效果如圖：
重點是判斷滑動到DataGrid底端（VerticalOffset等於ScrollableHeight），則繼續加載數據
private void DataGrid_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
var scrollViewer = e.OriginalSource as ScrollViewer;
if (e.VerticalOffset!=0 && e.VerticalOffset == scrollViewer.ScrollableHeight)
{
GenerateData();
}
}