Android Question ACToolBar shadow lost with background change

Sub CreateShadow (base As View, elevation As Int)
    Dim c As Canvas
    c.Initialize(base)
    
    Dim shadow As GradientDrawable
    Dim clrs(2) As Int
    clrs(0) = Colors.Gray
    clrs(1) = Colors.White
    shadow.Initialize("TOP_BOTTOM", clrs)
        
    Dim destRect As Rect
    destRect.Initialize(0, base.Height - elevation, base.Width, base.Height)
    c.DrawDrawable(shadow,destRect)
End Sub