ListView ScrollViewer問題

    我認爲您正在尋找的實際上 是一種ListView 將元素滾動到的頂部的方法。

• ScrollViewer 我創建了一個擴展方法是滾動到內的特定元素 。

  首先 ScrollViewer ，我需要找到   ListView 此類項目的實際實例，或滾動到next ListViewItem 。

  ScrollViewer 這是獲得的擴展方法：。

  public static ScrollViewer GetScrollViewer(this DependencyObject element)
  {
      if (element is ScrollViewer)
      {
          return (ScrollViewer)element;
      }
      for (int i = 0; i < VisualTreeHelper.GetChildrenCount(element); i++)
      {
          var child = VisualTreeHelper.GetChild(element, i);
          var result = GetScrollViewer(child);
          if (result == null)
          {
              continue;
          }
          else
          {
              return result;
          }
      }
      return null;
  }

  ScrollViewer 我創建了兩個擴展方法，以基於例如索引或鏈接對象滾動到某個項目。 ListView 並 共享GridView  相同的基類 ListViewBase。這兩種擴展方法 GridView 也適用。。

  更新

  基本上，該方法首先找到渲染的項目，然後立即滾動。如果該項爲 null ，則表示已啓用虛擬化並且尚未實現該項。ScrollIntoViewAsync 打電話給我們首先實現該項目 。（ 與ScrollIntoView 包裝內置 的基於任務的方法 ChangeViewAsync相同（提供了更簡潔的代碼））計算並存儲位置。現在您知道要滾動的位置了，您必須首先立即將項目滾動回到上一個位置（即無動畫），然後使用動畫滾動到所需的位置。

  public async static Task ScrollToIndex(this ListViewBase listViewBase, int index)
  {
      bool isVirtualizing = default(bool);
      double previousHorizontalOffset = default(double), previousVerticalOffset = default(double);
      // get the ScrollViewer withtin the ListView/GridView
      var scrollViewer = listViewBase.GetScrollViewer();
      // get the SelectorItem to scroll to
      var selectorItem = listViewBase.ContainerFromIndex(index) as SelectorItem;
      // when it's null, means virtualization is on and the item hasn't been realized yet
      if (selectorItem == null)
      {
          isVirtualizing = true;
          previousHorizontalOffset = scrollViewer.HorizontalOffset;
          previousVerticalOffset = scrollViewer.VerticalOffset;
          // call task-based ScrollIntoViewAsync to realize the item
          await listViewBase.ScrollIntoViewAsync(listViewBase.Items[index]);
          // this time the item shouldn't be null again
          selectorItem = (SelectorItem)listViewBase.ContainerFromIndex(index);
      }
      // calculate the position object in order to know how much to scroll to
      var transform = selectorItem.TransformToVisual((UIElement)scrollViewer.Content);
      var position = transform.TransformPoint(new Point(0, 0));
      // when virtualized, scroll back to previous position without animation
      if (isVirtualizing)
      {
          await scrollViewer.ChangeViewAsync(previousHorizontalOffset, previousVerticalOffset, true);
      }
      // scroll to desired position with animation!
      scrollViewer.ChangeView(position.X, position.Y, null);
  }
  public async static Task ScrollToItem(this ListViewBase listViewBase, object item)
  {
      bool isVirtualizing = default(bool);
      double previousHorizontalOffset = default(double), previousVerticalOffset = default(double);
      // get the ScrollViewer withtin the ListView/GridView
      var scrollViewer = listViewBase.GetScrollViewer();
      // get the SelectorItem to scroll to
      var selectorItem = listViewBase.ContainerFromItem(item) as SelectorItem;
      // when it's null, means virtualization is on and the item hasn't been realized yet
      if (selectorItem == null)
      {
          isVirtualizing = true;
          previousHorizontalOffset = scrollViewer.HorizontalOffset;
          previousVerticalOffset = scrollViewer.VerticalOffset;
          // call task-based ScrollIntoViewAsync to realize the item
          await listViewBase.ScrollIntoViewAsync(item);
          // this time the item shouldn't be null again
          selectorItem = (SelectorItem)listViewBase.ContainerFromItem(item);
      }
      // calculate the position object in order to know how much to scroll to
      var transform = selectorItem.TransformToVisual((UIElement)scrollViewer.Content);
      var position = transform.TransformPoint(new Point(0, 0));
      // when virtualized, scroll back to previous position without animation
      if (isVirtualizing)
      {
          await scrollViewer.ChangeViewAsync(previousHorizontalOffset, previousVerticalOffset, true);
      }
      // scroll to desired position with animation!
      scrollViewer.ChangeView(position.X, position.Y, null);
  }
  public static async Task ScrollIntoViewAsync(this ListViewBase listViewBase, object item)
  {
      var tcs = new TaskCompletionSource<object>();
      var scrollViewer = listViewBase.GetScrollViewer();
      EventHandler<ScrollViewerViewChangedEventArgs> viewChanged = (s, e) => tcs.TrySetResult(null);
      try
      {
          scrollViewer.ViewChanged += viewChanged;
          listViewBase.ScrollIntoView(item, ScrollIntoViewAlignment.Leading);
          await tcs.Task;
      }
      finally
      {
          scrollViewer.ViewChanged -= viewChanged;
      }
  }
  public static async Task ChangeViewAsync(this ScrollViewer scrollViewer, double? horizontalOffset, double? verticalOffset, bool disableAnimation)
  {
      var tcs = new TaskCompletionSource<object>();
      EventHandler<ScrollViewerViewChangedEventArgs> viewChanged = (s, e) => tcs.TrySetResult(null);
      try
      {
          scrollViewer.ViewChanged += viewChanged;
          scrollViewer.ChangeView(horizontalOffset, verticalOffset, null, disableAnimation);
          await tcs.Task;
      }
      finally
      {
          scrollViewer.ViewChanged -= viewChanged;
      }
  }
   

  沒有動畫的更簡單方法

  ScrollIntoView 您還可以對第二個參數使用新的重載，以使該項目與頂部邊緣對齊。但是，這不會導致以前的擴展方法中的平滑滾動過渡。

  MyListView?.ScrollIntoView(MyListView.Items[5], ScrollIntoViewAlignment.Leading);

   

• 二

  ScrollIntoView使該項進入視圖，並且不會滾動到句點。

  當您呼叫成員並位於顯示列表的底部時，該項向下滾動，直到您成爲顯示列表的最後一個成員。

  當您呼叫成員並位於列表頂部時，該項目將向上滾動，直到您成爲列表的第一個成員。

  如果我呼叫了一個成員並且當前顯示該成員，則根本不起作用。

   

• 三

  Var sv = new ScrollViewerHelper().GetScrollViewer(listView);
          sv.UpdateLayout();
          sv.ChangeView(0, sv.ExtentHeight, null);
   
  

  GetScrollViewer方法： 

  
  public ScrollViewer GetScrollViewer(DependencyObject element)
      {
          if (element is ScrollViewer)
          {
              return (ScrollViewer)element;
          }
          for (int i = 0; i < VisualTreeHelper.GetChildrenCount(element); i++)
          {
              var child = VisualTreeHelper.GetChild(element, i);
              var result = GetScrollViewer(child);
              if (result == null)
              {
                  continue;
              }
              else
              {
                  return result;
              }
          }
          return null;
      }