Animations

First steps with Animations in Avalonia.

Sample

I created a very simple example below. A standard Avalonia application with only a couple of elements in the MainWindow.axaml file. For simplicity, I don’t have any view models, only a binding directly to the checkbox to control whether or not to show the Footer element.

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<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AvaloniaTransitions.MainWindow"
        Title="AvaloniaTransitions">
    <Grid RowDefinitions="*, Auto">
        
        <Border Grid.Row="0" Background="Navy">
            <StackPanel Orientation="Vertical" VerticalAlignment="Top">
                <CheckBox Name="CheckBox" Margin="20">Show Footer</CheckBox>
            </StackPanel>
        </Border>
        
        <Border Grid.Row="1" 
                IsVisible="{Binding #CheckBox.IsChecked}"
                BorderThickness="0 1 0 0"
                BorderBrush="Red"
                Background="DarkRed">
            <TextBlock Margin="20">Footer</TextBlock>
        </Border>
        
    </Grid>
</Window>

This is what happens when you toggle the checkbox:

Desired Result

It would be nice if the footer panel would fade in and slide in from the bottom when it’s shown and fade out as well as slide out when it’s hidden.

Failed Approaches

SplitView

My very first approach was to use the SplitView control. Unfortunately this only works for scenarios where the content to be shown is either on the left or right hand side. There’s a suggestion to expand the SplitView control, but it’s still not implemented.

KeyFrame Animations

My attempts to do this animation using KeyFrame animations all failed at some point. The whole thing was very unreliable and unpredictable. I wasn’t able to create the proper animation using KeyFrames to work in all scenarios. If you happen to know how to do it with KeyFrame animations, let me know in the comments.

Solution

I was able to successfully implement the animation using Transitions. To make this work, the following needs to be done:

Style Classes

In Avalonia, there’s a very convenient way to apply style classes based on binding values. Take a close look at lines 2 and 3 in the updated Footer border code:

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<Border Grid.Row="1" 
        Classes.show="{Binding #CheckBox.IsChecked}"
        Classes.hide="{Binding !#CheckBox.IsChecked}"
        BorderThickness="0 1 0 0"
        BorderBrush="Red"
        Background="DarkRed">
        <TextBlock Margin="20">Footer</TextBlock>
</Border>

Whenever the IsChecked property from the CheckBox is true, the show class is applied to the border. When the IsChecked property is false, the hide class is applied to the border. If you open the Dev Tools using F12, you can see the classes being applied when you toggle the check box.

Fade-In and Fade-Out

Now that we have those two classes applied to the border depending on the checked state of our checkbox, we need to create two styles and one transition to make the border fade-in and fade-out.

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<Border Grid.Row="1" 
        Classes.show="{Binding #CheckBox.IsChecked}"
        Classes.hide="{Binding !#CheckBox.IsChecked}"
        BorderThickness="0 1 0 0"
        BorderBrush="Red"
        Background="DarkRed">
    <Border.Styles>
        <Style Selector="Border.show">
            <Setter Property="Opacity" Value="1" />
        </Style>
        <Style Selector="Border.hide">
            <Setter Property="Opacity" Value="0" />
        </Style>
    </Border.Styles>
    <Border.Transitions>
        <Transitions>
            <DoubleTransition Property="Opacity" Duration="0:0:1" />
        </Transitions>
    </Border.Transitions>
    <TextBlock Margin="20">Footer</TextBlock>
</Border>

• In lines 8 and 9 we create a style which selects the border with the show class applied and set the Opacity property value to 1.
• In lines 11 and 12 we create a style for the border with thehide class applied and set the Opacity property value to 0.
• Lastly, in lines 15 through 19 we create a DoubleTransition for the Opacity property. For demonstration, I used 0:0:1 (1 second). In a real world application you may want this animation to be more subtle and set it to 0:0:0.2 (200 ms).

Handle Visibility

The challange we have now is that we need to set the IsVisible to false whenever the Opacity is 0 and set IsVisible to true whenever the Opacity is greater than 0. This can be achieved with another Style:

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<Border Grid.Row="1" 
        Classes.show="{Binding #CheckBox.IsChecked}"
        Classes.hide="{Binding !#CheckBox.IsChecked}"
        BorderThickness="0 1 0 0"
        BorderBrush="Red"
        Background="DarkRed">
    <Border.Styles>
        <Style Selector="Border.show">
            <Setter Property="Opacity" Value="1" />
        </Style>
        <Style Selector="Border.hide">
            <Setter Property="Opacity" Value="0" />
        </Style>
        <Style Selector="Border[Opacity=0]">
            <Setter Property="IsVisible" Value="False" />
        </Style>
    </Border.Styles>
    <Border.Transitions>
        <Transitions>
            <DoubleTransition Property="Opacity" Duration="0:0:1" />
        </Transitions>
    </Border.Transitions>
    <TextBlock Margin="20">Footer</TextBlock>
</Border>

Lines 14 through 16 demonstrate how to create a style like that.

Sliding Animation

Sliding animations can be done using RenderTransforms and the corresponding translate methods. In our case we only need the translateY methods.

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<Border Grid.Row="1" 
        Classes.show="{Binding #CheckBox.IsChecked}"
        Classes.hide="{Binding !#CheckBox.IsChecked}"
        BorderThickness="0 1 0 0"
        BorderBrush="Red"
        Background="DarkRed">
    <Border.Styles>
        <Style Selector="Border.show">
            <Setter Property="Opacity" Value="1" />
            <Setter Property="RenderTransform" Value="translateY(0px)" />
        </Style>
        <Style Selector="Border.hide">
            <Setter Property="Opacity" Value="0" />
            <Setter Property="RenderTransform" Value="translateY(75px)" />
        </Style>
        <Style Selector="Border[Opacity=0]">
            <Setter Property="IsVisible" Value="False" />
        </Style>
    </Border.Styles>
    <Border.Transitions>
        <Transitions>
            <DoubleTransition Property="Opacity" Duration="0:0:0.2" />
            <TransformOperationsTransition Property="RenderTransform" Duration="0:0:0.2" />
        </Transitions>
    </Border.Transitions>
    <TextBlock Margin="20">Footer</TextBlock>
</Border>

In lines 10 and 14 I added another Setter for the show and hide class of our border. The property RenderTransform should have a translateY offset of 75px on the y-axis when hidden and no offset (0px) when shown. In line 23 we need to add a TransformOperationsTransition for the RenderTransform property.

That’s it. The result (with shorter timings) looks like this:

Conclusion

Note, that I intentionally set those background colors for demonstration purposes to show the impact on the layout when handling the IsVisible property. The effect that the Footer bottom area is hidden after the animation may not be that visible depending on your layout.

So one improvement to make this smoother would be to not deal with the IsVisible property the way I did and simply just let the Footer seemlessly slide-in and out. Not sure how this can be done. Another thing which is not ideal is the hardcoded translateY value (75px). A solution where just the desired height of the Footer is taken would be much better. Also not sure how this can be accomplished.

I’m still learing Avalonia and there’s a good chance that there are other ways to accomplish a similar effect. If you have suggestions or different solutions on how to implement these animations, let me know in the comments.