Dynamic View Animations using MVVM

Question

I've been trying to figure out how to effectively trigger animations in a View when a property in the ViewModel updates, where the animation depends on the value of said property.

I've recreated my problem in a simple application with a single View and ViewModel. The goal here is to transition the color change of a rectangle by using a ColorAnimation. For reference, I've been using the MVVM Foundation package by Josh Smith.

The example project can be downloaded here.

To summarize, I want to animate the color transition in the View whenever the Color property changes.

MainWindow.xaml

<Window x:Class="MVVM.ColorAnimation.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:ColorAnimation="clr-namespace:MVVM.ColorAnimation" Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <ColorAnimation:MainWindowViewModel />
    </Window.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="40" />
        </Grid.RowDefinitions>

        <Rectangle Margin="10">
            <Rectangle.Fill>
                <SolidColorBrush Color="{Binding Color}"/>
            </Rectangle.Fill>
        </Rectangle>

        <StackPanel Orientation="Horizontal" Grid.Row="1">
            <Button Command="{Binding BlueCommand}" Width="100">Blue</Button>
            <Button Command="{Binding GreenCommand}" Width="100">Green</Button>
        </StackPanel>
    </Grid>
</Window>

MainWindowViewModel.cs

namespace MVVM.ColorAnimation
{
    using System.Windows.Input;
    using System.Windows.Media;

    using MVVM;

    public class MainWindowViewModel : ObservableObject
    {
        private ICommand blueCommand;
        private ICommand greenCommand;

        public ICommand BlueCommand
        {
            get
            {
                return this.blueCommand ?? (this.blueCommand = new RelayCommand(this.TurnBlue));
            }
        }

        private void TurnBlue()
        {
            this.Color = Colors.Blue;
        }

        public ICommand GreenCommand
        {
            get
            {
                return this.greenCommand ?? (this.greenCommand = new RelayCommand(this.TurnGreen));
            }
        }

        private void TurnGreen()
        {
            this.Color = Colors.Green;
        }

        private Color color = Colors.Red;

        public Color Color
        {
            get
            {
                return this.color;
            }

            set
            {
                this.color = value;
                RaisePropertyChanged("Color");
            }
        }     
    }
}

Is there anyway from the View to trigger a ColorAnimation instead of an instant transition between the values? The way I'm currently doing this is another application is quite messy, in that I set the ViewModel through a ViewModel property, and then using a PropertyObserver to monitor value changes, then create the Animation and trigger it from the codebehind:

this.colorObserver = new PropertyObserver<Player>(value)
    .RegisterHandler(n => n.Color, this.CreateColorAnimation);

In a situation where I'm dealing with many colors and many potential animations, this becomes quite a mess, and messes up the fact that I'm manually passing in the ViewModel to the View than simply binding the two through a ResourceDictionary. I suppose I could do this in the DataContextChanged event as well, but is there a better way?

Answer 1

If just for a few animations I would recommend using Visual States. Then you can use GoToAction behavior on the view to trigger different animations. If you are dealing with a lot of similar animations, creating your own behavior would be a better solution.

Update I have created a very simple behaivor to give a Rectangle a little color animation. Here is the code.

   public class ColorAnimationBehavior : TriggerAction<FrameworkElement>
    {
        #region Fill color
        [Description("The background color of the rectangle")]
        public Color FillColor
        {
            get { return (Color)GetValue(FillColorProperty); }
            set { SetValue(FillColorProperty, value); }
        }

        public static readonly DependencyProperty FillColorProperty =
            DependencyProperty.Register("FillColor", typeof(Color), typeof(ColorAnimationBehavior), null);
        #endregion

        protected override void Invoke(object parameter)
        {
            var rect = (Rectangle)AssociatedObject;

            var sb = new Storyboard();
            sb.Children.Add(CreateVisibilityAnimation(rect, new Duration(new TimeSpan(0, 0, 1)), FillColor));

            sb.Begin();
        }

        private static ColorAnimationUsingKeyFrames CreateVisibilityAnimation(DependencyObject element, Duration duration, Color color)
        {
            var animation = new ColorAnimationUsingKeyFrames();

            animation.KeyFrames.Add(new SplineColorKeyFrame { KeyTime = new TimeSpan(duration.TimeSpan.Ticks), Value = color });

