title: how to set CompositionObject by variable
body: <p>i want to add the animation for many object ,and i'm try this one
  <a href="https://stackoverflow.com/questions/46021277/fill-ellipse-with-wave-animation/46057193#46057193">Fill Ellipse with wave animation</a> 
  it work ,but i want to use it in the same page by many object, each object have different value ,
  the point is how do i Set Reference Parameter by variable or something i can dyanmic setting</p>
  
  <pre><code>var waveOffsetYExpressionAnimation = _compositor.CreateExpressionAnimation("Lerp(135.0f, 0.0f, Value_" + i + ")");
  waveOffsetYExpressionAnimation.SetReferenceParameter("Value_" + i, _percentPropertySet);
  imageSurfaceBrush.StartAnimation("Offset.Y", waveOffsetYExpressionAnimation);
  </code></pre>