Event Trigger not working on Storyboard

Question

I was just hoping you could help fill a gap in my WPF knowledge.

(please forgive the generic naming, not sure if it helps)

I've a custom object, MyObject, that implements INotifyPropertyChanged. It has a property called MyCustomProperty, as follows;

public int MyCustomProperty
{
 get { return this._myCustomProperty; }
 set
 {
  if (this._myCustomProperty == value)
    return;
  this._myCustomProperty= value;
  OnPropertyChanged("MyCustomProperty");
 }
}

This all works. In my WPF app I have these 3 functions;

private void DoStuff()
{
   AddItemsToCanvas();
   ChangeValues();
}

private void AddItemsToCanvas()
{
   DataTemplate dt = (DataTemplate)FindResource("myDataTemplate");
   foreach (MyObject temp in ListOfMyObjects)
   {
     ContentControl cc = new ContentControl();
     cc.ContentTemplate = dt;
     cc.Content = temp;
     myCanvas.Children.Add(cc);
   }
}

private void ChangeValues()
{
  // this simply changes the MyCustomPropery in each of the objects
}

The DataTemplate looks like this;

<DataTemplate x:Key="myDataTemplate">
 <Canvas>
  <TextBlock Name="tb_debug" Text="{Binding Path=MyCustomProperty, NotifyOnTargetUpdated=True}">
   <TextBlock.Triggers>
    <EventTrigger RoutedEvent="Binding.TargetUpdated">
     <BeginStoryboard>
      <Storyboard>
       <DoubleAnimation Storyboard.TargetName="tb_debug" Storyboard.TargetProperty="(Canvas.Top)" From="0" To="350" Duration="0:0:1.6" />
      </Storyboard>
     </BeginStoryboard>
    </EventTrigger>
   </TextBlock.Triggers>
  </TextBlock>
 </Canvas>
</DataTemplate>

It simply moves the textbox from the top to the bottom of the canvas when MyCustomProperty changes.

When the UserControl is Loaded, I call both AddItemsToCanvas() and ChangeValues(). The Text value updates and displays the correct value, but the Trigger doesn't fire (ie the TextBox doesn't move). Any time after that, when I call ChangeValues() the Text updates AND the TextBox moves.

Why would the EventTrigger be failing that initial time? thanks in advance

Answer 1

I'm not sure why the Binding.TargetUpdated does not get called, but it could be only called when a the target is updated once already set and not when its first set (maybe).

But you could just add another EventTrigger on TextBlock Loaded in your DataTremplate to make sure it fires on UserControl Load.

Example:

<DataTemplate x:Key="myDataTemplate">
    <DataTemplate.Resources>
        <Storyboard x:Key="animation" >
            <DoubleAnimation Storyboard.TargetName="tb_debug" Storyboard.TargetProperty="(Canvas.Top)" From="0" To="350" Duration="0:0:1.6" />
        </Storyboard>
    </DataTemplate.Resources>
    <Canvas>
        <TextBlock Name="tb_debug" Text="{Binding Path=MyCustomProperty, NotifyOnTargetUpdated=True}">
           <TextBlock.Triggers>
            <EventTrigger RoutedEvent="Binding.TargetUpdated">
                <BeginStoryboard Storyboard="{StaticResource animation}" />
            </EventTrigger>
               <EventTrigger RoutedEvent="Loaded">
                <BeginStoryboard Storyboard="{StaticResource animation}" />
            </EventTrigger>
           </TextBlock.Triggers>
        </TextBlock>
    </Canvas>
</DataTemplate>