UWP 自定义 ListView 向下滚动

Question

所以，我有一个列表视图，我希望它在创建项目时滚动到该项目（底部）。因为我使用的是 MVVM，所以我找到了关于如何制作一个继承自向下滚动列表视图的新控件的非常好的解释。问题是 this answer (the third) 指的是 6 年前的 WPF。我正在制作一个 UWP 应用程序，所以我复制了代码并尝试根据我的需要对其进行格式化。下面的代码没有给出任何错误或异常，而是加载 "ChatListView" 我完美地调用它然后什么都不做。与原始代码相比，注释仅进行了一些编辑。

我能做什么？提前致谢！

public class ChatListView : ListView
{
    //Define the AutoScroll property. If enabled, causes the ListBox to scroll to 
    //the last item whenever a new item is added.
    public static readonly DependencyProperty AutoScrollProperty =
        DependencyProperty.Register(
            "AutoScroll",
            typeof(Boolean),
            typeof(ChatListView),
            new PropertyMetadata(
                true, //Default value.
                new PropertyChangedCallback(AutoScroll_PropertyChanged)));

    //Gets or sets whether or not the list should scroll to the last item 
    //when a new item is added.
    public bool AutoScroll
    {
        get { return (bool)GetValue(AutoScrollProperty); }
        set { SetValue(AutoScrollProperty, value); }
    }

    //Event handler for when the AutoScroll property is changed.
    //This delegates the call to SubscribeToAutoScroll_ItemsCollectionChanged().
    //d = The DependencyObject whose property was changed.</param>
    //e = Change event args.</param>
    private static void AutoScroll_PropertyChanged(
        DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        SubscribeToAutoScroll_ItemsCollectionChanged(
            (ChatListView)d,
            (bool)e.NewValue);
    }

    //Subscribes to the list items' collection changed event if AutoScroll is enabled.
    //Otherwise, it unsubscribes from that event.
    //For this to work, the underlying list must implement INotifyCollectionChanged.
    //
    //(This function was only creative for brevity)

    //listBox = The list box containing the items collection.
    //subscribe = Subscribe to the collection changed event?
    private static void SubscribeToAutoScroll_ItemsCollectionChanged(
        ChatListView listView, bool subscribe)
    {
        INotifyCollectionChanged notifyCollection =
            listView as INotifyCollectionChanged;
        if (notifyCollection != null)
        {
            if (subscribe)
            {
                //AutoScroll is turned on, subscribe to collection changed events.
                notifyCollection.CollectionChanged +=
                    listView.AutoScroll_ItemsCollectionChanged;
            }
            else
            {
                //AutoScroll is turned off, unsubscribe from collection changed events.
                notifyCollection.CollectionChanged -=
                    listView.AutoScroll_ItemsCollectionChanged;
            }
        }
    }

    //Event handler called only when the ItemCollection changes
    //and if AutoScroll is enabled.

    //sender = The ItemCollection.
    //e = Change event args.
    private void AutoScroll_ItemsCollectionChanged(
        object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.Action == NotifyCollectionChangedAction.Add)
        {
            int count = Items.Count;
            ScrollIntoView(Items[count - 1]);
        }
    }

    //Constructor a new ChatListView.
    public ChatListView()
    {
        //Subscribe to the AutoScroll property's items collection 
        //changed handler by default if AutoScroll is enabled by default.
        SubscribeToAutoScroll_ItemsCollectionChanged(
            this, (bool)AutoScrollProperty.GetMetadata(typeof(ChatListView)).DefaultValue);
    }
}

Answer 1

如果您想创建一个聊天应用程序，您可以使用 ItemsStackPanel 的 ItemsUpdatingScrollMode 特定的属性到 KeepLastItemInView 值来滚动到最新项目。

用法：

<ListView>
    <ListView.ItemsPanel>
        <ItemsPanelTemplate>
            <ItemsStackPanel ItemsUpdatingScrollMode="KeepLastItemInView" />
        </ItemsPanelTemplate>
    </ListView.ItemsPanel>
</ListView>

注意：KeepLastItemInView枚举成员是在14393 SDK中引入的。

相关link： https://docs.microsoft.com/en-us/uwp/api/Windows.UI.Xaml.Controls.ItemsStackPanel#properties_

Answer 2

接受的答案很不错。但是我有一件事它不会做（至少如果我简单地复制并粘贴上面的XAML）：如果用户在新页面时离开了该页面，它就不会进行预期的滚动添加了项目，然后他们导航到该页面。

为此我不得不连接到

protected override void OnNavigatedTo(NavigationEventArgs e)
{
   base.OnNavigatedTo(e);

   if (MyListView.Items.Count == 0)
      return;

   object lastItem = MyListView.Items[MyListView.Items.Count - 1];
   MyListView.ScrollIntoView(lastItem);
}

UWP 自定义 ListView 向下滚动

UWP Custom ListView to scroll down

dependency-properties

custom-controls

observablecollection

mvvm

uwp