Windows 10 ScrollIntoView() 没有滚动到列表视图中间的项目

I think what you are looking for is a method to actually scroll an element to the top of the ListView.

在这篇文章中，我创建了一个滚动到 ScrollViewer 中特定元素的扩展方法.

In this post, I created an extension method that scrolls to a particular element within a ScrollViewer.

你的想法是一样的.

您需要首先在 ListView 中找到 ScrollViewer 实例，然后是要滚动到的实际项目，即 ListViewItem.

You need to first find the ScrollViewer instance within your ListView, then the actual item to scroll to, that is, a ListViewItem.

这是获取ScrollViewer的扩展方法.

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;
}

获得 ScrollViewer 实例后，我创建了另外两个扩展方法，分别根据其索引或附加对象滚动到项目.由于 ListView 和 GridView 共享相同的基类 ListViewBase.这两个扩展方法也应该适用于 GridView.

Once I get the ScrollViewer instance, I have created two more extension methods to scroll to an item based on its index or attached object respectively. Since ListView and GridView are sharing the same base class ListViewBase. These two extension methods should also work for GridView.

基本上，这些方法将首先找到该项目，如果它已经渲染，然后立即滚动到它.如果item为null，则表示虚拟化开启，item尚未实现.所以首先要实现item，调用ScrollIntoViewAsync(基于任务的方法来包装内置的ScrollIntoView，和ChangeViewAsync一样，提供了很多更清晰的代码)，计算位置并保存.因为现在我知道要滚动到的位置，所以我需要先将项目一直滚动到它之前的位置立即(即没有动画)，然后最后用动画滚动到所需的位置.

Basically, the methods will first find the item, if it's already rendered, then scroll to it right away. If the item is null, it means the virtualization is on and the item has yet to be realized. So to realize the item first, call ScrollIntoViewAsync (task-based method to wrap the built-in ScrollIntoView, same as ChangeViewAsync, which offers much cleaner code), calculate the position and save it. Since now I know the position to scroll to, I need to first scroll the item all the way back to its previous position instantly (i.e. without animation), and then finally scroll to the desired position with animation.

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;
    }
}

<小时>

您还可以通过指定第二个参数来使用 ScrollIntoView 的新重载，以确保项目在顶部边缘对齐；但是，这样做并没有我以前的扩展方法中的平滑滚动过渡.

You can also use the new overload of ScrollIntoView by specifying the second parameter to make sure the item is aligned on the top edge; however, doing so doesn't have the smooth scrolling transition in my previous extension methods.

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

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