WP7 - Issues with delegates whilst using a storyboard - windows-phone-7

Question

I have introduced some animation into my WP7 app via a storyboard.
It is all working as expected the first time through - the event occurs, and I call the code to do the animation in the code behind and I get exactly the behaviour I want.
But when I trigger the second time, it is like the the delegate code involved in this is being called twice. And then I trigger the same event and the delegate code is called 3 times. Its cumulative.
Code that is called is as follows
private void WeekForward()
{
FadeTitleOut.Begin();
FadeTitleOut.Completed += delegate
{
StartingMonday = StartingMonday.AddDays(7);
BuildPage();
FadeTitleIn.Begin();
};
}
FadeTitleOut and FadeTitleIn are my storyboards and the animation effect is fine. It's just that the first time the event happens StartingMonday is incremented by 7 days. The next time the event fires its incremented by 14 days and so on.
Am I doing something really dumb with my delegate code?
Probably should add that I am doing most of the stuff here in the code behind, including the generation and deletion of controls dynamicaly, but the Storyboards are defined in the XAML and they reference a control that is defined in the XAML (and not generated in the code behind)
Storyboard XAML is
<phone:PhoneApplicationPage.Resources>
<Storyboard x:Name="FadeTitleOut">
<DoubleAnimation Storyboard.TargetName="LayoutRoot" Storyboard.TargetProperty="Opacity" From="1.0" To="0.0" Duration="0:0:0.3" />
</Storyboard>
<Storyboard x:Name="FadeTitleIn">
<DoubleAnimation Storyboard.TargetName="LayoutRoot" Storyboard.TargetProperty="Opacity" From="0.0" To="1.0" Duration="0:0:0.3" />
</Storyboard>
</phone:PhoneApplicationPage.Resources>
Any ideas?

Answer 1

Have a look through your code, my guess is that "WeekForward()" is getting called more often than you want it to and you are subscribing the FadeTitleOut.Completed each time it is called (hence the behaviour you are describing). One way to fix this would be to have this code else where so it only subscribed to the Completed event once.
Also, you should write up your events before firing the Begin method as you have a potential race condition. eg,
FadeTitleOut.Completed += delegate
{
StartingMonday = StartingMonday.AddDays(7);
BuildPage();
FadeTitleIn.Begin();
};
FadeTitleOut.Begin();
Put this code where it will be fired once during the life of the XAML page and it should be fine. Hope that helps.