            Storyboard.SetTargetProperty(animation, new PropertyPath("(Shape.Fill).(SolidColorBrush.Color)"));
            Storyboard.SetTarget(animation, element);

            return animation;
        }

    }

In xaml, you simply attach this behavior like this,

    <Rectangle x:Name="rectangle" Fill="Black" Margin="203,103,217,227" Stroke="Black">
        <i:Interaction.Triggers>
            <i:EventTrigger EventName="MouseLeftButtonDown">
                <local:ColorAnimationBehavior FillColor="Red"/>
            </i:EventTrigger>
        </i:Interaction.Triggers>
    </Rectangle>

When you click the Rectangle, it should go from Black color to Red.

Answer 2

I used the code that Xin posted, and made a few very minor tweeks (code is below). The only 3 material differences:

I created the behavior to work on any UIElement, not just a rectangle

I used a PropertyChangedTrigger instead of an EventTrigger. That let's me Monitor the color property on the ViewModel instead of listening for click events.

I bound the FillColor to the Color property of the ViewModel.

To use this, you will need to download the Blend 4 SDK (it's free, and you only need it if you don't already have Expression Blend), and add references to System.Windows.Interactivity, and Microsoft.Expression.Interactions

Here's the code for the behavior class:

// complete code for the animation behavior
using System.Windows;
using System.Windows.Interactivity;
using System.Windows.Media;
using System.Windows.Media.Animation;

namespace ColorAnimationBehavior
{
    public class ColorAnimationBehavior: TriggerAction<UIElement>
    {
        public Color FillColor
        {
            get { return (Color)GetValue(FillColorProperty); }
            set { SetValue(FillColorProperty, value); }
        }

        public static readonly DependencyProperty FillColorProperty =
            DependencyProperty.Register("FillColor", typeof(Color), typeof(ColorAnimationBehavior), null);

        public Duration Duration
        {
            get { return (Duration)GetValue(DurationProperty); }
            set { SetValue(DurationProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Duration.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty DurationProperty =
            DependencyProperty.Register("Duration", typeof(Duration), typeof(ColorAnimationBehavior), null);

        protected override void Invoke(object parameter)
        {
            var storyboard = new Storyboard();
            storyboard.Children.Add(CreateColorAnimation(this.AssociatedObject, this.Duration, this.FillColor));
            storyboard.Begin();
        }

        private static ColorAnimationUsingKeyFrames CreateColorAnimation(UIElement element, Duration duration, Color color)
        {
            var animation = new ColorAnimationUsingKeyFrames();
            animation.KeyFrames.Add(new SplineColorKeyFrame() { KeyTime = duration.TimeSpan, Value = color });
            Storyboard.SetTargetProperty(animation, new PropertyPath("(Shape.Fill).(SolidColorBrush.Color)"));
            Storyboard.SetTarget(animation, element);
            return animation;
        }
    }
}

Now here's the XAML that hooks it up to your rectangle:

<UserControl x:Class="MVVM.ColorAnimation.Silverlight.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    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"
    xmlns:ColorAnimation="clr-namespace:MVVM.ColorAnimation"
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
    xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"
    xmlns:ca="clr-namespace:ColorAnimationBehavior;assembly=ColorAnimationBehavior"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <UserControl.DataContext>
        <ColorAnimation:MainWindowViewModel />
    </UserControl.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="40" />
        </Grid.RowDefinitions>

        <Rectangle x:Name="rectangle" Margin="10" Stroke="Black" Fill="Red">
            <i:Interaction.Triggers>
                <ei:PropertyChangedTrigger Binding="{Binding Color}">
                    <ca:ColorAnimationBehavior FillColor="{Binding Color}" Duration="0:0:0.5" />
                </ei:PropertyChangedTrigger>
            </i:Interaction.Triggers>
        </Rectangle>
        <StackPanel Orientation="Horizontal" Grid.Row="1">
            <Button Command="{Binding BlueCommand}" Width="100" Content="Blue"/>
            <Button Command="{Binding GreenCommand}" Width="100" Content="Green"/>
        </StackPanel>
    </Grid>
</UserControl>

It was really Xin's idea -- I just cleaned it up a bit.