How to get collection of visible rows at System.Windows.Controls.Grid?

Question

I have treeview wrapper of System.Windows.Controls.Grid. I can get this object:

var winControl = tvObject.Content as System.Windows.Controls.Grid;

if(winControl != null)
{                     
    Console.WriteLine(winControl);
    // Here I want to get only visible rows.
}

So, when i touch Page Down button- i get new visible rows. How to get this at System.Windows.Controls.Grid programmatically? Thank you!