Xamarin.Forms ListView 大小到内容？

Question

我有一个相当大的表单（主要适用于平板电脑），它有一个 TabbedPage 嵌套 ScrollView 和一个包含许多控件的垂直 StackPanel。

我很少会遇到 ListView 包含一些单行项目的情况，我需要根据内容调整大小。
我想摆脱它的滚动条，但无论如何我不希望它占用比其项目所需的更多 space。
有没有一种方法（即使是一种丑陋的方法）可以实现这一点而不必编写渲染器 x3 平台？

这是对我的树的伪描述：

<ContentPage>
  <MasterDetailPage>
    <MasterDetailPage.Detail>
      <TabbedPage>
        <ContentPage>
          <ScrollView>
            <StackPanel>
              <!-- many controls-->
              <ListView>

渲染时，ListView 之后有一个巨大的间隙。我怎样才能避免这种情况？

我试着摆弄 VerticalOptions 和 HeightRequest，但没有一个有效。

我正在寻找一种动态方式（最好没有继承）来实现这一点，而不涉及自定义渲染器。

Answer 1

好的，假设您的 ListView 中填充了 NewsFeed，让我们使用 ObservableCollection 来包含我们的数据来填充 ListView，如下所示：

XAML代码：

<ListView x:Name="newslist"/>

C# 代码

ObservableCollection <News> trends = new ObservableCollection<News>();

然后将趋势列表分配给 ListView :

newslist.ItemSource = trends;

然后，我们在 ListView 和数据上做了一些逻辑，让 ListView 包装数据，随着数据的增加，ListView 也增加，反之亦然 :

int i = trends.Count;
int heightRowList = 90;
i = (i * heightRowList);
newslist.HeightRequest = i;

因此完整的代码是：

ObservableCollection <News> trends = new ObservableCollection<News>();
newslist.ItemSource = trends;
int i = trends.Count;
int heightRowList = 90;
i = (i * heightRowList);
newslist.HeightRequest = i;

希望对您有所帮助。

Answer 2

基于 Lutaaya 的 , I made a behavior that automates this, determining and setting the row-height (Gist).

行为：

namespace Xamarin.Forms
{
  using System;
  using System.Linq;
  public class AutoSizeBehavior : Behavior<ListView>
  {
    ListView _ListView;
    ITemplatedItemsView<Cell> Cells => _ListView;

    protected override void OnAttachedTo(ListView bindable)
    {
      bindable.ItemAppearing += AppearanceChanged;
      bindable.ItemDisappearing += AppearanceChanged;
      _ListView = bindable;
    }

    protected override void OnDetachingFrom(ListView bindable)
    {
      bindable.ItemAppearing -= AppearanceChanged;
      bindable.ItemDisappearing -= AppearanceChanged;
      _ListView = null;
    }

    void AppearanceChanged(object sender, ItemVisibilityEventArgs e) =>
      UpdateHeight(e.Item);

    void UpdateHeight(object item)
    {
      if (_ListView.HasUnevenRows)
      {
        double height;
        if ((height = _ListView.HeightRequest) == 
            (double)VisualElement.HeightRequestProperty.DefaultValue)
          height = 0;

        height += MeasureRowHeight(item);
        SetHeight(height);
      }
      else if (_ListView.RowHeight == (int)ListView.RowHeightProperty.DefaultValue)
      {
        var height = MeasureRowHeight(item);
        _ListView.RowHeight = height;
        SetHeight(height);
      }
    }

    int MeasureRowHeight(object item)
    {
      var template = _ListView.ItemTemplate;
      var cell = (Cell)template.CreateContent();
      cell.BindingContext = item;
      var height = cell.RenderHeight;
      var mod = height % 1;
      if (mod > 0)
        height = height - mod + 1;
      return (int)height;
    }

    void SetHeight(double height)
    {
      //TODO if header or footer is string etc.
      if (_ListView.Header is VisualElement header)
        height += header.Height;
      if (_ListView.Footer is VisualElement footer)
        height += footer.Height;
      _ListView.HeightRequest = height;
    }
  }
}

用法：

<ContentPage xmlns:xf="clr-namespace:Xamarin.Forms">
  <ListView>
    <ListView.Behaviors>
      <xf:AutoSizeBehavior />

Answer 3

我在这里可能过于简单化了，但仅将 HasUnevenRows="True" 添加到 ListView 对我有用。

Answer 4

我可以创建一个事件处理程序来考虑 ListView 单元格大小的变化。在这里：

    <ListView ItemsSource="{Binding Items}"
         VerticalOptions="Start"
         HasUnevenRows="true"
         CachingStrategy="RecycleElement"
         SelectionMode="None" 
         SizeChanged="ListView_OnSizeChanged">
               <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell >
                           <Frame Padding="10,0" SizeChanged="VisualElement_OnSizeChanged">

