Amadeusz Wieczorek

Let’s build a sleek way to display notifications to the user. The messages won’t show up in traditional pop-up windows, but instead slide in at the top of the screen. User can dismiss each one of them by clicking at it.

Complete project is available on GitHub and this blog post covers only bits related to animation.

Each message animates twice: first, when it’s added to the collection and first rendered, and second time, disappearing after user dismissed it.

Both animations are defined as resources of ItemsControl, and invoked by DataTrigger in ItemsControl.ItemContainerStyle. We are using ItemsControl element because it offers the most rendering flexibility.

<ItemsControl.Resources>
  <Storyboard x:Key="enterStoryboard">
      <ThicknessAnimation Storyboard.TargetProperty="Margin"
                                                   Duration="00:00:00.25"
                                                   From="0, 0, 500, 0"
                                                   To="0, 0, 0, 0"
                                                   DecelerationRatio="1"
                                                   FillBehavior="HoldEnd" />
      <DoubleAnimation Storyboard.TargetProperty="Opacity"
                                                   Duration="00:00:00.25"
                                                   From="0"
                                                   To="1"
                                                   FillBehavior="HoldEnd" />
  </Storyboard>
</ItemsControl.Resources>
 
<ItemsControl.ItemContainerStyle>
  <Style>
    <Style.Triggers>
      <DataTrigger Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=DataContext.IsAlive}" Value="True">
        <DataTrigger.EnterActions>
          <BeginStoryboard>
            <StaticResource ResourceKey="enterStoryboard"/>
          </BeginStoryboard>
        </DataTrigger.EnterActions>
      </DataTrigger>
    </Style.Triggers>
  </Style>
</ItemsControl.ItemContainerStyle>

The DataTrigger fires the animation whenever property IsAlive is found to be true. Here, we instantiate it to true in each MessageObject.

Let’s see how it looks like:

The slide-out effect for removed messages gets a bit more tricky. Once the element is removed from the collection, we can’t animate it, because its graphical representation disappears.

We will use the property IsAlive to trigger the slide-out animation, and we will actually remove the message from the collection after the animation completes. Similarly to the enter animation, the exit animation is triggered by setting isAlive to false. This can be defined twofold in XAML:

<!-- Simple approach, but may break if IsAlive is not initialized to true -->
 
<DataTrigger Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=DataContext.IsAlive}" Value="True">
    <DataTrigger.EnterActions>
        <BeginStoryboard>
            <StaticResource ResourceKey="enterStoryboard"/>
        </BeginStoryboard>
    </DataTrigger.EnterActions>
    <DataTrigger.ExitActions>
        <BeginStoryboard>
            <StaticResource ResourceKey="exitStoryboard"/>
        </BeginStoryboard>
    </DataTrigger.ExitActions>
</DataTrigger>
 
<!-- Another approach. In our case, it has the same result.
     It may be used to create more complex and robust triggers -->
 
<DataTrigger Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=DataContext.IsAlive}" Value="True">
  <DataTrigger.EnterActions>
    <BeginStoryboard>
      <StaticResource ResourceKey="enterStoryboard"/>
    </BeginStoryboard>
  </DataTrigger.EnterActions>
</DataTrigger>
<DataTrigger Binding="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=DataContext.IsAlive}" Value="False">
  <DataTrigger.EnterActions>
    <BeginStoryboard>
      <StaticResource ResourceKey="exitStoryboard"/>
    </BeginStoryboard>
  </DataTrigger.EnterActions>
</DataTrigger>        

Ideally, we would listen to StoryboardCompleted event and remove the item once the framework tell us that the animation has completed. However, the StoryboardCompleted event doesn’t have a reference to the removed item or the collection. How would we know which item to remove from the collection?

We need to wait agreed amount of time in the code-behind before the element can be removed from the collection. This is not an ideal solution, but I can’t think of another way to achieve this.

internal void RemoveMessage(object parameter)
{
    var message = parameter as MessageObject;
    if (message != null)
    {
        // Trigger the move-out animation
        message.IsAlive = false;
        // Remove the element after animation completes
        Task.Run(() =>
            {
                Thread.Sleep(250);
                acutallyRemoveMessageFromCollection(message);
            }
        );
    }
}
 
private void acutallyRemoveMessageFromCollection(MessageObject message)
{
    // Make sure this runs on the UI thread.
    Application.Current.Dispatcher.BeginInvoke
        (System.Windows.Threading.DispatcherPriority.Normal,
        (Action)(() =>
            {
                Messages.Remove(message);
            }
        ));
}

Here’s how it looks like:

We can see the item occupies its space as it animates out, and other items abruptly jump in its space after the item is removed. We can address that by adding a slide-up animation to the exitStoryboard. The second animation starts only when the first animation ends (note the BeginTime). The animation changes the top margin of the item to -50, effectively dragging it up. We’re happy to see that all items below it also move up!

<Storyboard x:Key="exitStoryboard">
    <ThicknessAnimation Storyboard.TargetProperty="Margin"
                                                     Duration="00:00:00.25"
                                                     From="0, 0, 0, 0"
                                                     To="500, 0, 0, 0"
                                                     AccelerationRatio="1"                                        
                                                     FillBehavior="HoldEnd" />
    <DoubleAnimation Storyboard.TargetProperty="Opacity"
                                                 Duration="00:00:00.25"
                                                 From="1"
                                                 To="0"
                                                 FillBehavior="HoldEnd" />
    <!-- After sliding out to the right, slide up so that remaining items 
         in the collection smoothly fill the gap -->
    <ThicknessAnimation Storyboard.TargetProperty="Margin"
                                                     BeginTime="00:00:00.25"
                                                     Duration="00:00:00.25"
                                                     From="500, 0, 0, 0"
                                                     To="500, -50, 0, 0"
                                                     DecelerationRatio="1"                                        
                                                     FillBehavior="HoldEnd" />                
</Storyboard>

Now we only need to change the code-behind to wait for both animations to finish before removing the element, so we change the delay from 250ms to 500ms. That’s the final effect:

For best results, this control can be placed above the rest of your app with help of Z-index