private void _list_PreviewTouchDown(object sender, TouchEventArgs e)
{
if (!is_tap.ContainsKey(e.TouchDevice.Id))
this.is_tap.Add(e.TouchDevice.Id, true);
ScrollViewer scroll = configurations.GetDescendantByType(this._list, typeof(ScrollViewer)) as ScrollViewer;
last_scroll_offset = scroll.VerticalOffset;
//scroll.Elasticity = new Vector(0.0, 0.4);
bool r = e.TouchDevice.Capture(this._list as IInputElement, CaptureMode.SubTree);
e.Handled = true;
}