WPF - Storyboard événement terminé

Je suis de la conception d'un jeu de ce type

class Anima
{
Storyboard story;
Random rand;
Canvas canvas;
Ellipse target;
public Anima() {
rand = new Random();
canvas = new Canvas();
target = new Ellipse();
target.Fill = Brushes.Red;
target.Width = 50;
target.Height = 50;
Canvas.SetLeft(target, rand.NextDouble() * 300);
Canvas.SetTop(target, rand.NextDouble() * 300);
canvas.Children.Add(target);
story = new Storyboard();
story.BeginTime = TimeSpan.FromMilliseconds(rand.Next(500, 5000));
DoubleAnimation a = new DoubleAnimation();
a.To = rand.NextDouble() * 300;
a.Duration = new Duration(TimeSpan.FromMilliseconds(50));
Storyboard.SetTarget(a, target);
Storyboard.SetTargetProperty(a, new PropertyPath(Canvas.LeftProperty));
story.Children.Add(a);
DoubleAnimation b = new DoubleAnimation();
b.To = rand.NextDouble() * 300;
b.Duration = new Duration(TimeSpan.FromMilliseconds(50));
Storyboard.SetTarget(b, target);
Storyboard.SetTargetProperty(b, new PropertyPath(Canvas.TopProperty));
story.Children.Add(b);
story.Completed += new EventHandler(story_Completed);
Window win = new Window();
win.Loaded += delegate(object sender, RoutedEventArgs e) {
story.Begin();
};
win.Content = canvas;
win.Show();
}
void story_Completed(object sender, EventArgs e) {
int next = rand.Next(500, 5000);
double left = rand.NextDouble() * 300;
double top = rand.NextDouble() * 300;
Console.WriteLine("position: ({0:G6}, {1:G6})", Canvas.GetLeft(target), Canvas.GetTop(target));
Console.WriteLine("state   : wait for " + next + " ms");
Console.WriteLine("next    : ({0:G6}, {1:G6})", left, top);
Console.WriteLine();
(story.Children[0] as DoubleAnimation).To = left;
(story.Children[1] as DoubleAnimation).To = top;
story.BeginTime = TimeSpan.FromMilliseconds(next);
story.Begin();
}
}

Tout est parfait, mais j'ai trouvé que l'ellipse n'a pas la bonne position et de faire une erreur d'environ 2% à 50%. Il semble que la table de montage séquentiel.Terminé événement déclenché avant la fin de l'animation.

Quel est le problème avec elle?