Animation on ContentPresenter ContenSource Changed

[ezazazel]

Hello! Could anyone please help me with this: XAML:

ViewModel

private UserControl navigation;
public UserControl Navigation{
get {return navigation;}
set {navigation = value; OnPropertyChanged(()=>Navigation);}
}

void LoadControl(UserControl control)
{
//control can either be null or typeof(UserControl)
this.Navigation = control;
}

What I want to do is create an animation which changes the Width of the ContentPresenter stepwise. If control != null step from 0 => control.Width else step from control.Width => 0 Any ideas how this can be done (can this be done in XAML soley)? Help, as always, would be highly appreciated.