How to use trigger in datatemplate in WPF

Question

How to change property Fill of icon of current selected element in checkbox depending on IsChecked property?

My resource dictionary:

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <!-- (...) --> 
        </ResourceDictionary.MergedDictionaries>
        <DataTemplate x:Key="FooTemplate">
            <Icons:ExIcon Fill="Red"
                          Width="12"
                          Height="Auto">
                <!--
                <Icons:ExIcon.Triggers>
                    <DataTrigger Binding="{Binding RelativeSource={???}}, Path=???}" Maybe here?
                       Value="???"/>
                </Icons:ExIcon.Triggers>
                -->
            </Icons:ExIcon>
        </DataTemplate> 
        <!-- (...) -->
        <styles:IconSelector x:Key="IconSelector"
                             FooTemplate="{StaticResource FooTemplate}"
                             FooTemplateSSecond="{StaticResource FooTemaplteSecond}"/>

And listbox:

<ListBox ItemsSource="{Binding DataSources}">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">
                <!-- (...) --> 
                <CheckBox IsChecked="{Binding IsSelected, Mode=TwoWay}">
                    <CheckBox.Template>
                        <!-- (...) --> 
                        <ContentControl Name="Icon"
                                        Content="{Binding}"
                                        ContentTemplateSelector="{StaticResource IconSelector}"
                                        HorizontalAlignment="Right"
                                        Grid.Column="1"/>
                        <!-- (...) --> 
                    </CheckBox.Template>
                </CheckBox>
            </StackPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

Is it possible?

Answer 1

It's not completely clear to me how your CheckBox.Template looks like. Also the Icons:ExIcon Control is a mystery to me. Anyway here is a small working example. It uses a Rectangle instead of your ExIcon. But you can easily replace it ;-) I've bound directly to IsSelected via a DataTrigger instead of searching by ElementName or RelativeSource.

XAML:

       <ListBox ItemsSource="{Binding}">
        <ListBox.Resources>
            <DataTemplate x:Key="template1">
                <StackPanel Orientation="Horizontal">
                    <CheckBox IsChecked="{Binding Path=IsSelected}" />
                    <Rectangle Height="20"
                               Width="20">
                        <Rectangle.Style>
                            <Style TargetType="Rectangle">
                                <Setter Property="Fill"
                                        Value="Blue" />
                                <Style.Triggers>
                                    <DataTrigger Binding="{Binding Path=IsSelected}"
                                                 Value="True">
                                        <Setter Property="Fill"
                                                Value="Red" />
                                    </DataTrigger>
                                </Style.Triggers>
                            </Style>
                        </Rectangle.Style>
                    </Rectangle>
                </StackPanel>
            </DataTemplate>
        </ListBox.Resources>
        <ListBox.ItemTemplate>
            <DataTemplate>
                <ContentControl ContentTemplate="{StaticResource template1}" Content="{Binding}">

                </ContentControl>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

NOTE Content="{Binding}". This sets the DataContext of the ContentPresenter to the "actual" DataContext of the ListBoxItem. If you use VS2015 you can investigate this with the VisualTreeViewer otherwise you can use https://snoopwpf.codeplex.com/

You should also consider to rename IsSelectedto IsChecked. Noramlly you use the IsSelected property for indicating if the row is selected or not.