C# – How to add an Event Trigger to a data template for a business object

cdatatemplateeventseventtriggerwpf

I have a custom class named BlinkingLight.
I also have a static ObservableCollection BlinkingLightCollection.
In the UI, I have a ListBox that is bound to BlinkingLightCollection.

In my ListBox I want to essentially display each BlinkingLight object as a custom control that looks like box with an LED light that has an animation that makes the LED look like it just flashed on for a second then goes back to normal.

My BlinkingLight class has third party "LED" object that raises an event called 'Flash'.

I am looking for ideas or solutions to get this to work!

My failed attempt:

I created a custom control (BlinkingLightControl) that can bind to the data of my BlinkingLight class when a BlinkingLight is the DataContext of my custom control.

I created a DataTemplate for my ListBox:

<Window.Resources>
  <DataTemplate x:Key="blinkingLightItemTemplate" >
    <local:BlinkingLightControl />
  </DataTemplate>
</Window.Resources>

<ListBox ItemsSource={Binding Source={x:Static local:Data.BlinkingLightCollection}}
         ItemTemplate="{StaticResource blinkingLightItemTemplate}" />

Note: I can just put the xaml for my custom control into the datatemplate instead having a completely different control if that makes things easier.

Now I want to have an EventTrigger in my BlinkingLightControl (or DataTemplate) who's RoutedEvent is the LED.Flash event. Unfortunately I can't seem to figure this part out. I've tried to create a RoutedEvent in my BlinkingLight class and just raise it whenever I handle the LED.Flash event. However my class is not a UIElement or ContentElement, and per MSDN: MSND Link

"The routed event owner can be any class, but routed events must be raised by and handled by UIElement or ContentElement derived classes in order to be useful. For more information about custom events, see How to: Create a Custom Routed Event."

Any help would be greatly appreciated!!
Thanks,
Scott

Best Answer

In this case, the best way is to use WPF Commanding and create a "BlinkTheLights" RoutedCommand - your BlinkingLightControl will handle the BlinkTheLights command, and respond by starting a StoryBoard which does the light blink.

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No matter which TabItem I set to be selected at startup, that tab will be displayed fine, but when I click on the other tab the application crashes with an 'Exception has been thrown by the target of an invocation' error pointing toward the DataTemplate for the tab I'm switching to.

Enable first-chance exceptions, so you can find out the actual exception instead of the wrapped outer exception - what's going wrong will be much clearer.

Wpf – How to have a ListBox auto-scroll when a new item is added

You can extend the behavior of the ListBox by using attached properties. In your case I would define an attached property called ScrollOnNewItem that when set to true hooks into the INotifyCollectionChanged events of the list box items source and upon detecting a new item, scrolls the list box to it.

Example:

class ListBoxBehavior
{
    static readonly Dictionary<ListBox, Capture> Associations =
           new Dictionary<ListBox, Capture>();

    public static bool GetScrollOnNewItem(DependencyObject obj)
    {
        return (bool)obj.GetValue(ScrollOnNewItemProperty);
    }

    public static void SetScrollOnNewItem(DependencyObject obj, bool value)
    {
        obj.SetValue(ScrollOnNewItemProperty, value);
    }

    public static readonly DependencyProperty ScrollOnNewItemProperty =
        DependencyProperty.RegisterAttached(
            "ScrollOnNewItem",
            typeof(bool),
            typeof(ListBoxBehavior),
            new UIPropertyMetadata(false, OnScrollOnNewItemChanged));

    public static void OnScrollOnNewItemChanged(
        DependencyObject d,
        DependencyPropertyChangedEventArgs e)
    {
        var listBox = d as ListBox;
        if (listBox == null) return;
        bool oldValue = (bool)e.OldValue, newValue = (bool)e.NewValue;
        if (newValue == oldValue) return;
        if (newValue)
        {
            listBox.Loaded += ListBox_Loaded;
            listBox.Unloaded += ListBox_Unloaded;
            var itemsSourcePropertyDescriptor = TypeDescriptor.GetProperties(listBox)["ItemsSource"];
            itemsSourcePropertyDescriptor.AddValueChanged(listBox, ListBox_ItemsSourceChanged);
        }
        else
        {
            listBox.Loaded -= ListBox_Loaded;
            listBox.Unloaded -= ListBox_Unloaded;
            if (Associations.ContainsKey(listBox))
                Associations[listBox].Dispose();
            var itemsSourcePropertyDescriptor = TypeDescriptor.GetProperties(listBox)["ItemsSource"];
            itemsSourcePropertyDescriptor.RemoveValueChanged(listBox, ListBox_ItemsSourceChanged);
        }
    }

    private static void ListBox_ItemsSourceChanged(object sender, EventArgs e)
    {
        var listBox = (ListBox)sender;
        if (Associations.ContainsKey(listBox))
            Associations[listBox].Dispose();
        Associations[listBox] = new Capture(listBox);
    }

    static void ListBox_Unloaded(object sender, RoutedEventArgs e)
    {
        var listBox = (ListBox)sender;
        if (Associations.ContainsKey(listBox))
            Associations[listBox].Dispose();
        listBox.Unloaded -= ListBox_Unloaded;
    }

    static void ListBox_Loaded(object sender, RoutedEventArgs e)
    {
        var listBox = (ListBox)sender;
        var incc = listBox.Items as INotifyCollectionChanged;
        if (incc == null) return;
        listBox.Loaded -= ListBox_Loaded;
        Associations[listBox] = new Capture(listBox);
    }

    class Capture : IDisposable
    {
        private readonly ListBox listBox;
        private readonly INotifyCollectionChanged incc;

        public Capture(ListBox listBox)
        {
            this.listBox = listBox;
            incc = listBox.ItemsSource as INotifyCollectionChanged;
            if (incc != null)
            {
                incc.CollectionChanged += incc_CollectionChanged;
            }
        }

        void incc_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (e.Action == NotifyCollectionChangedAction.Add)
            {
                listBox.ScrollIntoView(e.NewItems[0]);
                listBox.SelectedItem = e.NewItems[0];
            }
        }

        public void Dispose()
        {
            if (incc != null)
                incc.CollectionChanged -= incc_CollectionChanged;
        }
    }
}

Usage:

<ListBox ItemsSource="{Binding SourceCollection}" 
         lb:ListBoxBehavior.ScrollOnNewItem="true"/>

UPDATE As per Andrej's suggestion in the comments below, I added hooks to detect a change in the ItemsSource of the ListBox.

Best Answer

Related Solutions

Wpf – App crashes on changing tabs that contain ListBoxes with ItemTemplates

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