如何在 Xamarin.Forms 中显示和隐藏基于 ListView 滚动方向的 StackLayout?

问题描述：

我有一个带有 ListView 的屏幕，它显示了一组评论.另外，我有一个 StackLayout 与 ListView 的末尾重叠，它有一个 Entry 和一个 Button 要添加一条新评论.

I have a screen with a ListView that shows a collection of comments. Also, I have a StackLayout overlapping the end of the ListView, which has an Entry and a Button to add a new comment.

我想根据 ListView 滚动方向隐藏/显示这个 StackLayout:

I want to hide/show this StackLayout depending on the ListView scrolling direction:

如果用户向下滚动 -> 隐藏 StackLayout.
如果用户向上滚动 -> 显示 StackLayout.

有人知道实现这种行为的方法吗?

Anyone knows a way to accomplish that behavior?

提前致谢！

答

Xamarin.Forms ListView 提供了可以订阅的 OnItemAppearing 事件.有了这个，您可以通过查找出现的项目的索引并将其与最后出现的项目进行比较来跟踪滚动方向.尝试这样的事情:

Xamarin.Forms ListView provides an OnItemAppearing event you can subscribe to. With this you can track your scroll direction by finding the index of the item that appeared and comparing it to the last item that appeared. Try something like this:

public partial class MainPage : ContentPage
{
    public ObservableCollection<MyItemType> Items { get; set; } = new ObservableCollection<MyItemType>();
    int lastItemIndex;
    int currentItemIndex;

    public MainPage()
    {
        ...
        listView.ItemAppearing += ListView_ItemAppearing;
    }

    void ListView_ItemAppearing(object sender, ItemVisibilityEventArgs e)
    {
        MyItemType item = e.Item as MyItemType;

        currentItemIndex = Items.IndexOf(item);
        if (currentItemIndex > lastItemIndex)
        {
            stackLayout.IsVisible = false;
        }
        else
        {
            stackLayout.IsVisible = true;
        }
        lastItemIndex = currentItemIndex;
    }
}

闪烁实际上是由于 ListView 在 StackLayout 显示和隐藏时调整了大小，因此请确保 ListView 没有调整大小.也许将 ListView 和 StackLayout 放在网格中，以便在显示和隐藏 StackLayout 时 ListView 不会调整大小，例如:

Flickering is really due to ListView being resized when the StackLayout shows and hides, so make sure that the ListView is not getting resized. Perhaps put the ListView and the StackLayout in a grid so that when you show and hide the StackLayout the ListView does not get resized, e.g.:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*" />
        <RowDefinition Height="35" />
    </Grid.RowDefinitions>
    <ListView x:Name="listView"
              ItemsSource="{Binding Items}" 
              Grid.Row="0">
        <ListView.ItemTemplate>
            ...
        </ListView.ItemTemplate>
    </ListView>
    <StackLayout x:Name="stackLayout"
                 Grid.Row="1">
        ...
    </StackLayout>
</Grid>

使用上述方法，不再出现闪烁.

With the above, the flickering no longer occurs.

如何在 Xamarin.Forms 中显示和隐藏基于 ListView 滚动方向的 StackLayout?

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