Datatrigger to change a ListView ItemsSource not firing - wpf

Question

I am trying to change the ItemsSource Property of a ListView, depending on if a collection contains elements or is null.
The ListView currently looks like this:
<Control.Resources>
<Style TargetType="{x:Type GridViewColumnHeader}">
<Setter Property="HorizontalContentAlignment" Value="Left" />
</Style>
<Style TargetType="{x:Type ListViewItem}">
<EventSetter Event="MouseDoubleClick" Handler="GridFiles_MouseDoubleClick" />
</Style>
<CollectionViewSource x:Key="DirectoryFiles" Source="{Binding Path=CurrentDirectory.Files}"/>
<CollectionViewSource x:Key="DirectorySubDirs" Source="{Binding Path=CurrentDirectory.SubDirectories}"/>
<CompositeCollection x:Key="CombinedCollection">
<CollectionContainer Collection="{Binding Source={StaticResource DirectorySubDirs}}" />
<CollectionContainer Collection="{Binding Source={StaticResource DirectoryFiles}}" />
</CompositeCollection>
</Control.Resources>
<Grid>
<ListView x:Name="gridFiles" Grid.Row="2" Grid.Column="1" Margin="2"
SelectedItem="{Binding Path=SelectedFileGridItem}"
ItemsSource="{StaticResource CombinedCollection}">
<ListView.View>
<GridView>
<GridViewColumn Header="Name" DisplayMemberBinding="{Binding Name}" />
<GridViewColumn Header="Extension" DisplayMemberBinding="{Binding Extension}" />
<GridViewColumn Header="Checksum" DisplayMemberBinding="{Binding Checksum}" />
</GridView>
</ListView.View>
</ListView>
<Grid>
In my viewmodel there is an IEnumerable<FileNodeViewModel> property called SearchResults which is filled with results after performing a search, otherwise its null.
If I manually set the ItemsSource of the ListView property to {Binding SearchResults} the search results are displayed as intended, so I am sure that the data is correctly formated and available to the Listview.
In this thread I found a similar question and appended the xaml code:
<Control.Resources>
<Style TargetType="{x:Type GridViewColumnHeader}">
<Setter Property="HorizontalContentAlignment" Value="Left" />
</Style>
<Style TargetType="{x:Type ListViewItem}">
<EventSetter Event="MouseDoubleClick" Handler="GridFiles_MouseDoubleClick" />
</Style>
<Style TargetType="ListView">
<Style.Triggers>
<DataTrigger Binding="{Binding SearchResults}" Value="All">
<Setter Property="ItemsSource" Value="{Binding SearchResults}" />
</DataTrigger>
</Style.Triggers>
</Style>
<CollectionViewSource x:Key="DirectoryFiles" Source="{Binding Path=CurrentDirectory.Files}"/>
<CollectionViewSource x:Key="DirectorySubDirs" Source="{Binding Path=CurrentDirectory.SubDirectories}"/>
<CompositeCollection x:Key="CombinedCollection">
<CollectionContainer Collection="{Binding Source={StaticResource DirectorySubDirs}}" />
<CollectionContainer Collection="{Binding Source={StaticResource DirectoryFiles}}" />
</CompositeCollection>
</Control.Resources>
<Grid>
<ListView x:Name="gridFiles" Grid.Row="2" Grid.Column="1" Margin="2"
SelectedItem="{Binding Path=SelectedFileGridItem}"
ItemsSource="{StaticResource CombinedCollection}">
<ListView.View>
<GridView>
<GridViewColumn Header="Name" DisplayMemberBinding="{Binding Name}" />
<GridViewColumn Header="Extension" DisplayMemberBinding="{Binding Extension}" />
<GridViewColumn Header="Checksum" DisplayMemberBinding="{Binding Checksum}" />
</GridView>
</ListView.View>
</ListView>
<Grid>
However, nothing changes. No matter if the SearchResults property is null or not, the ListView always displays the combined collection. I also tried to create a boolan property to indicate if the second collection should be displayed or not, with the same result:
<Style TargetType="ListView">
<Style.Triggers>
<DataTrigger Binding="{Binding ShowSearchResults}" Value="True">
<Setter Property="ItemsSource" Value="{Binding SearchResults}" />
</DataTrigger>
</Style.Triggers>
</Style>
Does anybody know how what I should do to correctly change the ItemsSource property?