Frame可以通过Grid、StackLayout等来改变。 xaml.cs:

        static readonly Dictionary<ListView, Dictionary<VisualElement, int>> _listViewHeightDictionary = new Dictionary<ListView, Dictionary<VisualElement, int>>();

    private void VisualElement_OnSizeChanged(object sender, EventArgs e)
    {
        var frame = (VisualElement) sender;
        var listView = (ListView)frame.Parent.Parent;
        var height = (int) frame.Measure(1000, 1000, MeasureFlags.IncludeMargins).Minimum.Height;
        if (!_listViewHeightDictionary.ContainsKey(listView))
        {
            _listViewHeightDictionary[listView] = new Dictionary<VisualElement, int>();
        }
        if (!_listViewHeightDictionary[listView].TryGetValue(frame, out var oldHeight) || oldHeight != height)
        {
            _listViewHeightDictionary[listView][frame] = height;
            var fullHeight = _listViewHeightDictionary[listView].Values.Sum();
            if ((int) listView.HeightRequest != fullHeight 
                && listView.ItemsSource.Cast<object>().Count() == _listViewHeightDictionary[listView].Count
                )
            {
                listView.HeightRequest = fullHeight;
                listView.Layout(new Rectangle(listView.X, listView.Y, listView.Width, fullHeight));
            }
        }
    }

    private void ListView_OnSizeChanged(object sender, EventArgs e)
    {
        var listView = (ListView)sender;
        if (listView.ItemsSource == null || listView.ItemsSource.Cast<object>().Count() == 0)
        {
            listView.HeightRequest = 0;
        }
    }

Frame 显示时（ListView.ItemTemplate 正在应用），frame 大小改变。我们通过 Measure() 方法获取它的实际高度并将其放入 Dictionary，Dictionary 知道当前 ListView 并保存 Frame 的高度。当显示最后一帧时，我们将所有高度相加。如果没有项目，ListView_OnSizeChanged() 将 listView.HeightRequest 设置为 0。

Answer 5

我知道这是一个旧线程，但到目前为止我找不到更好的解决方案 - 两年多后:-(

我正在使用 Xamarin.Forms v5.0.0.2244。实施并应用了 Shimmy 在上面的“用法：”中建议的行为，但是，AppearanceChanged 是针对每个 listviewitem 执行的，而不是 listview 本身。因此，当在 UpdateHeight 中测量高度时，它测量单个项目的高度，并将列表视图的高度设置为该值——这是不正确的。该行为似乎是合理的，因为它附加到 ItemAppearing 和 ItemDisappering 事件。

但是，根据 Shimmy 的想法，这里有一个改进的（但远非完美的）实现。这个想法是处理项目外观，测量这些项目并计算它们的总和，同时更新父列表视图高度。

对我来说，初始高度计算不正确，好像有些项目不会出现，因此没有被计算在内。我希望有人可以改进它，或者指出更好的解决方案：

internal class ListViewAutoShrinkBehavior : Behavior<ListView>
{
    ListView _listView;
    double _height = 0;

    protected override void OnAttachedTo(ListView listview)
    {
        listview.ItemAppearing += OnItemAppearing;
        listview.ItemDisappearing += OnItemDisappearing;
        _listView = listview;
    }

    protected override void OnDetachingFrom(ListView listview)
    {
        listview.ItemAppearing -= OnItemAppearing;
        listview.ItemDisappearing -= OnItemDisappearing;
        _listView = null;
    }

    void OnItemAppearing(object sender, ItemVisibilityEventArgs e)
    {
        _height += MeasureRowHeight(e.Item);
        SetHeight(_height);
    }

    void OnItemDisappearing(object sender, ItemVisibilityEventArgs e)
    {
        _height -= MeasureRowHeight(e.Item);
        SetHeight(_height);
    }

    int MeasureRowHeight(object item)
    {
        var template = _listView.ItemTemplate;
        var cell = (Cell)template.CreateContent();
        cell.BindingContext = item;
        double height = cell.RenderHeight;
        return (int)height;
    }

    void SetHeight(double height)
    {
        //TODO if header or footer is string etc.
        if (_listView.Header is VisualElement header)
            height += header.Height;
        if (_listView.Footer is VisualElement footer)
            height += footer.Height;
        _listView.HeightRequest = height;
    }
}

Answer 6

特别是当您的项目数量较少并且不需要滚动时，只需使用 bindableLayout。 https://docs.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/layouts/bindable-layouts 这总是使它完全符合尺寸！如果你有复杂的项目，有互动，那么这些工作非常好。

