

    <Window.Resources>   
        <ControlTemplate x:Key="X" TargetType="{x:Type ContentControl}">
            <CheckBox Content="{Binding Text}" IsChecked="{Binding IsChecked}" />
        </ControlTemplate>
    </Window.Resources>
    
    <Grid>
        <ComboBox Name="comboBox" VerticalAlignment="Top" Margin="10">                  
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=CheckedItems}" />
                </DataTemplate>
            </ComboBox.ItemTemplate>
            <ComboBox.ItemContainerStyle>
                <Style TargetType="{x:Type ComboBoxItem}">
                    <Setter Property="Margin" Value="5"/>
                    <Setter Property="Template" Value="{StaticResource X}" />
                </Style>
            </ComboBox.ItemContainerStyle>
        </ComboBox>
    </Grid>

I am trying to bind the TextBlock Text property to the 'CheckedItems' property in my Window, I hope I'm not screwing that up...

Anyway, the result is a dropdown containing CheckBox items which I can check (correct), but after closing, the 'text area' remains empty. Even if I select an item (which isn't really defined for me) it displays nothing.



(Text too long, see post 2...)

[NickThissen]

Attempt 2:
Source
My XAML:

    <Window.Resources>   
        <DataTemplate x:Key="ComplexTemplate">
            <CheckBox Content="{Binding Text}" IsChecked="{Binding IsChecked}" />
        </DataTemplate>
        <ControlTemplate x:Key="SimpleTemplate">
            <TextBlock Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=CheckedItems}" />
        </ControlTemplate>
    </Window.Resources>
    
    <Grid>
        <ComboBox Name="comboBox" VerticalAlignment="Top" Margin="10">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel>
                        <ContentControl x:Name="content" Content="{Binding}" ContentTemplate="{StaticResource ComplexTemplate}"/>
                    </StackPanel>
                    <DataTemplate.Triggers>
                        <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=ComboBoxItem}}" Value="{x:Null}">
                            <Setter TargetName="content" Property="ContentTemplate" Value="{StaticResource SimpleTemplate}"/>
                        </DataTrigger>
                    </DataTemplate.Triggers>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </Grid>

This produces an error as soon as I start, saying it cannot convert a ControlTemplate to a DataTemplate...


Attempt 3:
Finally I simply tried to bind the Text property of the ComboBox to the property in my Window (same as before), but nothing... The text area stays empty.



I'm kinda out of ideas... What else can I do? This can't be so hard?

[Evil_Giraffe]

Sounds to me like you don't actually want a ComboBox, but rather an Expander. What you're describing is really strange behaviour for a ComboBox. The Expander control will give you that "hiding" behaviour that you're after, without the strangeness of the ComboBox implementation.

[Evil_Giraffe]

To fix your issue with the Count of items.

First off, I took your code and removed the code behind into its own class, MainWindowViewModel.

In the constructor of the Window, I instantiated one of them and set it as the DataContext of the Window:

Class MainWindow
 
    Public Sub New()
 
        ' This call is required by the designer.
        InitializeComponent()
 
        Me.DataContext = New MainWindowViewModel()
 
    End Sub
 
End Class

Normally, you'd set the DataContext from outside the Window, but this'll do for now.

Now, our XAML looks a bit like this:

<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <Expander>
            <ListView ItemsSource="{Binding Items}">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <CheckBox Content="{Binding Text}" IsChecked="{Binding IsChecked}" />
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </Expander>
        <TextBlock Text="{Binding CheckedCount}" />
    </StackPanel>
</Window>

(I've changed some of the property names slightly)

Now, the problem with the Binding for CheckedCount is that it needs to know when the CheckedCount property changes. Which it can't at the moment. So, get that ViewModel class implementing INotifyPropertyChanged so it can raise the PropertyChanged event when the items are checked on/off. But we don't yet know that so let's add an event to the ComboBoxItem class that lets us know when it gets checked:

Public Class ComboBoxItem
    ' ...
    Private _IsChecked As Boolean
    Public Property IsChecked() As Boolean
        Get
            Return _IsChecked
        End Get
        Set(ByVal value As Boolean)
            _IsChecked = value
            RaiseEvent IsCheckedChanged(Me, EventArgs.Empty)
        End Set
    End Property
 
    Public Event IsCheckedChanged(sender As Object, eventArgs As EventArgs)
 
End Class

That lets us hook on to the event, and raise our own event in the ViewModel:

Imports System.Collections.ObjectModel
Imports System.ComponentModel
 
Public Class MainWindowViewModel
    Implements INotifyPropertyChanged
 
    Private m_items As ObservableCollection(Of ComboBoxItem)
 
    Public Sub New()
 
        m_items = New ObservableCollection(Of ComboBoxItem)
        For i As Integer = 1 To 5
            Dim comboBoxItem As New ComboBoxItem() With {.Text = "Item " & i}
            AddHandler comboBoxItem.IsCheckedChanged, AddressOf Me.ComboBoxItemIsCheckedChanged
 
            m_items.Add(comboBoxItem)
        Next
 
    End Sub
 
    Public ReadOnly Property Items As ObservableCollection(Of ComboBoxItem)
        Get
            Return m_items
        End Get
    End Property
 
    Public ReadOnly Property CheckedCount As Integer
        Get
            Return m_items.Where(Function(cbi) cbi.IsChecked).Count()
        End Get
    End Property
 
    Public Event PropertyChanged(ByVal sender As Object, ByVal e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged
 
    Private Sub ComboBoxItemIsCheckedChanged(ByVal sender As Object, ByVal eventArgs As EventArgs)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs("CheckedCount"))
 
    End Sub
 
End Class