Answer 1

So you want to bind to SearchResults only if it's not null, or else you wan to bind to CombinedCollection? You could then use a DataTrigger with a value of {x:Null}:
<ListView x:Name="gridFiles" Grid.Row="2" Grid.Column="1" Margin="2"
SelectedItem="{Binding Path=SelectedFileGridItem}">
<ListView.Style>
<Style TargetType="ListView">
<Setter Property="ItemsSource" Value="{Binding SearchResults}" />
<Style.Triggers>
<DataTrigger Binding="{Binding SearchResults}" Value="{x:Null}">
<Setter Property="ItemsSource" Value="{StaticResource CombinedCollection}" />
</DataTrigger>
</Style.Triggers>
</Style>
</ListView.Style>
<ListView.View>
<GridView>
<GridViewColumn Header="Name" DisplayMemberBinding="{Binding Name}" />
<GridViewColumn Header="Extension" DisplayMemberBinding="{Binding Extension}" />
<GridViewColumn Header="Checksum" DisplayMemberBinding="{Binding Checksum}" />
</GridView>
</ListView.View>
</ListView>
Make sure that you only set the ItemsSource property in the Style Setter.

Answer 2

So, after some trial and error I found the issue. Everything was pretty much set up correctly, but I was not aware that styles can only append properties to an element, not replace existing ones.
To test this, I created a new ListView:
<ListView Grid.Row="2" Grid.Column="1" Margin="2"
ItemsSource="{StaticResource CombinedCollection}"
Foreground="Blue">
<ListView.Style>
<Style TargetType="ListView">
<Style.Triggers>
<DataTrigger Binding="{Binding ShowSearchResults}" Value="True">
<Setter Property="Foreground" Value="Red" />
</DataTrigger>
</Style.Triggers>
</Style>
</ListView.Style>
<ListView.View>
<GridView>
<GridViewColumn Header="Name" DisplayMemberBinding="{Binding Name}" />
</GridView>
</ListView.View>
</ListView>
The data was displayed correctly, with blue foreground color. If I changed ShowSearchResults to True, nothing happened, the color still was blue. So I removed the foreground property:
<ListView Grid.Row="2" Grid.Column="1" Margin="2"
ItemsSource="{StaticResource CombinedCollection}">
I retried the test, and suddenly the ListView items would turn red. Knowing this, I removed the explicit data source from the ListView definition and used two data triggers instead.
For reference, here is the working markup. Thanks a lot to mm8 for providing some great input about the formatting!
<ListView Grid.Row="2" Grid.Column="1" x:Name="gridFiles" Margin="2"
SelectedItem="{Binding Path=SelectedFileGridItem}">
<ListView.Style>
<Style TargetType="ListView">
<Setter Property="ItemsSource" Value="{StaticResource CombinedCollection}" />
<Style.Triggers>
<DataTrigger Binding="{Binding ShowSearchResults}" Value="True">
<Setter Property="ItemsSource" Value="{Binding SearchResults}" />
</DataTrigger>
</Style.Triggers>
</Style>
</ListView.Style>
<ListView.View>
<GridView>
<GridViewColumn Header="Name" DisplayMemberBinding="{Binding Name}" />
<GridViewColumn Header="Extension" DisplayMemberBinding="{Binding Extension}" />
<GridViewColumn Header="Checksum" DisplayMemberBinding="{Binding Checksum}" />
</GridView>
</ListView.View>
</ListView>