Answer 7

这里的大多数答案都有正确的想法，但似乎对 ViewCell 元素不太适用。这是我能想到的最有效的解决方案：

public class AutoSizeBehavior : Behavior<ListView>
{
    ListView _listView;
    VisualElement _parent;

    ITemplatedItemsList<Cell> _itemsList;
    Dictionary<int, double> _cells = new Dictionary<int, double>();
    Dictionary<VisualElement, int> _elMap = new Dictionary<VisualElement, int>();

    double _parentHeight;
    double _contentHeight;

    protected override void OnAttachedTo(ListView bindable)
    {
        bindable.ItemAppearing += HandleItemAppearing;

        _listView = bindable;
        _itemsList = ((ITemplatedItemsView<Cell>)_listView).TemplatedItems;

        if (_listView.Parent is VisualElement parent)
        {
            AttachToParent(parent);
        }
        else
        {
            _listView.PropertyChanged += HandleListViewPropertyChanged;
        }

        for (int i = 0; i < _itemsList.Count; i++)
            UpdateItemHeight(i);
    }

    protected override void OnDetachingFrom(ListView bindable)
    {
        _listView.ItemAppearing -= HandleItemAppearing;
        _listView.PropertyChanged -= HandleListViewPropertyChanged;

        for (int i = 0; i < _itemsList.Count; i++)
        {
            var cell = _itemsList[i];
            cell.PropertyChanged -= HandleCellPropertyChanged;
        }

        foreach (var pair in _elMap)
        {
            var el = pair.Key;
            el.SizeChanged -= HandleCellSizeChanged;
        }

        if (_parent != null)
            _parent.SizeChanged -= HandleParentSizeChanged;

        _elMap.Clear();
        _cells.Clear();

        _cells = null;
        _itemsList = null;
        _listView = null;
        _parent = null;
        _parentHeight = _contentHeight = -1;
    }

    #region Handlers
    private void HandleItemAppearing(object sender, ItemVisibilityEventArgs e)
    {
        UpdateItemHeight(e.ItemIndex);
    }

    private void HandleListViewPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == nameof(ListView.Parent))
        {
            if (_listView.Parent is VisualElement parent)
            {
                AttachToParent(parent);
                _listView.PropertyChanged -= HandleListViewPropertyChanged;
            }
        }
    }

    private void HandleParentSizeChanged(object sender, EventArgs e)
    {
        if (_parent == null) return;

        _parentHeight = _parent.Height;
        AdjustListHeight();
    }

    private void HandleCellPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (!(sender is Cell cell)) return;

        if (e.PropertyName == nameof(Cell.RenderHeight))
        {
            int index = _itemsList.IndexOf(cell);

            if (index < 0)
                return;

            UpdateItemHeight(index);
        }
    }

    private void HandleCellSizeChanged(object sender, EventArgs e)
    {
        if (!(sender is VisualElement el)) return;

        if (_elMap.TryGetValue(el, out int index))
            UpdateItemHeight(index);
    }
    #endregion

    private void AttachToParent(VisualElement parent)
    {
        _parent = parent;
        _parentHeight = _parent.Height;
        _parent.SizeChanged += HandleParentSizeChanged;
    }

    private void UpdateItemHeight(int index)
    {
        Cell cell = _itemsList[index];
        double newHeight;

        if (!_cells.TryGetValue(index, out double oldHeight))
        {
            oldHeight = 0.0;
            _cells[index] = oldHeight;

            if (cell is ViewCell viewCell)
            {
                _elMap[viewCell.View] = index;
                viewCell.View.SizeChanged += HandleCellSizeChanged;
            }
            else
            {
                cell.PropertyChanged += HandleCellPropertyChanged;
            }
        }

        if (cell is ViewCell vCell)
        {
            newHeight = vCell.View.Height;
        }
        else
        {
            newHeight = cell.RenderHeight;
        }

        if (newHeight >= 0)
        {
            double delta = newHeight - oldHeight;
            if (delta == 0) return;

            _cells[index] = newHeight;

            _contentHeight += delta;
            AdjustListHeight();
        }
    }

    private void AdjustListHeight()
    {
        if (_contentHeight > 0 && _contentHeight < _parentHeight)
        {
            _listView.HeightRequest = _contentHeight;
        }
        else if (_parentHeight >= 0)
        {
            if (_listView.HeightRequest != _parentHeight)
                _listView.HeightRequest = _parentHeight;
        }
    }
}

AutoSizeBehavior 的这种实现可以很好地调整大小并自行清理。

Xamarin.Forms ListView 大小到内容？

Xamarin.Forms ListView size to content?

listview

xamarin.forms

height

sizetocontent

行为：

用法：