how to set fade function with scrollview position

I have a page and when it scrolling up I need a header part of my page should fade out and when scrolling out it should fade in. I tried to do this with fade in function actually I did it but its using time instead of the position.

The problem is Fade function support the time. I need to to fade away with position

I have tried

     <ScrollView Scrolled="ScrollView_OnScrolled">
   <StackLayout >
   <AbsoluteLayout x:Name="AbsoluteLayout1"   >

      // header part

        </AbsoluteLayout>


    <StackLayout >
    // body part

     </StackLayout>        
  </StackLayout>

In my code behind i tried this

     private double previousScrollPosition = 0;
   private async  void  ScrollView_OnScrolled(object sender, ScrolledEventArgs e)
   {
    if (previousScrollPosition < e.ScrollY)
    {
        //scrolled up
        previousScrollPosition = e.ScrollY;           
        await AbsoluteLayout1.FadeTo(0.2, 400, Easing.SinInOut);
      }

      }
    else
    {
        //scrolled down

        if (Convert.ToInt16(e.ScrollY) == 0)
            previousScrollPosition = 0;
        await AbsoluteLayout1.FadeTo(1, 400,Easing.SinIn);          
    }
   }

How to set fadeaway with scrolling position on xamarin forms