Creating animations in XAML is pretty easy using Storyboards, but after you create them, you have to trigger them somehow. There are a bunch of ways to do this, but each has its own shortcomings if you want to trigger your animation based on viewmodel changes:

DataTrigger & Trigger - Not supported in WinRT apps so they are non-starters.

EventTrigger - Triggers your animation in response to an event. These are still supported in WinRT but they can only hook up to the Loaded event so it is effectively inaccessible from a viewmodel.

Storyboard.Begin() method - straight to the point, but not easy to databind to since it is a method.

VisualStateManager - This is essentially the replacement for DataTriggers, but again, it requires calling a method (VisualStateManager.GoToState()) to trigger it.

Behaviors - Code like this can trigger a storyboard using a custom behavior, but (and maybe this is just me) I rarely use behaviors and find them kind of clunky for some reason. Just my opinion.

So where does that leave us? How about this:

<Storyboard common:StoryboardHelper.BeginIf="{Binding IsLoggedIn}">
    <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.RenderTransform).(CompositeTransform.TranslateY)" Storyboard.TargetName="StartSessionOverlay">
        <EasingDoubleKeyFrame KeyTime="0:0:0" Value="640"/>
        <EasingDoubleKeyFrame KeyTime="0:0:0.3" Value="0"/>
    </DoubleAnimationUsingKeyFrames>
</Storyboard>

Using a custom attached property, we can set up a mechanism that allows declarative databinding in the XAML and yet runs arbitrary code behind the scenes. The implementation is straight-forward:

using System;
using System.Collections.Generic;
using System.Text;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media.Animation;

namespace Demo
{
    public class StoryboardHelper : DependencyObject
    {
        public static bool GetBeginIf(DependencyObject obj)
        {
            return (bool)obj.GetValue(BeginIfProperty);
        }

        public static void SetBeginIf(DependencyObject obj, bool value)
        {
            obj.SetValue(BeginIfProperty, value);
        }

        public static readonly DependencyProperty BeginIfProperty = DependencyProperty.RegisterAttached("BeginIf", typeof(bool), typeof(StoryboardHelper), new PropertyMetadata(false, BeginIfPropertyChangedCallback));

        private static void BeginIfPropertyChangedCallback(DependencyObject s, DependencyPropertyChangedEventArgs e)
        {
            var storyboard = s as Storyboard;
            if (storyboard == null)
                throw new InvalidOperationException("This attached property only supports Storyboards.");

            var begin = (bool)e.NewValue;
            if (begin) storyboard.Begin();
            else storyboard.Stop();
        }
    }
}

Attached properties have been solving a lot of little things like this for me lately so I will probably write about them some more in the future.