Question

I have an observable collection of Song objects. These song objects have a property called “Playing” that is a bool (bad naming, I know). The songs display in a ListBox in my application. I want the song that is Playing to be colored red. I have been working with triggers all day trying to make this work. So far, I have gotten to the point where they are colored based on what Playing is set to when the song is added to the list. Is it possible to make it change when the Playing property changes?
Here is my XAML:

<Window x:Class="MusicPlayer.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:MusicPlayer="clr-namespace:MusicPlayer" Title="Music" Height="350" Width="525" Name="Main">
    <Grid>
        <ListBox HorizontalAlignment="Stretch"  Name="Playlist" VerticalAlignment="Stretch" Margin="0,23,0,79" MouseDoubleClick="Playlist_MouseDoubleClick">
            <ListBox.Resources>
                <Style TargetType="ListBoxItem">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding Path=Playing}" Value="True">
                            <Setter Property="Background" Value="Red" />
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
                <DataTemplate DataType="{x:Type MusicPlayer:Song}">
                    <TextBlock Text="{Binding Path=Title}"/>
                </DataTemplate>
            </ListBox.Resources>

        </ListBox>
        <Button Content="Play" Height="23" HorizontalAlignment="Center" Margin="0,0,330,20" Name="PlayButton" VerticalAlignment="Bottom" Width="75" Click="PlayButton_Click" />
        <Button Content="Stop" Height="23" HorizontalAlignment="Center" Margin="0,0,165,20" Name="stopButton" VerticalAlignment="Bottom" Width="75" Click="stopButton_Click" />
        <Button Content="Next" Height="23" HorizontalAlignment="Center" Margin="165,0,0,20" Name="nextButton" VerticalAlignment="Bottom" Width="75" Click="nextButton_Click" />
        <Button Content="Add Songs..." Height="23" HorizontalAlignment="Center" Margin="330,0,0,20" Name="AddButton" VerticalAlignment="Bottom" Width="75" Click="AddButton_Click" />
        <Button Content="Previous" Height="23" HorizontalAlignment="Center" Margin="0,0,0,20" Name="PreviousButton" VerticalAlignment="Bottom" Width="75" Click="PreviousButton_Click" />
        <Button Content="Pause" Height="23" HorizontalAlignment="Center" Margin="0,0,330,20" Name="PauseButton" VerticalAlignment="Bottom" Width="75" Visibility="Hidden" Click="PauseButton_Click" />
        <Menu Height="23" HorizontalAlignment="Stretch" Name="menu1" VerticalAlignment="Top">
            <MenuItem Header="File">
                <MenuItem Header="Quit" Click="MenuItem_Click" />
            </MenuItem>
        </Menu>
            <Slider Height="23" HorizontalAlignment="Stretch" Margin="10,0,10,50" Name="ProgressBar" VerticalAlignment="Bottom" />
    </Grid>
</Window>

Edit:

After implementign INotifyPropertyChanged, The color will change to red using either the above or below XAML. Now, it won’t change back white using either method. I am also having a problem where my Play method will not change the background color to red, but my next method will. Here is my code: http://pastebin.com/EMTUpTin http://pastebin.com/LuK78zGp
Here is my new XAML:

<Window x:Class="MusicPlayer.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:MusicPlayer="clr-namespace:MusicPlayer" Title="Music" Height="350" Width="525" Name="Main">
    <Grid>
        <ListBox HorizontalAlignment="Stretch"  Name="Playlist" VerticalAlignment="Stretch" Margin="0,23,0,79" MouseDoubleClick="Playlist_MouseDoubleClick">
            <ListBox.Resources>
                <DataTemplate DataType="{x:Type MusicPlayer:Song}">
                    <TextBlock Text="{Binding Path=Title}">
                        <TextBlock.Background>
                            <SolidColorBrush Color="{Binding BackgroundColor}"/>
                        </TextBlock.Background>
                    </TextBlock>
                </DataTemplate>
            </ListBox.Resources>
        </ListBox>
        <Button Content="Play" Height="23" HorizontalAlignment="Center" Margin="0,0,330,20" Name="PlayButton" VerticalAlignment="Bottom" Width="75" Click="PlayButton_Click" />
        <Button Content="Stop" Height="23" HorizontalAlignment="Center" Margin="0,0,165,20" Name="stopButton" VerticalAlignment="Bottom" Width="75" Click="stopButton_Click" />
        <Button Content="Next" Height="23" HorizontalAlignment="Center" Margin="165,0,0,20" Name="nextButton" VerticalAlignment="Bottom" Width="75" Click="nextButton_Click" />
        <Button Content="Add Songs..." Height="23" HorizontalAlignment="Center" Margin="330,0,0,20" Name="AddButton" VerticalAlignment="Bottom" Width="75" Click="AddButton_Click" />
        <Button Content="Previous" Height="23" HorizontalAlignment="Center" Margin="0,0,0,20" Name="PreviousButton" VerticalAlignment="Bottom" Width="75" Click="PreviousButton_Click" />
        <Button Content="Pause" Height="23" HorizontalAlignment="Center" Margin="0,0,330,20" Name="PauseButton" VerticalAlignment="Bottom" Width="75" Visibility="Hidden" Click="PauseButton_Click" />
        <Menu Height="23" HorizontalAlignment="Stretch" Name="menu1" VerticalAlignment="Top">
            <MenuItem Header="File">
                <MenuItem Header="Quit" Click="MenuItem_Click" />
            </MenuItem>
        </Menu>
        <Slider Height="23" HorizontalAlignment="Stretch" Margin="10,0,10,50" Name="ProgressBar" VerticalAlignment="Bottom" />
    </Grid>
</Window>

Edit 2:
I just noticed I am having another problem. If I highlight one of the items with a white background, the text is invisible. How do I fix this?
Edit 3:
Fixed this problem by setting the background to Colors.Transparent instead of Colors.White.

Answer 1

Does your object with the Playing property implement INotifyPropertyChanged ? If it does, then your UI should auto-update based on the DataTrigger approach you are using.

Another approach is to use ViewModels instead of Triggers (easier to understand and work with – when things don’t go as expected) An example

Update:
Just looked at your code snippets. One thing I found – you need to trigger the event after you’ve applied the new value.

public Color BackgroundColor
        {
            get { return _backgroundColor; }

            set
            {
                _backgroundColor = value;
                NotifyPropertyChanged("BackgroundColor"); // must be after so that new value is readable by the GUI
            }
        }

Also the dataTemplate must be applied to the ItemTemplate property of the listbox

<ListBox.ItemTemplate> <!-- not Resources property -->
                <DataTemplate DataType="{x:Type MusicPlayer:Song}">

I basically changed the snippet from the example that I posted to use your song class and then modified the button click to toggle between 2 colors. (Also turned ListView into ListBox)

private bool _isGreen = false;
        private void Button_Click(object sender, RoutedEventArgs e)
        {

            foreach (var item in Items)
                item.BackgroundColor = (_isGreen ? Colors.Cyan : Colors.PaleGreen );
            _isGreen = !_isGreen;
        }

My listboxes change color on every click! 🙂